ABSTRACT: Fiber optic based distributed acoustic sensors (DAS) provide a new approach for monitoring signals of interest in the subsurface with unprecedented spatiotemporal resolution. These sensors produce measurements that are fundamentally different from their traditional counterparts, such as geophones, and produce significantly larger volumes of data. To interpret these data, we begin by using the physics-based thermal-hydraulic-mechanical (THM) model in the GEOS code to simulate synthetic DAS measurements for a range of subsurface conditions (fracture propagation, fault slip, etc.) and sensor configurations (e.g.: horizontal or vertical well deployments). These synthetic DAS measurements are then algorithmically labeled based upon features of interest within their parent models, such as the extent of any generated hydraulic fractures or the distribution of proppant particles, and are compiled into a database. The synthetic database can be used to train and test an initial deep neural network (DNN) representation of the subsurface, which can then be optimized by incorporating any available field measurements through transfer learning. This hybrid, physics informed DNN model is capable of interpreting DAS measurements in near-real time, making it a useful tool for decision making by field engineers, and works under both data-rich and data-poor conditions. To demonstrate this approach, we consider the problem of imaging hydraulic fracture propagation in an unconventional oil and gas reservoir. Our results indicate that we are can use a trained DNN model to accurately estimate the extents of a hydraulic fracture using the location information and DAS measurements for a single fiber-optic sensor as inputs.

ABSTRACT: In this paper, the algorithm of the hybrid finite-element meshfree method (FEMM) is first used to simulate three-dimensional thermal cracking in the context of the TOUGH-FEMM simulator. The TOUGH-FEMM simulator combines two existing codes sequentially, TOUGH2 and FEMM. The temperature distribution of the system is analyzed using TOUGH2, which is able to solve coupled problems of nonisothermal, multiphase, multicomponent fluid flow in complex geological systems. Recently, hydro-mechanical models based on FEMM and TOUGH2 have been coupled together for the analysis of hydraulic fracturing in fractured rock. The thermal stress, rock deformation and cracking induced by temperature change are modeled by FEMM. The finite element meshfree method (FEMM) is presented to model mechanical deformation and discontinuities. With the concept of partition of unity, the accuracy of global approximation can be improved without adding extra degrees of freedom (DOF), and various local approximation functions can be used for modeling strong or weak discontinuities. Linear elastic fracture mechanics (LEFM) with mixed-mode stress intensity factors are employed to determine the initiation and direction of crack propagation. Several benchmark examples, including stress intensity factor calculation, steady-state heat conduction, transient heat conduction, and deformation induced by thermal stress, are provided to validate TOUGH-FEMM in dealing with thermo-mechanical problems with cracking. Additionally, a numerical experiment of crack propagation induced by thermal stress is also given, and its results are compared with the experimental results. The simulation results agree well with the analytical solutions or experimental results, indicating that the proposed approach is effective for thermal cracking simulation.

ABSTRACT: Sand production is a serious problem in heavy oil thermal recovery wells in the Bohai Bay, China. In this study, a comprehensive thermal-hydro-mechanical sand production model combining the processes of drilling, screen insertion, and gravel packing was established based on the production data of the Bohai Bay. The model was firstly verified by comparing the data from field. And then the model was used to analyze the influence of multiple factors on formation stress and sand production in the process of heavy oil thermal recovery. Several findings are obtained in this study: (1)The circumferential stress, radial stress, and vertical stress around the wellbore all reduce when the injection velocity is increased. (2) The heat injection rate plays the significant roles in the formation stress and pore pressure, especially in near-wellbore region. The pore pressure changes by 8.84 MPa between the maximum and minimum thermal injection velocities. And the plastic zone areas along the direction of maximum principal stress increases exponentially while the thermal injection rate increasing. This study is applicable and meaningful to optimize production strategy of heavy oil thermal recovery wells in Bohai Bay.

ABSTRACT: In this study, we present the results of numerical simulations of washout behavior of granular geomaterials. The washout of soil grains, which is driven by a fluid flow, is considered to be one of the most significant reasons of ground subsidence particularly in urban areas accompanies by underground excavations since the excavation may disturb and cause a groundwater gradient. For this, a computational fluid dynamics (CFD) module was implemented into particle flow code (PFC) so as to include a seepage force in the particle displacement calculation of the conventional PFC. Through this coupling, both particle dislocation created by fluid flow and porosity/permeability change induced by the particles redistribution are considered. To verify the validity of the current coupled PFC-CFD analysis, we compared our numerical estimation of washout rate to that of the previous laboratory experiments. They showed well comparable results for two comparative cases of stable and unstable conditions. We then performed a parameter sensitivity analysis to identify the influential parameters on internal stability against a washout behavior and showed it is the effect of particle size distribution, the area where fluid flows out and infiltration flow rate that affect a washout rate.

