Integrated wellbore-reservoir-geomechanics modeling for enhanced interpretation of distributed fiber-optic strain sensing data in hydraulicfracture analysis

Fiber-optic distributed strain sensing(FO-DSS)has been successful in monitoring strain changes along horizontal wellbores in hydraulically fractured reservoirs.However,the mechanism driving the various FO-DSS responses associated with near-wellbore hydraulic fracture properties is still unclear.To address this knowledge gap,we use coupled wellbore-reservoir-geomechanics simulations to study measured strain-change behavior and infer hydraulic fracture characteristics.The crossflow among fractures is captured through explicit modeling of the transient wellbore flow.In addition,local grid refinement is applied to accurately capture strain changes along the fiber.A Base Case model was designed with four fractures of varying properties,simulating strain change signals when the production well is shut-in for 10 d after 240 d of production and reopened for 2 d.Strain-pressure plots for different fracture clusters were used to gain insights into inferring fracture properties using DSS data.When comparing the model with and without the wellbore,distinct strain change signals were observed,emphasizing the importance of incorporating the wellbore in FO-DSS modeling.The effects of fracture spacing and matrix permeability on strain change signals were thoroughly investigated.The results of our numerical study can improve the understanding of the relation between DSS signals and fracture hydraulic properties,thus maximizing the value of the dataset for fracture diagnostics and characterization.

Geomechanics model test and numerical simulation of 2G-NPR bolt support effect in an active fault tunnel

Active faults are a common adverse geological phenomenon that can occur during tunnel excavation and has a very negative impact on the construction and operation of the tunnel.In this paper,the grade IV rock surrounding the cross-fault tunnel with poor geological conditions has been chosen for the study.The support capacity of 2^(nd) Generation-Negative Poisson’s Ratio(2G-NPR)bolt in an active fault tunnel has been carried out on the basis of relevant results obtained from the geomechanical model test and numerical investigations of failure model for existing unsupported fault tunnel.The investigation shows that surrounding rock of the tunnel is prone to shear deformation and crack formation along the fault,as a result,the rock mass on the upper part of the fault slips as a whole.Furthermore,small-scale deformation and loss of blocks are observed around the tunnel;however,the 2G-NPR bolt support is found to be helpful in keeping the overall tunnel intact without any damage and instability.Due to the blocking effect of fault,the stress of the surrounding rock on the upper and lower parts of the fault is significantly different,and the stress at the left shoulder of the tunnel is greater than that at the right shoulder.The asymmetrical arrangement of 2G-NPR bolts can effectively control the asymmetric deformation and instability of the surrounding rock.The present numerical scheme is in good agreement with the model test results,and can reasonably reflect the stress and displacement characteristics of the surrounding rock of the tunnel.In comparison to unsupported and ordinary PR(Poisson’s Ratio)bolt support,2G-NPR bolt can effectively limit the fault slip and control the stability of the surrounding rock of the fault tunnel.The research findings may serve as a guideline for the use of 2G-NPR bolts in fault tunnel support engineering.

Geology and geomechanics of hydraulic fracturing in the Marcellus shale gas play and their potential applications to the Fuling shale gas development

Geological characteristics,geomechanical behavior and hydraulic fracture propagation mechanism in the Marcellus shale gas play are analyzed and compared with China’s Fuling shale play.Successful experiences in hydraulic fracturing and shale gas development in the Marcellus shale gas play are summarized,which might be applicable in other shale plays.The main factors contributing to the successful development of the Marcellus shale gas play include adoption of advanced drilling and completion technologies,increases of hydraulic fracturing stages,proppant concentration and fluid injection volume.The geological and geomechanical mechanisms related to those technologies are analyzed,particularly the in-situ stress impacts on hydraulic fracturing.The minimum horizontal stress controls where the fractures are initiated,and the maximum horizontal stress dominates the direction of the hydraulic fracture propagation.Hydraulic fracturing performed in the shale reservoir normally has no stress barriers in most cases because the shale has a high minimum horizontal stress,inducing hydraulic fractures propagating beyond the reservoir zone,resulting in inefficient stimulation.This is a common problem in shale plays,and its mechanism is studied in the paper.It is also found that the on-azimuth well has a higher productivity than the off-azimuth well,because shear fractures are created in the off-azimuth well,causing main fractures to kink and increasing fracture tortuosity and friction.The Fuling shale gas play has a markedly higher minimum horizontal stress and much smaller horizontal stress difference.The high minimum horizontal stress causes a much higher formation breakdown pressure;therefore,hydraulic fracturing in the Fuling shale gas play needs a higher treatment pressure,which implies higher difficulty in fracture propagation.The small difference in the two horizontal stresses in the Fuling shale gas play generates shorter and more complex hydraulic fractures,because hydraulic fractures in this case are prone to curve to preexisting fractures.To overcome these difficulties,we recommend reducing well spacing and increasing proppant concentration to increase gas productivity for the Fuling shale gas development.

