Different measurement methods have been utilized to investigate the quality of the cement sheath inside two cemented sandwich sections including high-resolution ultrasonic cement evaluation logs, analysis of samples, mechanical loading, and fluid seepage measurements. The sections were recovered from a North Sea well during a permanent plug and abandonment operation. The measurements have been analyzed with an aim to describe in detail the spatial variations in the cement properties and relate them to the logs. Ultrasonic cement evaluation logs were recorded to map the acoustic properties of the annular cement in the casing sections. Logging passes were recorded using different annular fluids and with different internal casing pressures to investigate the potential effect on the casing to cement bond response. The casing annulus was pressure tested using water and gas, and seepage rates were recorded whilst varying the annulus and the inner casing pressures. The sections tested were instrumented with an array of annulus pressure sensors. On one section, strain gauges were installed on the casing outer surface to record the transfer of strain through the cement sheath to the outer casing. The eccentricity of the inner casing was up to 70% compared with the outer casing which results in a substantial variation of the cement sheath thickness. Accordingly, the pressure sensors and strain gauge arrays were positioned to capture both axial and azimuthal variations of the cement sealing properties. Cement mechanical, chemical, and acoustic bulk properties were also measured on core plugs taken from the cement sheath. The log recordings and sensor measurements showed that the cement sheath properties vary considerably, both along the section length and from the narrow to the wide side of the annulus in the casing sandwich sections. The sealing quality of the cement sheath measured by pressure testing could be correlated with the log response. We observed a nearly linear reduction in seepage rates when increasing the inner casing pressure due to the reduction in size of the annular leakage path. Analysis of bulk properties confirm the presence of cement defects such as mud contamination and microannuli. The logs identified features related to the test cell construction that demonstrated the log spatial resolution and enabled an accurate spatial comparison to be made between the logs and cement sheath sealing properties. A comprehensive data set has been recorded on casing in casing-cemented sandwich sections with axial and azimuthal variations in the cement sheath quality. The data analysis has improved the understanding of the cement sheath mechanical properties, the seal quality, and the response of the ultrasonic cement evaluation logs.

As part of plug and abandonment (P&A) operations, several acceptance criteria need to be considered by operators to qualify barrier elements. In casing annuli, highly bonded material is occasionally found far above the theoretical top of cement. This paper aims to describe how the highly bonded material can be identified using a combination of ultrasonic logging data, validated with measurements in lab experiments using reference cells and how this, in combination with data from the well construction records. can contribute to lowering the costly toll of P&A operations. Ultrasonic and sonic log data was acquired in several wells to assess the bond quality behind multiple casing sizes in an abandonment campaign. Data obtained from pulse-echo and flexural sensors was interactively analyzed with a cross-plotting technique to distinguish gas, liquid, barite, cement, and formation in the annular space. Within the methodology used, historical data on each well was considered as an integral part of the analysis. During the original well construction, either water-based or synthetic oil-based mud was used for drilling and cementing operations, and some formation intervals consistently showed high bonding signature under specific conditions, giving clear evidence of formation creep. Log data from multiple wells confirms formation behavior is influenced by the type of mud used during well construction. The log data provided information of annulus material with a detailed map of the axial and azimuthal variations of the annulus contents. In some cases, log response showed a clear indication of formation creep, evidenced by a high bond quality around the production casing where cement cannot be present. Based on observations from multiple fields in the Norwegian continental shelf, a crossplot workflow has been designed to distinguish formation from cement as the potential barrier element. NORSOK D-010 has initial verification acceptance criteria both for annulus cement and creeping formation as a well barrier element, both involving bond logs; however, in the case of creeping formation it is more stringent stating that "two independent logging measurements/tools shall be applied." This paper aims to demonstrate how this can be done with confidence utilizing ultrasonic and sonic log data, validated against reference barrier cells (SPE-199578). Logging responses like those gathered during full-scale experiment of reference barrier cells with known defects were observed in multiple wells in the field. Understanding the phenomenon of formation creep and its associated casing bond signature could have a massive impact on P&A operations. With a successful qualification of formation as an annulus barrier, significant cost and time savings can be achieved.

