Abstract Production of sand from a reservoir is a well known problem in the oil industry. Classically, it is solved by a gravel-pack or frac-and-pack type of completion. This approach, although resolving the sand production problem, has the important drawback of severely reducing the output of the completed well. Recently, two types of completions have been proposed for sanding prone reservoirs: slotted liners for horizontal wellbores and hydraulic fracturing without gravel packing in vertical wellbores. These completions have a less negative impact on reservoir production, but require high quality data acquisition to complete successfully. This paper focuses on how a combination of sonic measurements and stress tests carried out with the micro-hydraulic fracturing technique enabled the design of a successful hydraulic fracturing treatment for sand control offshore California. Hydraulic fracturing specialists believe that a hydraulic fracture alone can successfully perform sand control if one has proper knowledge of the stresses acting over the zone of interest. The direction of the maximum principal stress is needed to ensure that all perforations are connected to the hydraulic fracture and therefore protected against sand production. Magnitudes of the minimum principal stress are needed for the proper design of the hydraulic fracturing job and, in particular, to ensure proper placement of the proppant which will stop the produced sand from reaching the wellbore. Such an approach was successfully applied to a sanding- prone turbidite sand-shale reservoir. Critical knowledge of the stresses was acquired by first determining the azimuth of the preferred fracture plane from anisotropy processing of the sonic logs. This direction was validated from local knowledge of the active fault system. It is recognized that stress magnitudes are best measured with the micro-hydraulic fracturing technique. Thus, a special cased hole program was designed to measure the magnitude of the minimum principal stress with a wireline testing tool. Measurements were obtained in the two reservoir sands and three bounding shale layers. These measurements were then used to calibrate a stress log obtained with the processing of a sonic log. This provided a profile of minimum stress magnitudes along the zone of interest. Analysis of the micro-hydraulic fracturing tests showed very little stress contrast existed between the reservoir rock and the bounding layers. This gave the client an option to design a single hydraulic fracturing treatment for the two layers, more efficient that the two separate treatments initially proposed. The interpretation also confirmed from a hydraulic fracturing standpoint that the azimuth derived from acoustic anisotropy was indeed that of the preferred fracturing plane. This allowed the client to orient the perforations with confidence. The unique combination of measurements described in this paper enabled the client to design and successfully carry out a hydraulic fracturing treatment for sand control. P. 219

Abstract Core damage is a permanent alteration of rock properties as a result of drill-out and retrieval of the core from the in situ environment to the surface. For reservoir rocks, stress release during coring is thought to be a major core damage mechanism. In this paper, we present a systematic approach to correcting core compaction data for effects of such damage. Our core damage quantification strategy has the following 3 steps: – A synthetic rock, analogous to the reservoir rock with respect to rock mechanical and petrophysical properties, plus texture, is created under stress. Comparative tests are performed to simulate virgin compaction behavior, and the behavior of an unloaded and reloaded simulated core. – Based on these experiments, a simple elastoplastic model has been developed, describing both the core and virgin compaction behavior of the synthetic analogues. The model is currently in a preliminary stage, but we present it here as an important element in our strategy. – Tests with natural reservoir rocks are performed. Obtained parameters are fed to the mathematical model above, permitting a prediction of the expected compaction of the same rock sample placed in the reservoir. A field example illustrating the use of this approach is presented. The results, based on measurements on synthetic and real cores, indicate that core damage corrections may be substantial, reducing the expected compaction by 25 - 50 % during reservoir depletion. P. 311

Abstract This paper presents relationships for estimating horizontal stresses based on the assumptions that the in situ stress state in a petroleum basin is controlled by the bounding normal or thrust faults at a limit equilibrium and that the fault block is linear elastic and plane strain condition applies in the direction parallel to the strike of the fault. These relationships are an extension of an earlier study and include the effect of residual friction angles on the estimation of horizontal stresses at depth. The result shows that re-orientation of the minimum principal stress is possible after faulting depending on the Poisson's ratio of the formation. Predictions based on the relationships are compared with the stress data obtained in normal and thrust fault conditions as well as with the change in the minimum horizontal stress induced by the pore pressure depletion. The results show that to match the field stress data, a relatively low residual friction angle (100 – 300) on the fault is required. This is further supported by the numerical modelling of the in situ stresses in the Cusiana field in Colombia, and is consistent with the residual friction angles measured in laboratory or back-calculated based on earthquake mechanism. P. 59