ABSTRACT : The estimation of the in-situ stress state is required for the design and execution of deep engineering operations related to Enhanced Geothermal System (EGS). Borehole failures, often referred as borehole breakouts, which are controlled by local stress concentration around the wellbore, are recognized being a useful indicator to assess in-situ stress conditions. However, breakouts evolve with time and this may affect our ability to use them for quantifying the stress state. We use a unique data set from the deep geothermal well of Rittershoffen GRT-1 in order to verify the hypothesis concerning wellbore breakout geometrical evolution. In GRT-1 wellbore, imaging has been acquired 4 days, 348 days and 946 days after drilling completion. Thermal, hydraulic and chemical stimulations have been performed between the first and the second image acquisition. Using this data set, we were able to describe in-situ the breakout evolution with time. We show increase in the extension of breakouts along the well. Contrary to the common assumptions, we also show that breakout widen, but within the limit of the accuracy of our analysis they do not deepen. The consequences of the breakout evolution for stress characterization are significant and add up to other important uncertainties in such analyses like the estimation of strength parameters.

ABSTRACT: The dissolution of rocks in underground flow paths is transport-controlled if their dissolution rates are relatively high, while the dissolution is reaction-controlled if their dissolution rates are relatively low. Transport-controlled dissolution is a common process in the formation of gypsum karst and oil reservoir acid stimulations. As an initial step to understand groundwater flow and dissolution in fractures and in wormholes, pipe flow is used as a simplified representation of groundwater flow. The extended Graetz solution was developed by Li and Einstein, 2017 to simulate the transport-controlled dissolution process when water flows through a tube in gypsum. This model can predict both the effluent concentration and the evolving geometry of the tube during the dissolution process. This paper focuses on the experimental validation of this model. An effluent chemistry monitoring system (ECMS) was developed and integrated in the triaxial system at MIT. Flow tests were conducted with cast gypsum tubes in this triaxial system. The experimental results of the effluent chemistry and tube geometries confirmed the validity of the extended Graetz solution.

ABSTRACT: The knowledge of the fluid flow behaviour is crucial for the success of several energy recovery and geotechnical processes such as enhanced geothermal system (EGS), enhanced oil recovery (EOR), extraction of natural gas, CO2 sequestration, and disposal of nuclear waste in deep geological repositories (DGR). In this study, a 2-dimensional numerical model has been developed to analyse the fluid flow through a micro-porous section of an Indian sandstone. The flow behaviour of various fluids (water, oil and CO2) through the microphotographic section of sandstone sample were analysed using an FEM based software, COMSOL Multiphysics 4.3a. Prior to numerical analysis, the geometry of the section was developed using CorelDraw v.17. The Navier-Stokes equation was used to simulate the flow through the porous media. Flow velocity and Reynolds number were calculated at different planes and locations of the geometry. Due to the difference in the viscosities and densities of the fluids, a higher flow velocity was observed in the case of CO2 when compared to that of water and oil. As expected, a higher magnitude of flow was observed in the pores that have a smaller cross-sectional area. The profiles for flow velocities and fluid pressures have been plotted throughout the sandstone section. Pressure built-up was observed prior to the section having the smallest cross-sectional area. As the fluid flows through the channel, a substantial increase and decrease in the velocity and the pressure has been recorded, respectively.

ABSTRACT: Account for fluid-solid interaction based on a locally averaged approach (Anderson & Jackson, 1967, Tsuji et al., 2008, Furuichi et al., 2014) Directly simulate hydrodynamic forces on the solid particles (Noble & Torczynksi, 1998, Holdych, 2003, Holzer & Sommerfeld, 2009) Scour of rock in unlined rock spillway channels is a critical issue facing many of the world’s dams. From a modeling point of view this poses a challenging and interesting problem that combines rock mechanics and hydraulics of turbulent flow. We analyze this interaction between the blocky rock mass and water by directly modeling the solid and fluid phases—the individual polyhedral blocks are modeled using the Discrete Element Method (DEM) while the water is modeled using the Lattice Boltzmann Method (LBM). The LBM mesh is entirely independent of the DEM discretization, making it possible to refine the LBM mesh such that transient and varied fluid pressures acting of the rock surface are directly modeled. This provides the capability to investigate the effect of water pressure inside the fractured rock mass, along potential sliding planes, and can be extended to rock falls and slides into standing bodies of water such as lakes and reservoirs. Herein we present preliminary results to demonstrate the capabilities of the methodology.