The Extended Leak Off Test (XLOT) is a sophisticated formation integrity test that can be performed during drilling, recompletion, or at the well abandonment stage. The test is usually characterized by multiple cycles, creating and manipulating a fracture that can extend several meters away from the wellbore. The test can provide more data (both formation stress and fracture mechanics) compared to traditional leak-off tests. This data is used extensively both for determination of the in-situ formation stress for well barrier integrity assessment and for more general rock mechanical work such as quantifying fracture gradient for use in wellbore stability programs for drilling and completion operations. The interpretation is performed by analysis of the surface pressure and, often with downhole data from memory gauges (or, increasingly, with real-time data from wired pipe) at different stages of the XLOT test. The typical XLOT pressure analysis chart is shown below (see Fig.1 ). The key determined parameters are: – Leak Off Pressure (LOP) – Fracture Initiation Pressure (FIP) – Formation Break Down Pressure (FBR) – Formation Propagation Pressure (FPP) – Instantaneous Shut-In Pressure (ISIP) – Formation Closure Pressure (FCP) – Fracture Reopening Pressure (FRP) Figure 1 The traditional XLOT interpretation plot. A key requirement of the test is to ensure hydraulic connectivity to the targeted formation only. This can be achieved in the case where annulus barriers are in place and perform well. Unintentional communication to non-targeted zones may result in abnormal behavior, more complex interpretation of obtained data, larger uncertainty in the meaning of the results and ultimately failure of the XLOT test. To verify the well barriers integrity prior to the XLOT different techniques can be utilized. The main one is cement bond logging across the cemented barriers. This indicates the condition of the cement behind the first casing and increases the level of confidence the test will be conducted successfully. "However, recent case studies have shown that an indication of good bond above and/or below the target formation from a cement bond log cannot guarantee the isolation required to sufficiently hold the applied pressure [Maxim Volkov]." The paper demonstrates an approach taken by Equinor in a special application where XLOT testing was advanced by adding downhole monitoring during the test. This targeted the following parameters to evaluate the new essential components of XLOT interpretation: – depth and capacity of opened and re-opened fractures, – actual sealing of the cement barriers above and below the targeted zone, – failure investigation in case the FBP cannot be achieved.

We build systems to automatically interpret cement evaluation logs using supervised machine learning (ML). Such systems can provide instant rough interpretations that may then be used as a basis for human interpretation. Here, we compare the performance of two approaches: A previously published approach based on deep convolutional neural networks (CNNs) that autonomously learn to extract features from well log data, and a feature-engineering approach where we use our own domain knowledge to extract features. We base this work on a dataset of around 60 km of well log data. Specialist interpreters have classified these logs according to the bond quality (6 ordinal classes) and hydraulic isolation (2 classes) of solids outside the casing. We train the ML systems to reproduce these reference interpretations in segments of 1 m length. The CNNs directly receive log data as a collection of 2D images and 1D curves. In the feature-engineering approach, we combine the extracted features with various classifiers. For bond quality, the CNNs’ interpretation exactly matches the reference 51.6% of the time. 88.5% of the time, it does not miss by more than one class. For hydraulic isolation, the CNNs match the reference 86.7% of the time. The best-performing feature-based classifier, which is an ensemble of individual classifiers, provides better results of 57.4%, 89.5%, and 88.9%, respectively. Our results indicate two main reasons why feature-based classifiers may perform particularly well on this task. First, there is some subjectivity inherent in the well log interpretations that are used to train and test ML systems. Second, well logs comprise many different and complex pieces of data. For these reasons, this dataset may be particularly liable to overfitting. This may favour approaches based on feature engineering, where we apply our domain knowledge to extract a few pieces of essential information from the data instead of leaving the job of understanding the data to an ML system that may misinterpret spurious patterns as generalisable. It may also favour simpler classifiers with less overfitting capacity. This article shows how petroleum researchers and engineers can implement automatic interpretation systems for cement evaluation logs using ML methods that are relatively easy to apply and deploy, with better results than an approach based on autonomous feature extraction. This approach could also be adapted for automatic interpretation of other types of well log data.

Shale is an effective barrier material. It has a proven track record of acting as a seal (barrier) for oil and gas reservoirs for millions of years. Shale with high clay content and especially high smectite has low permeability, in the nanodarcy range, compared to standard class G laboratory cement that has permeability in the 10–20 microdarcy range. Weak ductile shales will also have a self-healing behavior should fractures be induced at some point. Shale is approved by regulators to be used as well barriers and part of permanent plug and abandonment (P&A) for oil and gas wells. Examples of regulations are Norsok D-010, 2013 in Norway and O&G UK, 2012. In Norsok D-010, one suggests the formation of shale barriers to happen due to creep in ductile shales. Creep occurs in many materials and is observed as deformation under constant load and is also well described in rock mechanics literature. In a previous paper ( Kristiansen et al., 2018 ), it was discussed how shale can be activated as a barrier to form around the wellbore in some shale types. This can be done by inducing a pressure drop in the open annulus (rapid drawdown), by heating the shale by a couple of hundred degrees Celsius, or by chemical processes. In that paper, the process found most effective and practical at that time was demonstrated: the activation of shale barriers with a rapid pressure drop in the annulus. It was also shown that the barrier can be verified days after by standard verification methods used in the industry (pressure testing and bond logging). The shale barrier verification criteria are analogous to cement barriers. In this paper we share the experience from the implementation of a strategy to use shale as well barriers in new wells at Valhall and a second field, Ula, around 100 km away. The method used to activate the shale barriers has revealed some challenges from a well control point of view, but it has also shown that waiting a couple of weeks, or in some cases a couple of months, shale barriers are forming with the same quality as when they were activated or logged later as part of P&A. From this work it can be concluded that the shale barriers logged during P&A are, in some cases, in place only weeks or months after the wells have been drilled. The activation seems to induce an acceleration of time-dependent deformation that will naturally happen over longer time and is consistent with rock mechanics principles of time-dependent deformations in rocks (like creep and consolidation).