Abstract In response to increasing concerns over the environmental effects of the disposal of drill cuttings to the sea, a number of North Sea operators have begun to dispose of cuttings through injection into underground strata. Phillips Petroleum, as part of the Eko II redevelopment program for the Ekofisk field, conducted an evaluation of cuttings disposal options prior to selecting cuttings reinjection for all cuttings disposal from the new 2/4X wellhead platform. In support of the selection of the final alternative for reinjection of cuttings, reinjection of slurrified cuttings via a dedicated well into the mid-overburden overlying the northeastern flank of the reservoir, an extensive engineering study was performed. Within the study, a number of issues were evaluated and reviewed: the relevant experiences of other North Sea operators; the economic evalution of various mud systems and disposal alternatives; a complete review of the geologic structure of the overburden for determining the optimal location of reinjection; numerical simulations of fracture behavior and performance; well design considerations; final selection of the well location; contingency planning; and a subsidence risk assessment. The subsidence risk assessment, performed in cooperation with an outside consultant, evaluated the unique concerns of reinjection into the overburden overlying a compacting reservoir. Cuttings reinjection was initiated late in 1996 and by early 1998 more than 700,000 barrels of slurry had been reinjected into a single completion interval. Difficulties with the well have been relative minor, though the well has plugged-up several times, and the reinjection operation is considered successful. Building upon these results, plans are now being implemented for cuttings reinjection via dedicated wells at the Eldfisk field, another compacting reservoir lying to the southeast of Ekofisk. P. 95

Abstract This paper describes the hierarchical approach used to characterize the regional state of stress in tectonically active settings. The example featured here is the Cusiana field where wellbore instability was a serious problem. The Cusiana field is located in the foothill of the Colombian Andes, South America. A key to solving the instability the was to identify what stress information was needed to improve drilling performance. Using drilling records, caliper logs, cavings samples, cores from the reservoir and a few meters of borehole image, an internally consistent working hypothesis of the state of stress was formulated, tested and refined. Simple calculations based on elastic-brittle model, confirmed also b, y drilling data, showed that the minimum principle stress is horizontal and the overburden was the intermediate stress. Elastoplastic modeling of borehole deformation in sands and shales based on core testing and finite element analysis placed tighter constraints on the relative principal stress magnitudes and the magnitude of the maximum horizontal stress. P. 179

Abstract The principal function of leak-off tests is to assess casing integrity, but the test data are commonly used beyond this original purpose, for stress estimations in exploration and drilling. The value of leak-off test data for stress estimation is questionable and extended leak-off tests have been proposed as a more appropriate method for stress estimation. The paper for the first time compares leak-off test (LOT) and extended leak- off test (ELOT) data from two areas, the North West Self of Australia and the Norwegian North Sea. Standard LOTs have a characteristically large scatter associated with the leak-off pressures (Plo), minimum stress and fracture gradient estimates in both areas. However, the minimum stress determined from ELOTs are consistent and form a lower bound to the standard fl0 data in both areas. The ELOT takes about an hour to perform, but provides far superior data than that obtained from a LOT, and is recommended where stress data are required. P. 131

Abstract A model of induced seismicity caused by water injection is considered. Analysis is based on the assumption of the existence of listric (curvilinear) faults. This presumes that shear stresses acting on the fault may be close to shear resistance at a certain part of the fault surface which is favorably orientated with respect to the principal tectonic stresses. Shear stress on the other parts of the fault is much less than the shear resistance. It is supposed that the pore pressure caused by water injection can propagate down the fault decreasing its shear resistance until shear stresses initiate propagation of shear fracture. Thus slip may occur on a part of the fault which is situated deeper than water injection depth. The slip, if happens dynamically, can be associated with a major seismic event. Analysis has been performed to estimate possible locations of the event, its magnitudes and occurrence time. Results of these calculation agree with observations of seismic events reported for the Tatarstan oil province (Russia). It is believed that similar results can be found for other oil production regions. P. 265