The limit analysis in soil and rock:a mature discipline of geomechanics

The solution of a slope stability problem can be approached by its least upper-bound and maximum lower-bound with high accuracy. The limit equilibrium methods that employ vertical slices imply a lower bound of the factor of safety. It has been successfully extended to the area of active earth pressure analysis that accounts for different input of locations of earth pressure applications. Those methods that employ slices with inclined interfaces give an upper-bound approach to the stability analysis. It enjoys a sound mechanical background and is able to provide accurate solutions of soil plasticity. It has been successfully extended to the area of bearing capacity analysis in which various empirical coefficients are no longer necessary. The 3D upper- and lower-bound methods under this framework have been made possible and show great potential for solving various engineering problems.

Geomechanics of Hazardous Landslides

A catalogue of possible landslide initial failure mechanisms, taking into account the geological setting and the geometry of the slope, the joint structure, the habitus of the rock blocks, as well as the mechanical behaviour of the rocks and of the rock mass (deformation and strength parameters), is presented. Its aim is to give geologists as well as engineers the opportunity to compare phenomena in the field and phenomena belonging to particular mechanisms and to find the mechanism occurring. The presented catalogue of initial landslide mechanisms only comprises the mechanisms having a clearly defined mechanical model that can be divided into empirical relations and into mechanical models, as well as an overview of run out models, which can be divided into empirical relations and into mechanical models.

On the four-dimensional lattice spring model for geomechanics

Recently, a four-dimensional lattice spring model(4D-LSM) was developed to overcome the Poisson’s ratio limitation of the classical LSM by introducing the fourth-dimensional spatial interaction. In this work, some aspects of the 4D-LSM on solving problems in geomechanics are investigated, such as the ability to reproduce elastic properties of geomaterials, the capability of solving heterogeneous problems,the accuracy on modelling stress wave propagation, the ability to solve dynamic fracturing and the parallel computational efficiency. Our results indicate that the 4D-LSM is promising to deal with problems in geomechanics.

Carbon capture and storage,geomechanics and induced seismic activity

Injection of large volumes of carbon dioxide(CO) for the purposes of greenhouse-gas emissions reduction has the potential to induce earthquakes.Operators of proposed projects must therefore take steps to reduce the risks posed by this induced seismicity.In this paper,we examine the causes of injection-induced seismicity(IIS),and how it should be monitored and modelled,and thereby mitigated.Many US case studies are found where fluids are injected into layers that are in close proximity to crystalline basement rocks.We investigate this issue further by comparing injection and seismicity in two areas where oilfield wastewater is injected in significant volumes:Oklahoma,where fluids are injected into a basal layer,and Saskatchewan,where fluids are injected into a much shallower layer.We suggest that the different induced seismicity responses in these two areas are at least in part due to these different injection depths.We go on to outline two different approaches for modelling IIS:a statistics based approach and a physical,numerical modelling based approach.Both modelling types have advantages and disadvantages,but share a need to be calibrated with good quality seismic monitoring data if they are to be used with any degree of reliability.We therefore encourage the use of seismic monitoring networks at all future carbon capture and storage(CCS) sites.

AN AXIOMATICS OF GEOMECHANICS

Taking the theory of mixture as a basic .framework, the paper merges the primesof rational mechanics irreivrsible thermodynamics and soil meehanics into an organicsystem and proposes an axiomatics of geomechanics .The theoretical system consistsof 5 basis laws and 8 constitutive principles .and it erects a bridge across the gapbetween the pure theory of mechanics and engineering practice .