ABSTRACT: Dynamic friction slip of rock joint is prevalent in seismic wave propagation and plate sliding. Great concerns have been taken to reveal the whole process of dynamic friction slip, i.e. the wave-induced micro slip, the inertial slip, and the plate sliding. Series of measuring techniques including the multi-point acoustic emission method and the multi-point infrared temperature method are established based on the modified split Hopkinson pressure device (SHPB), and the characteristics of acoustic emission and heat production in the initial stage of friction sliding on the joint surfaces are investigated. The results show that wave profiles corresponding to the wave-induced micro slip is located at the higher frequency domain of the acoustic emission signals, e.g. frequencies ranging from 78.125 kHz to 312.5 kHz for granite specimens with size 50×50×50 mm 3 . While the signal measured at the lower frequency domain characterizes the propagating of the Rayleigh wave. Wave profiles corresponding to the heat production is located at the lower frequency domain in the temperature signals. The measured amplitudes of elevated temperature are in order of 0.01 °C, which increases slightly with increasing the joint angle. To reveal the intrinsic rules of these measured signals, a reasonable analysis model based on the numerical method is established. The results show that when dynamic slip occurs, a slow front generates and propagates along the joint surfaces. With increasing the joint angle, the amplitude of elevated temperature on the joint surface increases. The calculation results are in agreement with the experimental results.

ABSTRACT: In the framework of the In-situ Stimulation and Circulation (ISC) experiment Fiber-Bragg-Grating (FBG) and Brillouin strain sensing systems were installed to monitor deformation during six hydraulic shearing and six hydraulic fracturing experiments. Three boreholes were dedicated to strain monitoring. Both systems are installed in the same boreholes, offering a unique opportunity to compare these systems with respect to their applicability in hydraulic stimulation tests. A total of 60 FBG sensors with 1 m base length were installed across fractures, shear zones and intact rock. Along the entire borehole length, pre-stressed optical cables for Brillouin distributed strain (DBS) sensing were embedded in grout with two installation methods: a bare cable and a cable packed and fixed with glue every 0.65 m. The strain signals were compared as time series for a given borehole depth and as profiles along the borehole axis. The study reveals that the FBG system gives a high accuracy (0.04 µ-strain) and temporal resolution (>1s) with pointwise measurements. The bare DBS leg yield good quantitative strain data with poorer strain accuracy (>500 times poorer than FBG) and poorer temporal resolution (factor of >100). The packed DBS leg provide no meaningful information about the strain field

ABSTRACT: The aim of this article is to understand and improve the predictive power of our previously developed empirical model (Hettema et al., 2017) to be able to better predict the seismicity of the Groningen gas reservoir. A specific characteristic of the gas production from the Groningen field has been the large seasonal variations in production. Four issues are addressed, namely the effects of delay time and production fluctuations, the application to regions in the field and understanding the causality between the production and seismicity. The model predicts inter-event volumes, which is the product of inter-event time and production rate, which decreases with increasing production. The analysis of four classes of minimum magnitudes with different data densities, requires a proper choice of the analysis window, sometimes requiring single event analysis. The causality of the model is understood and allows determination of inter-event times. Furthermore we come to the conclusion that the delay time is currently limited to days. Applying the model to different regions of the field gave reasonable descriptive results, but the applicability is still not proven.With a P of 2900 billion normal cubic meters, the Groningen field in The Netherlands is the largest onshore gas field in Europe. Continuous production since 1963 has led to induced seismicity starting in the early 90's. Production measures aimed at lowering the level of seismicity have been implemented since 2014. Based on an empirical relationship between the cumulative number of seismic events and cumulative gas production, a basic empirical predictive model has been developed (Hettema et al. 2017) . The two empirical parameters and the confidence interval of the prediction have been determined by analyzing the yearly ratio of seismic activity over production versus the cumulative production. Predictions have only been made one year ahead. The present article aims to further develop the model with better predictive power and its control possibilities by increasing the confidence of the model. This requires incorporation of four refinements/extensions: the effects of delay time and production fluctuations, application to regions of the field and understanding the causality between the production and the seismicity. The effect of delay times between production and seismicity has been included by analyzing the pressure transient effects based on semi-steady-state diffusion times. Hettema et al. (2017) have shown that this delay-time increases with increasing reservoir pressure depletion. A specific characteristic of the gas production from the Groningen field has been the large seasonal variation in production. With the demand being high in winter and low in summer, the field played the role of ‘swing producer’ for the Dutch gas market. Since the previous model relates the seismic activity rate to the volume produced, the production rates are averaged within the analyzed window. Consequently, the rate fluctuations and time delays are suppressed. The present model performs better than the basic empirical model in that the goodness-of-fit shows a better confidence interval. It is desirable to be able to apply a model to different parts of the field as to optimize the production control based on localized risk. Finally, we aim to understand the physical causality of the model by considering the material balance and the well production profiles of the depleting gas reservoir. With this new model we gain confidence that better seismicity rate and inter-event time predictions can be made, with the goal to enable the decision makers to make science-based decisions to optimize the safety of the people that live in the province of Groningen.