Abstract A system for acoustic velocity and static Young's modulus measurements on small (mm-size) shale samples is presented. The equipment has been tested on both soft and hard shales, including cuttings. Even though more experimental testing, must be carried out to confirm and improve its performance, it is demonstrated that reliable measurements can actually be performed on the small samples. Moreover, the results appear consistent with results attained from larger plugs using more conventional test methods. This technology represents a relatively fast and inexpensive method of attaining dynamic and static data from sub-surface formations. Use of cuttings and small samples can greatly improve the data availability, particularly where proper cores are not available. P. 23

Abstract In laboratory experiments, an evolution of the measured rock properties is often observed when the core undergoes cyclic solicitations. There is still a debate in order to define which value should be chosen, and basically the question is to define which value is intrinsic to the rock and which value should be considered as an artifact. Experiments have been performed on a reservoir sandstone and 9 loading cycles have been applied. It is shown that the sandstone behavior can be fully described using Biot's semi-linear poroelasticity. Within this theoretical framework, it is shown that cyclic loading leads to intrinsic values of the rock constitutive law. P. 321

Abstract Stress trajectories around faults have been simulated with a 3-D finite element model, based on 8-model brick elements in five layers. Preliminary simulations imply that stress trajectory deflections in the neighbourhood of fault zones can be expected if there is a large geomechanical contrast between the fault zone and the adjacent rocks and if far field stress anisotropy is small. On the other hand, if the horizontal stress anisotropy is large, significant stress deflections appear unlikely to occur near a fault. Within a softer' fault zone H will be aligned approximately parallel to the trend. Magnitudes of maximum and minimum stress are modelled for soft and hard fault simulations. Significant modifications in stress magnitudes developed in the vicinity of the fault zones. P. 33

Abstract We conducted laboratory-drilling experiments in Berea sandstone blocks of two different porosities in which borehole breakouts were induced under several truly triaxial far-field stress conditions. We used a petrographic microscope to study thin sections of the drilled boreholes and observed the mechanism of failure that ultimately resulted in breakout formation. In lower porosity Berea sandstone (17%) we identified a breakout failure mechanism that is similar in its final V' shape to that observed in granite and limestone, and occurring along the ah spring line. This shape results from successive spalling of slabs initiated by dilatant intragranular microcracking subparallel to a~ direction, The cracks extend intergranularly due to the much lower toughness of the rock matrix, and reach the borehole wall along tortuous diagonal paths, forming a wide-angle V'- shaped notch. A dramatically different failure mechanism was observed in the higher porosity sandstone (22.5%), a more weakly cemented but still competent Berea sandstone. Breakout failure occurs at the same locations around the borehole as above, but its final shape is best described as a narrow linear slot or fracture perpendicular to aft direction. Our interpretation of the mechanism forming this type of breakout is that it is related to the weak bonding between the quartzitic grains as inferred from the high porosity of the rock, which also implies less cementing matrix material between the grains. Under sufficient far-field stresses, high stress concentration around the hole (along the ah springline), causes the collapse of the weak and sparse matrix, and partially or totally debonds the quartz grains. The disaggregated grains spall off in non-dilatant fashion, creating a higher stress concentration behind them. This in turn facilitates further disintegration of the cementing matrix, and removal of additional grains along the ah spring line. The newly observed breakout type, different from any previously experienced, could have major significance in studying instability problems in wellbores intersecting poorly consolidated sandstones. P.229

Abstract This paper is based on laboratory investigations of synthetic sandstone cemented under stress making it possible to perform mechanical tests directly from the simulated in situ stress state. We have mapped the acoustic emission damage surface, or Holcomb surface as we have called it here, around this stress state. The results show that this surface does not coincide with the "true" damage surface as damage is induced in the material even for very small load deviations from the forming stress state. The Holcomb surface mapping is done with a new technique that is able to better describe and quantify onset of acoustic emission. P. 23