Ultrasonic and sonic logs are increasingly used to evaluate the quality of cement placement in the annulus behind the pipe and its potential to perform as a barrier. Wireline logs are carried out in widely varying conditions and attempt to evaluate a variety of cement formulations in the annulus. The annulus geometry is complex due to pipe standoff and often affects the behavior (properties) of the cement. The transformation of ultrasonic data to meaningful cement evaluation is also a complex task and requires expertise to ensure the processing is correctly carried out as well interpreted correctly. Cement formulations can vary from heavy weight cement to ultralight foamed cements. The ultrasonic log-based evaluation, using legacy practices, works well for cements that are well behaved and well bonded to casing. In such cases, a lightweight cement and heavyweight cement, when bonded, can be easily discriminated from gas or liquid (mud) through simple quantitative thresholds resulting in a Solid(S) - Liquid(L) - Gas(G) map. However, ultralight and foamed cements may overlap with mud in quantitative terms. Cements may debond from casing with a gap (that is either wet or dry), resulting in a very complex log response that may not be amenable to simple threshold-based discrimination of S-L-G. Cement sheath evaluation and the inference of the cement sheath to serve as a barrier is complex. It is therefore imperative that adequate processes mitigate errors in processing and interpretation and bring in reliability and consistency. Processing inconsistencies are caused when we are unable to correctly characterize the borehole properties either due to suboptimal measurements or assumptions of the borehole environment. Experts can and do recognize inconsistencies in processing and can advise appropriate resolution to ensure correct processing. The same decision-making criteria that experts follow can be implemented through autonomous workflows. The ability for software to autocorrect is not only possible but significantly enables the reliability of the product for wellsite decisions. In complex situations of debonded cements and ultralight cements, we may need to approach the interpretation from a data behavior-based approach, which can be explained by physics and modeling or through observations in the field by experts. This leads a novel seven-class annulus characterization [5S-L-G] which we expect will bring improved clarity on the annulus behavior. We explain the rationale for such an approach by providing a catalog of log response for the seven classes. In addition, we introduce the ability to carry out such analysis autonomously though machine learning. Such machine learning algorithms are best carried out after ensuring the data is correctly processed. We demonstrate the capability through a few field examples. The ability to emulate an "expert" through software can lead to an ability to autonomously correct processing inconsistencies prior to an autonomous interpretation, thereby significantly enhancing the reliability and consistency of cement evaluation, ruling out issues related to subjectivity, training, and competency.

As oil and gas fields mature, many wells are scheduled for permanent well abandonment or permanent abandonment of a section for subsequent slot recovery. We present a novel data interpretation workflow that integrates pulse-echo inversion to accurately predict borehole fluid properties and sonic attenuation with ultrasonic measurement to accurately define the annular material present behind the casing. Per the NORSOK D-010 standard, the requirement to qualify a well section for further drilling is to evaluate any degradation of the main bore casing, along with evaluation of the well barrier element. In an example slot recovery well, pulse-echo measurement and a cement bond log were recorded in the main borehole. The data was interpreted using an industry-available cement evaluation workflow to indicate the presence of very low impedance material (gas column) between two casings up to surface. The presence of a gas column required remedial action to mitigate well integrity issues during well construction. Advanced cement interpretation was conducted using a newly developed workflow that integrates new pulse-echo inversion and sonic attenuation measurement. The novel pulse-echo inversion accurately determines azimuthal as well as depth variations in the acoustic impedance of the borehole mud. The inversion is validated using 3D models without prior knowledge of mud properties. With the help of in situ measurement of mud properties, we can now visualize features such as mud deposition and segregation in deviated pipes as well. This new processing enables easier and more accurate interpretation of the cement sheath together with providing essential information about the logging fluid. The obtained results clearly identified the presence of solids and liquids in the annular space, which helped the operator to make informed decisions for continued slot recovery and new well drilling operations, rather than spending rig time on gas column mitigation measures. Advanced cement evaluation measurement and interpretation techniques reduce risks and enable accurate and critical decision making for well construction, intervention, and plugging and abandonment in many conditions that were previously ambiguous. Novel three-parameter inversion for pulse-echo measurements delivers unmatched accuracy for identifying the annulus material, as well as critical information on the logging mud, leading to a high level of confidence on cement placement. This aspect is especially critical in ensuring correct evaluation in complex new well designs including vertical, deviated, or horizontal pipes with a variety of mud types.