ABSTRACT: Acoustic image logs from a borehole located at Kimama, Idaho in the Snake River Province were interpreted using WellCAD™ to match and generate orientation data of the natural fractures intersected. One well known problem with such data, however, is the bias imposed towards perpendicular features from linear ‘scanline’ sampling along the borehole axis. This leads to an inaccurate representation of the one-dimensional (1-D) fracture intensity (P 10 ). To correct for this bias, the methodology relies on the assumption that the borehole is truly vertical. An analysis using the Terzaghi, 1965 correction gives 3% of all fracture orientations to be sub-parallel, 41% sub-horizontal and to demonstrate a uniform-biased distribution. The Fouché, 2004 correction yields the highest weighting factor of ~ 9 for a sub-parallel set, in comparison to ~ 14 using the Terzaghi, 1965 method. The chi-squared (? 2 ) test proves independence between observed and expected fracture distributions. The Kolmogorov-Smirnov (two-sample KS) test shows a substantial mismatch between the cumulative distribution functions (cdf) of the observed and modeled distribution for any sample chosen from the population. Results corroborate the scanline theory, but despite stronger theoretical justifications these methods do not substantially change the corrected statistics for the borehole.

ABSTRACT: A series of fracture and flow tests are being performed at the Sanford Underground Research Facility (SURF) as part of the EGS Collab project. The tests involve generating a communicating fracture(s) between two boreholes, and monitoring flow through the generated fracture(s). To perform these tests a robust, remotely operable pressure system is required, as much of the flow testing will be performed over long periods of time when the equipment is not monitored. The system utilizes several pumping systems to include air driven liquid pumps, syringe pumps, and a triplex pump. The syringe pumps and triplex pump are connected to a data acquisition and control system, and can be controlled remotely for pressure and flow. The triplex pump is controlled using a variable frequency drive and a pneumatically actuated back pressure control valve on an integrated flow bypass line. A secondary back pressure control valve can be used to generate back pressure in the production side of the system. The injection and production test intervals are connected at the surface via high-pressure tubing and a differential pressure gauge, and downstream of the production well a series of sensors are in place for detecting tracers injected into the system. The pressurization system is connected to the in situ fracture through a pair (one in each of the boreholes) of straddle packers with a proprietary measurement tool in the packer interval. The tool, dubbed the SIMFIP (Step-Rate Injection Measurement for Fracture In Situ Properties) packs off and allows flow into and out of the interval along with collecting data from numerous sensors. Data from the shakedown of the system, performed at Sandia National Laboratories prior to field deployment, will be presented.

ABSTRACT: For the storage of natural gas, compressed air or hydrogen in rock salt caverns it is necessary to carry out numerical calculations for the dimensioning of the relevant operation parameters. These are on the one hand the allowable maximum and necessary minimum internal pressure to ensure a gastight and stable cavern over decades and on the other hand the limitation of the withdrawal and refill rates between these geomechanical limits.

ABSTRACT: Borehole images have accurately recorded the geological information and the structure characteristics of rock mass, but structural plane parameters in borehole images are usually obtained by manual operation with large workload and affected results by human factors. To solve this problem, this paper presents a full-automatic recognition method of structural plane parameters in the obtained borehole image from borehole camera or imaging system. This method uses image gray, gradient values and projection method to distinguish the occurrence region of borehole structural planes, and then uses standard sine function matching method to search the structural plane in the region. Results show that this method can automatically identify the structural planes in the borehole image continuously and quickly, and obtain the corresponding structural parameters, such as the central positions, orientations and dip angles of structural planes in borehole images. It is stable and reliable, and greatly improves the working efficiency. This full-automatic recognition method can realize the automatic features picking of structural planes and the analysis of geometric parameters in borehole images after drilling engineering. This paper has provided an effective and reliable solution for the improvement of drilling information acquisition technique and borehole image signal processing technique.