Abstract A predictive in-situ stress model has been built using finite- element techniques for salt enclosed stringers located in the South Oman Salt Basin. The stringers comprise silicilyte reservoir and shale formation layers, in a semi-thrustlike configuration. The model provides input and support to the fracture distribution and the compaction assessment of the silicilyte reservoirs. The model has been build based on seismic, gravity and well data. Before building the in-situ stress model, two series of sensitivity analyses have been carried out concerning the geometrical parameters that influence the stress distribution and stability of in salt enclosed stringers. In the first analysis, the dominating parameters in an environment with an active rising salt dome (halokinesis) have been investigated. This analysis improved the motivation for the kinematic and geological development of the structures. In a second analysis, the stability of a single dipping stringer in a stationary salt body was investigated. The latter configuration is concordant with the current situation in the South Oman Sat Basin. The outcome of this analysis provided insight into the stress distribution of sinking stringers encased in salt. The insights obtained from these analyses were incorporated into the in-situ stress model. P. 43

Abstract Borehole failures like drilling-induced fractures and breakouts observed inimage logs offer the possibility to derive high quality stress orientations. Image logs from 16 wells in the Northern North Sea are analysed for theoccurrence of borehole failures. While in wells to the west of the Viking Graben very few stress related borehole failures are observed abundantdrilling-induced fractures are detected in all of the image logs in wells to the east of the graben. Also the orientation of S H appears to be slightly different for the western and the eastern part of the investigated area. An S H orientation of approximately NI000E is found in the western part, while the S H orientation is found to be approximately N800E for the wells in the eastern part. Investigating the conditions for fracture initiation in three of the wells allows to constrain the possible magnitude of S H . For all three wells the occurrence of drilling-induced fractures can only be explained if the S H magnitude is at least slightly greater than the vertical stress, which indicates a strike-slip tectonic regime (S h <S v <S H ). Introduction There is increasing awareness for the state of stress being an important parameter for engineering tasks in reservoir development, and for the geoscientific understanding of fault seal and hydrocarbon migration. While quite sophisticated tools for e.g. prediction of wellbore stability and sand production are available, the knowledge of the present-day in-situ stress field is still often based on scarce and poor quality data. Formerly, stress orientations were commonly derived from breakout orientations determined from the analysis of four-arm caliper data. As this tool provides only two diameters of the borehole cross section, the accuracy of derived stress orientations is generally low, and special care has to be taken to differ key seats from breakouts. This has in many former analyses lead to a significant scatter in the orientation of S H which often was explained by local influence of topography, faults, or salt layers, while it may be due to the immanent weaknesses of the four-arm caliper analysis.

This paper was prepared for presentation at the SPE/ISRM Eurock '98 held in Trondheim, Norway, 8–10 July 1998.

Abstract Based on a containment transport model developed for hydrogeological purposes, a numerical method for the analysis of intrusion of potassium ions into a shale has been developed. The scheme has been applied in the back-analysis of KCl-brine exposed specimens of smectite-rich Tertiary Paleocene shale from the North Sea. The specimens, exposed under effective confining stresses in a triaxial cell at 80 C, shrank during the KCl-exposure. Two tests with different KCl concentrations (5wt% and 20wt%) have been back-analysed. The ion transport is modelled by diffusion Using a finite difference scheme. In the back-analysis it was assumed that the observed shrinkage is due to the ion exchange when the bound Na+-ions are exchanged with K+-ions from the exposure fluid. The agreement between simulated and measured shrinkage rate was good, indicating that the assumed relation between ion transport, ion exchange and shale shrinkage is a valid mechanism. The simulations showed tensile stresses near the specimen boundaries, where the shale first shrinks. A downhole situation was therefore analysed with a linear elastic model, and the development of tensile stresses with time was investigated. This linear elastic analysis showed that in the vicinity of the borehole wall, large tensile stresses develop as the front of the K+ ions progresses into the shale. Such tensile stresses may lead to the development of cracks and fissures, which in turn increases the surface area of the brine-exposed shale. An accelerating mechanism of cracking may thus develop, increasing the potential of destabilising the shale. For each practical case there is thus an upper limit to the KCl-concentration which should be used in smectite-rich shale. Up to this limit, stability is improved due to a reduced stress concentration. Above this limit, stability problems will increase with increasing KCl-concentration. P. 273