This paper is based on the analysis of the ultrasonic/sonic data of the 9 5/8-in. casing logging of the 14 wells of the Varg field within the Norwegian Continental Shelf. While writing this papper Varg field was undergoing a plug and abandonment (P&A) phase after 19 years of production. High-quality bonding is observed behind the 9 5/8-in. casing far above expected theoretical top of cement within single casing areas. This bonding is attributed to the formation influence. Formation is used as an alternative to traditional cement barriers during P&A, and its use is regulated by the legislation. The paper aims to develop better understanding of the mechanisms responsible for formation bonding development. The percentage of observed bonding at "high" and "high and moderate-to-high" quality is calculated within each well and is related to the various factors that could influence formation bonding development. Factors such as duration of time lapsed from well completion to well logging, type of well (producer versus injector), geological formation, type of drilling mud used, duration of production periods, volumes of production, and well deviation and azimuth were looked at to determine observable trends and relationships. The results of the study allowed us to conclude which factors are critical or influence formation bonding. Based on the observations, recommendations can be made for the selection of the first well to be logged on the planned P&A campaigns. Correct selection of the first well saves time and resources on the formation testing for the qualification of the formation as a barrier.

The job objective in a UK North Sea field was to permanently abandon a well that had poor cement bonding behind the casing across the intended isolation intervals. The challenge was to provide lateral isolation across two separate intervals in the most efficient way possible. Two "perf-and-wash" operations were executed successfully during the well abandonment. The deeper barrier envelope was validated by tagging and pressure testing the plug. The shallower section had been logged prior to the operation and, on completion of the perf-and-wash job, the plug was drilled out to allow for relogging, which indicated more than 76% of the perforated interval had circumferential coverage. After the bond log results were confirmed, a further cement plug was set across the shallow interval by conventional methods and verified by tagging and pressure testing. This paper outlines the detailed design preparations and presents both case histories where these steps were implemented successfully.

In a Deepwater well off the Brazilian coast which presented a complex architecture with multiple drilling casings and liners, losses were expected during cement placement across a carbonate formation. This paper describes the use of a new real time monitoring and evaluation tool which takes the data acquired during the cement placement, then processes and simulates in real time to provide important job parameters such as estimation of fluid interface positions inside the casing and annular space, pressure match chart, density quality assurance and quality control (QA/QC), ECD and dynamic well security, among others. This manuscript present two cases history where the operator and the service company work together to define a decision tree for the possible contingencies related to unwanted TOC based on mud losses or unplanned cement placement. Later during the operation the new tool combines the design data with the cement unit and rig acquisition data to compare the job measured surface pressure, density, flowrate and volume with predicted data from simulations. Finally based on the information of real time estimation of the TOC outside the pipe and annulus space observed during the job execution a contingency from a decision tree is taken. The cementing service company provided real-time monitoring and evaluation tool that allowed the operator to identify the estimated TOC at the end of placement. With this information, the client was able to avoid the top of liner squeeze and save 2-3 days rig time Later a cement bond log showed that top of cement was found between the liner lap confirming the barrier element. In another case it was prevented leaving unplanned cement inside the casing with the analysis of the job and simulated pressure match trends at the end of the displacement and eliminated unexpected flat times for additional drill out time. Real-time monitoring and evaluation is a tool that can be deployed not only in Deepwater wells in Brazil, but in any section of wells being drilled around the world on land, on the shelf or in Deepwater, where the operator wants to visualize ether the deviation of job execution from job design parameters or a prompt estimation of top of cement as a first level of detection for the well barrier placement just after bumping the plug. In addition having the real time dynamic ECD will also aid in avoiding any potential well control situations (including lost circulation) during the cement operations at any time during this critical activity

This paper describes an approach for identifying formations as annular isolation barriers. It also presents details of a novel analysis of cement evaluation and casing integrity data sets as an alternative to casing milling and secondary cementing use in plug and abandonment operations. Recent ultrasonic cement evaluation logs, performed as part of the integrity diagnostics required for plug and abandonment operations, showed an increase in casing ovality when logging through a specific halite formation in the southern North Sea. These findings were used to postulate that the formation was coupled mechanically to the casing and could be used as an abandonment barrier. The circumferential cement evaluation data helped to identify the azimuthal coverage of the formation, and subsequent pressure testing confirmed the integrity of the halite formation as an isolating medium. Multiple offset wells were also analyzed with a focus on identifying additional horizons exhibiting mobility that could be detected using casing logging tools. It became evident that this halite formation showed a consistent and clear correlation between casing ovality and circumferential coverage. Case studies are presented in which the halite formation was identified as an appropriate barrier, based on the combined interpretation of both cement evaluation and casing inspection data. This phenomenon typically occurred when the top of cement was below the halite interval; however, in some cases, the formation movement actually improved the cement bond quality across the zone. By cross-referencing the cement evaluation and casing inspection logging data sets, the mobility of the halite formation can be identified, and subsequent integrity testing confirms its isolating capabilities as an appropriate barrier for plug and abandonment operations. The use of geological formations with suitable mobility as an alternative to a standard milled casing window with a cement plug can reduce rig time and consequently reduce operational costs.