ABSTRACT: Underground cavities in rock salt have received increased attention for the storage of oil, gas, and compressed air energy. In this study, the transition between secondary and tertiary creep in salt is determined by a micro-macro model: The initiation of grain breakage is correlated with the acceleration of viscoplastic deformation rate and with the initiation of damage at the macroscopic scale. Salt stiffness decreases when macroscopic damage increases, which allows predicting the evolution of the damage zone around salt caverns used for geological storage. After implementing the phenomenological model into the Finite Element Method (FEM) program POROFIS, two thermo-mechanical coupled stress paths are simulated to analyze stress concentrations and viscous damage around a 650-m-deep cavern in axisymmetric conditions. Numerical results indicate that, despite the pressurization or depressurization-induced temperature variation, internal gas temperature always tends to approach the primary surrounding rock mass value. The viscous deformation induced by thermo-mechanical couplings significantly affects the original stress field at the cavern wall and induces high damage at the most concave sections of the cavern. Results reveal the significant influences of idle time, gas pressure range, and injection and withdrawal cycles on stress, strain and temperature distributions in the vicinity of the cavern. More analyses are needed to confirm the influence of thermo-mechanical cycles of pressurization and depressurization, and to design long-term cavern operations. 1. INTRODUCTION

ABSTRACT: This paper considers the physics and solid and fluid mechanics relevant to sanding from weak or very weak sandstones, and then uses this understanding to suggest ways of designing screenless completions to take advantage of these aspects to avoid sand production. This is followed by a description of a scale-dependent approach to sand prediction. This takes grain size and production tunnel diameter into account within the conventional framework of in-situ stress, reservoir pressure and rock strength parameters. The influence of these factors on sand failure over the life-of-reservoir is treated. A brief discussion of the problem of predicting the rate of sand production is also included. The approach is demonstrated with a case study showing how wells in an old gas field, which were shut-in because of sand production, were recompleted using the sand failure prediction and design capabilities of the methodology presented to restore production, sand-free, to former rates.

ABSTRACT: Tracking the development of hydraulic fracture (HF) propagation in the laboratory and in the field has always been difficult, especially in cases where multiple HFs are propagating simultaneously. A new method is presented using fiber optic (FO) technology to monitor the hydraulic fracturing process during real time. The method has been proven using FO cables ‘woven’ in a grid pattern and cast inside 46 cm (18 inch) cubic blocks of cement. Some of these blocks were then placed in a large cubic polyaxial load frame and subjected to a true triaxial state of stress during the fracturing operation. The FO cable acts essentially as a long continuous ‘strain gage’ in the direction of the cable. As a HF propagates across each FO cable segment it sharply strains each one. This process allows a real time tracking of the position of the growing hydraulic fracture crack tip during testing. The technique is being used to track single HFs, multiple HFs, and leak- off haloes in polyaxial HF block testing.

ABSTRACT: Recent research at Montana Tech has focused on demonstrating the effectiveness of the emerging fiber-optic-based distributed strain and temperature (DST) and distributed acoustic sensing (DAS) technologies in an underground environment, facilitating their adoption by the mining industry and thereby contributing to mine safety. Deployment of a fiber optic array underground was used as base to compare the performance of this new technology to that of traditional instruments deployed at the site, and to compare performance of different brands of fiber optic cable. From the lessons and techniques learned during this deployment, an underground fiber optic array protocol has been developed to aid future engineers in the utilization of this tool.

Abstract: Understanding thermal properties is necessary for the development of both shallow and deep geothermal systems. Thermal properties can also be used for understanding the development and assessing weathering in a variety of materials. An experimental apparatus has been developed to determine the thermal properties of rock samples. The apparatus consists of a ring heater, aluminum adaptor, and a stainless steel base. Three thermocouples are used to measure heat flow through the base. The rock specimen sits on top of the stainless steel base. Twelve thermocouples, arranged vertically in sets of three at the cardinal positions around the rock sample, are used to measure heat flow through the sample. The apparatus is wrapped in insulation to prevent heat loss. The top of the apparatus is open so a thermal camera can capture the temperature increase at the top of the specimen. Thermal analyses conducted using finite element modeling have verified the design of the apparatus. Two limestone specimens have been tested and experimentally derived thermal conductivities are within experimental ranges of thermal conductivities presented in the literature. Introduction Understanding the thermal properties (thermal conductivity, thermal diffusivity, and heat capacity) of rock is important for regional geothermal and ground source heat pump applications. Ouali (2009) studied the variation in thermal conductivity of a stratigraphic column with varying porosity in Reykjavik, Iceland. Goto and Matsubayashi (2009) investigated the correlation of thermal properties in the Juan de Fuca Ridge as a function of porosity and depth. Additionally thermal properties may provide insight into the weathering state of specimens. Banks et al. (2010) studied the use of thermography for structural health monitoring of subsurface anomalies in porous materials. The thermal properties of rock can be estimated based on the rock porosity with varying accuracy.