Abstract Analysis of minifracs and pore pressure surveys from sand reservoirs in a Gulf of Mexico oil field show effective stress ratios, K, that scatter significantly and do not correlate with previously published fracture gradient models for this area. The lower-bound value of K is 0.33, which corresponds to the expected value for Coulomb failure for a coefficient of friction of 0.6 in normal faulting environments. However, in some sands K approaches unity, thus indicating an essentially isotropic stress field. Hence, the data indicate a highly variable state of stress that cannot be simply related to depth or pore pressure, but appears to reflect an interaction between deformational processes and material properties. Borehole breakout analysis in vertical wells reveals stress orientations that are predominantly perpendicular to normal faults and, hence, consistent with an extensional stress regime. Analysis of breakouts in inclined wells in two sand reservoirs allows to constrain the magnitude of the maximum horizontal principal stress, SHmax. and further indicates an active normal faulting environment with a clear, but small degree of horizontal stress anisotropy (i.e., Sv>SHmax>Shmin). P. 69

Abstract Analysing borehole breakouts is a well known method for obtaining horizontal stress directions. In this study dipmeter data from 46 wellbores in the Tampen area in the northern North Sea were analyzed for breakouts. The analysis identified a large problem of differentiating borehole breakouts from key- seats. Even at very low bole deviation elongations seemed to align with hole azimuth. This hampered breakout identification, and indicated that key seating possibly is a much larger problem than implied in earlier breakout work from the North Sea From our findings and also based on other stress data from the area it seems as though the wellbore very often deviates in the direction of the least horizontal stress, thereby masking breakouts as key seats. To examine this observation the effect of bedding and stress on the wellpath were examined, without yielding a solution to the problem. For future stress analyses in the northern North Sea enhanced use of imaging data will be important to avoid similar problems. Borehole breakouts identified by dipmeter analysis primarily seem to give an indication of the in-situ stress direction, as long as the number of observations are statistical significant. Traditional breakout analysis should not be trusted alone, but always be constrained by, for instance, image analysis. P. 151

Abstract Placement of solid wastes in dissolved caverns in salt is increasingly being viewed as an environmentally secure technology for low-toxicity wastes such as foundry sands (heavy metals), contaminated soil and other granular solid waste streams. Costs of salt cavern disposal probably eliminates their use for large volumes of non-toxic wastes, but permanent entombment of toxic materials in salt caverns is economically competitive to other alternative disposal or storage approaches. Salt caverns in Alberta and Saskatchewan at depths of 1,200-1,500 m are being used for disposal of non-toxic materials (non-hazardous oilfield wastes) and for storage of natural gas and liquids (propane, glycol, etc.). Low to medium toxicity oilfield wastes can be economically and securely placed in salt solution caverns using an engineered slurry approach. Effective cavern disposal requires a design and monitoring strategy to optimize cavern utilization and to demonstrate environmental security for waste containment. This article is conceptual in nature; however, its conclusions and observations are based firmly on extensive laboratory, field, and modeling experience in salt rocks, applied to caverns and to underground mines. P. 239

Abstract In this work we evaluate the role of lithospheric flexure resulting from post-glacial rebound as a possible source of regional stress variations in the northern North Sea. Compilations of leak-off and pore pressure data, estimates of the full stress tensor from observations of wellbore failure, and earthquake focal plane mechanisms indicate high horizontal compression (i.e., SHmax>Sv and Shmin=Sv) on the west side of the Viking Graben and decreased horizontal stresses on the east side. In this study we extend the available knowledge about regional stress magnitudes by analysis of leak-off and pore pressure data. We use numerical modeling of lithospheric flexure due to post glacial rebound to show that this is a possible cause of the observed high horizontal compression in the Tampen Spur, regional stress variations and possibly the high pore pressures observed at depth west of the Viking Graben. P. 189