A flowing or pressure-sustained a nn ulus is a live threat to the environment, population, vegetation, and natural habitat. In the current oilfield environment under strengthened regulations from regulatory authorities, zonal isolation must be assured before completely abandoning a well. In the west region of Sumatra, Indonesia, a large number of shallow injector wells are drilled; after passing a certain age and productive life, they are abandoned. Most of these wells have lost circulation and therefore need a cost- and time-efficient cement placement method that can demonstrate zonal isolation by means of cement bond logs. The conventional cement placement method across the perforations requires several steps: placing cement, squeezing, leaving the cement on top of the perforations, waiting on cement, drilling the cement, and running the cement bond log. A novel squeeze method was deployed as a solution by combining a chemical contaminant and a squeeze technique. The objective was to perform the cement bond log directly after wait on cement without spending time to drill out the cement. In this technique, cement was placed using a coiled tubing pump-and-pull method: A squeeze was performed in the perforations; then coiled tubing was run into the well below 30 ft of perforations and an engineered contaminant fluid was pumped to contaminate the cement in the tubing. After the recommended wait- on-cement time, the cement bond log was run to confirm zonal isolation across the perforations. This technique eliminated the need to drill the set cement in the slim tubing, saving several days of work time. In most of the wells, the cement bond log showed less than 5 mV, which helped to determine the achievement of zonal isolation. Furthermore pressure tests were conducted to ensure the zonal isolation across the perforated intervals. Engineered cement and contaminant design is required to ensure cement will gain and maintain designed mechanical properties behind the tubing. This paper will discuss the case histories from six wells in the west region of Sumatra field, where the novel coiled tubing placement technique has been applied and proven successful and cost-effective.

Cement evaluation is a critical part of a cementing operation. Recent industry events and the continuous tightening of regulations for cement placement have elevated the importance of cement evaluation to heights greater than ever. The initial verification of a primary cement job is performed using the pressure trend obtained while pumping, because the pressure increases as the cement rises in the annulus. The actual pressure trend can then be compared with the expected pressure to infer the actual cement height in the annulus; however, this methodology can only predict but not confirm the top of cement (TOC). In deepwater, the regulatory agencies may require physical evaluation of TOC in specific strings of the well that can require running cement bond log (CBL) tools for those strings. This would use the critical path time for every CBL run. In addition, this regulatory agency requirement can also be technically challenging because cement bond evaluation for casing as large as 18-in. that is placed in a 22-in. open hole is the upper threshold limit for conventional wireline conveyed CBL tools. Many operators are now runing logging-while-drilling (LWD) sonic tools for open hole compressional data. Using the same sonic tools, logging can be performed through the casing strings on the same run and the results can be analyzed to confirm the TOC. The data presented here seems to show that the results agreed well with the predicted TOC from the pumping data. The use of this tool for cement evaluation was further validated against CBL evaluation that was performed on deeper strings in example deepwater exploration wells. The paper will elaborate on how the integration of this technology can provide precise TOC evaluation and save the operators considerable rig time per section.

Cementing design and execution is key to achieving zonal isolation for safe and economic well production. The critical goal of long-term well integrity is a fundamental element of any well construction strategy especially when the well will be subject to cyclic loads and changing downhole stresses e.g from stimulations operations. Exploration and appraisal wells pose an additional challenge because many formation-specific parameters relating to the field may not be fully understood. This case describes the use of self-healing cement and flexible and expandable cement to optimize isolation and ultimately, well delivery on a critical exploration well project for a National Oil Company. Globally, self-healing cements have been increasing used in the last decade and this was the first application for this operator onshore Thailand. The self-healing cement system was used as a secondary barrier in the well in the case of cracks or deformation within the primary cement matrix. The combination would serve as a more robust solution to mitigate the impact of stresses generated during different well lifecycle phases. The design principle of self-healing cement is the ability to swell upon contact with hydrocarbons to restore well integrity. Upon installation in the well, cement evaluation logs and the physical indications showed that zonal isolation had been completely achieved for the entire production interval. In addition, no sustained casing pressure was experienced after the hydraulic fracturing operations were performed. Health, Safety & Environment (HSE) concerns were addressed ensuring no hydrocarbon gas leak to surface and no contamination of underground water zones.

Abstract During a sidetrack operation out of 9 7/8-in. casing on a deepwater well in Green Canyon Block 243, Gulf of Mexico, unexpected hole conditions were encountered that required the use of an additional casing string. The decision was made to run a 7 5/8 × 9 5/8-in. expandable liner in the 8 1/2 × 9 1/2-in. wellbore. The liner would expand to provide an inside diameter of 7.71 in., allowing space for a 7-in. production liner in the targeted interval. The 6,867-ft liner (pre-expansion length) is currently the world record for the longest expandable liner set to date and presented several challenges for cement job design. The liner would be cemented conventionally before the expansion operation. Expansion time was calculated to be approximately 17 hours, allowing the fluid time for the primary lead cement, with a safety factor, to exceed 19 hours. The tail cement had to exhibit good compressive strength around the shoe track, and the operator specified top of cement (TOC) at 17,000 ft to protect a secondary pay zone. Slurry properties were simulated to meet fluid times required for the liner expansion. Standard API lab tests used for cement testing were modified to accommodate this lengthy operation. The expandable liner was set at 20,605 ft measured depth (MD) and 19,930 ft true vertical depth (TVD) with a maximum hole angle of 36°. The liner was cemented successfully using an extended thickening-time lead slurry mixed at 15.7 lb/gal, followed by a 16.2-lb/gal tail slurry with a shorter pumping time to achieve good strength at the liner shoe. After drilling out the liner, the operator obtained a 16.8-lb/gal equivalent (PPGE) formation integrity test (FIT) and resumed drilling to the target depth. The cement-evaluation log showed excellent bonding behind the expandable liner with TOC at 17,000 ft as planned. Operational details and cement design considerations are provided in the paper. Emphasis is placed on wellbore configuration, expandable installation procedures, and hole preparation, a full understanding of which is the beginning of a successful cement job. Introduction The Aspen field is located in a prolific development area in the Green Canyon Block 243 in 3,000-ft water. This field development has been in progress since 2000 with five major production horizons drilled in the area. Through high-rate production, certain sands have seen some depletion and thus significant pressure regressions have been observed throughout the productive intervals. Operators following proper equivalent circulating density (ECD) management and using synthetic- based drilling fluids with optimized particle-size distributions to maximize the sealing of depleted sand packages have achieved success. This case-history well was a re-entry sidetrack to reach a lower objective known to be in a narrow pore pressure/fracture gradient window because of the pressure-depleted interval. The planned completion program consisted of sidetracking out of the original wellbore through a 10 ¾ × 9 7/8-in. tieback at 14,000 ft and directionally drilling an 8 ½-in. pilot hole to planned TD (Fig. 1). The hole would subsequently be opened to a 9 ½-in. hole size to run a 7 5/8-in. production liner. During the drilling operations through the production interval, wellbore pressures indicated that it would not be possible to continue without encountering significant losses. To still reach lower objectives and complete with a 7-in. production liner, the use of a 7 5/8 × 9 5/8-in. expandable liner installation became critical to the success of the well. Because there were secondary completion objectives behind this installation, it was also critical to obtain a top of cement (TOC) back to 17,000 ft as well.

Abstract The hydraulic isolation of the wellbore casing and cement is critical for the completion of production and injection wells. Zonal isolation prevents the production of fluids from non-completion intervals, contamination of ground water by fluids in the wellbore, and allows conformance control of injected fluids. Current acoustic evaluation techniques may be limited by the acoustic properties of the material behind casing and by the inability to see beyond the cemented region near the casing. A new ultrasonic imaging tool has been developed to address these limitations. The new imager tool combines the classical pulse-echo technique with a new ultrasonic technique that provides temporally compact echoes arising from propagation along the casing and also reflections at the cement-formation interface. Processing these signals yields unprecedented characterization of the cased-hole environment in terms of the nature and acoustic velocity of the material immediately behind casing, the position of the casing within the borehole, and the geometrical shape of the borehole.5 In order to provide answers to the casing/cement evaluation questions, a field study was performed to evaluate the results provided by both sonic and this new ultrasonic tool in the different cement materials, drilling fluids, and casing sizes. Field examples are presented to illustrate the actual response of the new ultrasonic tool to these various completion environments including wells cemented with conventional and lightweight cement. The results demonstrate enhanced cement evaluation for all cement types and a significant reduction in the uncertainty in making a squeeze or no-squeeze decision. The new cement evaluation tool implements both the traditional pulse-echo technique and the new flexural wave concept. The flexural mode enables deep imaging of the cement sheath up to the cement-formation interface. In addition, the measurement of the borehole geometrical shape makes it possible to evaluation double casing string conditions for potential damage. Introduction Sonic logging tools have been used since the 1960's to evaluate the placement of cement for hydraulic isolation of formations. There have been several advancements in the logging tools that improved the ability to evaluate the cement sheath since that time. During the same period of time there has been little change in the types of cement. In the past few years, however, there has been an emphasis on optimizing the cementing operation and reducing the overall cost of the completion. To the cementing operation, this meant developing lightweight and specialized cements that would allow setting casing strings deeper without worrying about lost returns. Other gains in efficiency were also achieved using lighter cements while drilling and completing weak formations. Changes in these cements, and their properties, have also brought about the need for re-evaluating the techniques and tools used for the evaluation of these cements with the sonic logging tools currently available. The Cement Bond Log (CBL) type tools, which include all tools that measure amplitude or attenuation, have a common theory of measurement, interpretation principles, strengths, and weaknesses. The principle of measurement of these tools is to measure the amplitude of a sonic signal, produced by a transmitter emitting a 20 kHz acoustic wave, after it has traveled through a section of the casing as an extensional mode. This amplitude is then converted into attenuation by either using a ratio of multiple transmitter and receiver amplitudes, or using chart book conversions. At this point the interpreter has to select a value for the attenuation of a 100% bonded interval. This can be done based on the CBL data collected in the well or it can come from the predicted cement properties. The value for the attenuation in a 100% bonded interval is the key to the interpretation of this type of log. Zonal isolation is estimated from an empirical data base. These tools also provide a qualitative indication of bond to the formation through the use of a Variable Density Log (VDL) waveform.

Abstract A promising oil source in South Oman is providing challenges to successful cementing operations. The formation is approximately 4200m deep with bottom hole pressure of 13000psi. There is a very narrow margin between the formation pore and fracture pressures. Also salt saturated mud with weights up to 17.5 lbm/gal is routinely required to drill these wells. There has always been a conflict with conventional oilfield cements between optimizing slurry properties for mixing and placement and resulting mechanical properties of set cement necessary for long-term zonal isolation. This conflict is even more evident in high-density environment where achieving slurry which is stable, mixable and pumpable frequently results in sacrificing set cement mechanical properties as compressive strength, permeability and porosity. The challenge has been successfully dealt with in cementing high-pressure wells in South Oman using a technology adapted from the concrete industry. Particle Size Distribution (PSD) optimizes both slurry and set cement properties simultaneously. Extensive lab testing and yard trial prior to field application demonstrated the superior performance of High Density High Performance Slurries (HDHPS) over conventional cements in both the liquid phase and when set. To date eight jobs have been performed and following improvements have been seen. Average waiting on cement time has been reduced by 50% for plugs. Much higher success for plug setting compared to three successful plugs out of the previous seven with conventional slurries. Reduced difference between the planned and actual cement heights for plugs. Well cemented critical production liners shown by Cement Bond Logs. Details of the slurry technology are presented along with data documenting the jobs done to date in South Oman. The system is widely accepted by operator in Oman and is the preferred system for critical high pressure cementing. Introduction In the southern Oman fields, Operator is exploring the potential of oil and gas from high-pressured carbonate stringers embedded in salt. To date the exploration has been highly successful with one of the recent wells flowing at facility constrained rates of 1000m3/day (6300 bbls/day). Drilling and completing such wells successfully is challenging. At depths of 3500 to 4800 m [11,483 to 15,748 ft], temperatures of 95 to 120°C [203 to 248°F] and bottom hole pressures of 13000 psi. Initially, the cement slurry densities required for these wells were achieved using hematite as the weighing agent. This substantially sacrificed the operations ease and set cement properties. Results were below expectations and following incidents were reported using the conventional system. The blend lost its homogeneous nature while transporting to the rig site as hematite being heavy separated out from cement in the blend. Fourteen hrs after a liner was cemented the well kicked. Four days were lost in controlling the well. On another well annulus pressure built up was observed probably due to micro annulus that could not be detected by CBL. Possible cause could be cement bulk shrinkage after it set or improper mud displacement.

Abstract The Boris prospect is a deepwater well drilled at Mississippi Canyon. This exploratory well was drilled in a subsalt environment, which provided many zonal-isolation challenges. Deepwater wells in this area can have shallow water flows (SWF), 1 long sections of salt, and disturbed zones or rubble zones above and below the salt section. While all these hazards were not encountered during the drilling of this well, foamed cement was an integral part of the plan to deal with them. The paper is a case study of how a new foamed-cementing process was used to solve the challenges associated with the zonal isolation and to help Shell meet the target for the number of drilling days. The paper will cover the cementing process used on three primary casing strings during well construction. Foamed Cement Foamed cement was developed in the late 1970's to obtain low-density cement with good compressive-strength development, especially in the 8- to 18-lbm/gal slurry density range. The literature thoroughly documents the use of foamed cement for its conventional, lightweight density uses. Cements are designed to provide zonal isolation, and a competent cement sheath can minimize buckling, parting, and elongation of the casing caused by stresses. The quality of the cement sheath is affected by several factors, including low formation fracture gradients, lost-circulation zones, tight annular clearances between previous casings, and variable holes sizes caused by washouts. In recent years, foamed cementing has become a solution to many different wellbore problems. In the Gulf of Mexico, one of the first special applications was reported in 1996 when foamed cement was used successfully to isolate problem formations behind a liner in a high-temperature high-pressure (HTHP) well in Mobile Bay. 2 Foam was first used as a solution to shallow water flow (SWF) in the Gulf of Mexico in 1994. The first well later became the model for 19 additional wells in a batch operation in which foamed cement was used on 39 total primary jobs. 3 Foamed cement quickly became popular as the cement of choice for the SWF. A 1997 paper presented the use of proper mud management techniques in conjunction with the foamed cement to solve SWF. 1 More recently, the use of foamed cement for controlling SWF was reported on another large batch-set operation. 4 Since foamed cement was first used to control SWF, several hundred jobs have been performed in the Gulf of Mexico. Foamed cement exhibits other desirable properties for obtaining short- and long-term zonal isolation in oil and gas wells. These properties are superior mud displacement, gas-migration control, and long-term sealing of the well's annulus by resistance to cement-sheath stress cracking. Cementing Equipment The process of foamed cementing has been enhanced by the introduction of a fully integrated cement quality control system. 5 Foamed cementing with this type of delivery process has been successfully used for steam producing wells in California. 6 The new system consolidates the controls of all units into one computerized system, allowing the operator to monitor and control all units on location simultaneously when making adjustments to the job. The importance of precision in controlling a job increases with the complexity of the job and the number of units involved. With the advent of foamed cementing, for example, a single job requires that the mixing system, liquid additive system, and nitrogen equipment be on location. More equipment usually requires more operators, and coordination and logistics become more difficult.

Abstract This paper presents the results of successful applications of polymer gels to control water production in Mexico. Three case studies are provided where a systematic methodology was employed to correctly diagnose near-wellbore water channeling behind casing. The methodology discusses the use of diagnostic plots based on the historical behavior of the water-oil ratio as a function of time. Including, correlation with information from original cement bond logs, oxygen activated logs during actual production to effectively determine the origin of the water, and saturation logs to determine the actual levels independently of the salinity of the water been produced. In addition, the paper presents successful applications of polymer gels to re-establish zone isolations in the three case studies mentioned above. It discusses gel placement and presents the procedure followed in each case, evaluation of a water injectivity test followed by a temperature log prior to gel placement to determine height propagation of the water and anticipate potential zone damage of adjacent producing intervals during gel placement. One case is discussed where a new interval was completed perforating through the gel, with excellent results. The other case, presents a zone abandonment with gel were positive pressure tested with 500 and 1,000 psi well head pressure at 2,500 meters. In all cases advantages of gel treatments over common cement squeeze are discussed. Finally, results of the treatments performed are discussed including the analysis of pressures recorded during gel placement. The oil and water production prior and after treatment are presented. Introduction One of the main problems encountered in old wells and in wells that were originally cemented under low reservoir pressure, consist in granting hydraulic isolations between the different intervals to allow proper production of the zones of interest. The lack of isolation has caused, among other things, undesired movements of fluids behind casing generating confusion of the actual levels of the oil-water contact and originating premature abandonment of oil reserves. This paper presents a methodology followed in the north of Mexico to correct channeling of water behind pipe using chromium crosslinked polymer gels. It presents the advantages of using gels over cement, including flexibility for their pumping without a workover rig, higher control in setting time, easy-to-clean, no milling time, and superiority regarding operation cost without risking treatment effectiveness. Included in the methodology is the candidate selection using diagnostic plots which allows to identify, among others, near wellbore flow which correlates with cement bond logs indicating poor cement. Finally, three field case studies corresponding to the north of Mexico, Poza Rica, are discussed. Analysis of the information available is discussed in each case, including saturation logs, production logs, density logs, and water flow, based on the activation of oxygen, to monitor the movement of water through a channel. Corrections to these flow of water is presented including a detail overview of the execution and of the results showing the effectiveness of the treatments. Near Wellbore Flow The problems associated with water production, and its control, represent a great challenge to the reservoir and workover engineers. The key of the problem lies in defining the origin of the water and determining whether the water production of a given interval is necessary to the associated oil production. Therefore it is required to define two kinds of water production: bad and good. P. 415^

SPE Members Abstract This paper describes and quantifies the importance of some geometrical parameters on the main output of the cement bond log, i.e. the attenuation of a sonic wave propagating along a casing to cement interface. These geometrical parameters are the cement thickness, the casing stand-off, the percentage of cemented area and the shape of the noncemented channels. Also the nature of the fluid in the noncemented channel has been investigated. These experiments were performed with a laboratory cement bond tool of variable spacing with between 0.5 and 3.0 feet, coupled to an oscilloscope and a data acquisition unit. It is observed that cement sheath thickness is an important parameter than can significantly affect the output of the tool up to a thickness of more than 2 inches. As a first approximation, the percentage of cemented area is confirmed to be in linear relationship with the attenuation rate. Casing stand-off can also change the log output by a factor of up to 30%. Conversely the shape of the channel is found to have no significant influence on the result provided the percentage of cemented area is the same. The nature, from gas to liquids, density and gel strength been found to be negligible factors. Some of the conclusions are illustrated through the interpretation of several field logs of cased hole or of open hole sections where both the cement and the caliper logs were available. Introduction Since the introduction of the Cement Bond Log (CBL) in the late 50's little experimental work has been performed in order to explain the output of the log in relation with the downhole annular configuration. The effect of vertical channels was studied and a linear relationship was observed between the attenuation rate and the percentage of circumference bonded for a centralized pipe. Later experimental and theoretical studies quantified the influence on the CBL amplitude of casing diameter, casing thickness, transmitter to receiver spacing and cement compressive strength leading to the construction of a nomograph well known as CBL interpretation chart. This last chart which has been modified to take foamed-cements into account suffers from some limitations with regards to tool and casing centralization, cement thickness, well history, etc… Later on, it was also found that the bond log amplitude was greatly influenced by the acoustic properties of the fluid in the casing and by downhole conditions affecting the transducer response, leading to the elaboration of a borehole-compensated cement bond tool. However not all the parameters have been considered and some field logs still cannot be explained with existing CBL interpretation techniques, resulting in totally unsuccessful squeeze cement jobs in wells showing poor bond logs. In the present study, we investigate the influence of critical borehole geometrical parameters which were not considered, or not completely covered in the past. These include casing stand-off, cement thickness, percentage of bonded area, shape of the channel and nature of fluid. P. 763^