A geomechanics classification for the rating of railroad subgrade performance

The type of subgrade of a railroad foundation is vital to the overall performance of the track structure.With the train speed and tonnage increase,as well as environmental changes,the evaluation and influence of subgrade are even more paramount in the railroad track structure performance.A geomechanics classification for subgrade is proposed coupling the stiffness(resilient modulus)and permanent deformation behaviour evaluated by means of repeated triaxial loading tests.This classification covers from fine-to coarse-grained soils,grouped by UIC and ASTM.For this achievement,we first summarize the main models for estimating resilient modulus and permanent deformation,including the evaluation of their robustness and their sensitivity to mechanical and environmental parameters.This is followed by the procedure required to arrive at the geomechanical classification and rating,as well as a discussion of the influence of environmental factors.This work is the first attempt to obtain a new geomechanical classification that can be a useful tool in the evaluation and modelling of the foundation of railway structures.

Geomechanics model test research on large deformation control mechanism of roadway disturbed by strong dynamic pressure

Comprehensive mechanized top-coal caving mining is one of the efficient mining methods in coal mines.However,the goaf formed by comprehensive mechanized top-coal caving mining is high,and the goaf roof collapse will cause strong dynamic pressure disturbance,especially the collapse of thick hard roof.Strong dynamic pressure disturbance has an influence on the stability of the roadway,which can lead to large deformation.In order to solve the above problem,a comprehensive pressure releasing and constant resistance energy absorbing control method is proposed.Comprehensive pressure releasing can change the roadway roof structure and cut off the stress transfer between goaf and roadway,which can improve the stress environment of the roadway.The constant resistance energy absorbing(CREA)anchor cable can absorb the energy of surrounding rock deformation and resist the impact load of gangue collapse,so as to ensure the stability of roadway disturbed by strong dynamic pressure.A three-dimensional geomechanics model test is carried out,based on the roadway disturbed by strong dynamic pressure of the extra-large coal mine in western China,to verify the control effect of the new control method.The stress and displacement evolution laws of the roadway with traditional control method and new control method are analyzed.The pressure releasing and energy absorbing control mechanism of the new control method is clarified.The geomechanics model test results show that the new control method can increase the range of low stress zone by 150%and reduce the average stress and the displacement by 34.7%and 67.8%respectively,compared with the traditional control method.The filed application results show that the new control method can reduce the roadway surrounding rock displacement by 67.4%compared with the traditional control method.It shows that the new control method can effectively control the displacement of the roadway disturbed by strong dynamic pressure and ensure that the roadway meets the safety requirements.On this basis,the engineering suggestions for large deformation control of this kind of roadway are put forward.The new control method can provide a control idea for the roadway disturbed by strong dynamic pressure.

Sealing capacity evaluation of underground gas storage under intricate geological conditions

Evaluating underground gas storage(UGS)sealing capacity is essential for its safe construction and operational efficiency.This involves evaluating both the static sealing capacity of traps during hydrocarbon accumulation and the dynamic sealing capacity of UGS under intensive gas injection and withdrawal,and alternating loads.This study detailed the methodology developed by Sinopec.The approach merges disciplines like geology,geomechanics,and hydrodynamics,employing both dynamic-static and qualitative-quantitative analyses.Sinopec's evaluation methods,grounded in the in situ stress analysis,include mechanistic studies,laboratory tests,geological surveys,stress analysis,and fluid-solid interactions.Through tests on the static and dynamic sealing capacity of UGS,alongside investigations into sealing mechanisms and the geological and geomechanical properties of cap rocks and faults,A geomechanics-rock damage-seepage mechanics dynamic coupling analysis method has been developed to predict in situ stress variations relative to pore pressure changes during UGS operations and evaluate fault sealing capacity and cap rock integrity,thereby setting the maximum operational pressures.Utilizing this evaluation technique,Sinopec has defined performance metrics and criteria for evaluating the sealing capacity of depleted gas reservoirs,enabling preliminary sealing capacity evaluations at UGS sites.These evaluations have significantly informed the design of UGS construction schemes and the evaluation of fault sealing capacity and cap rock integrity during UGS operations.

Assessing the Viability of Gandhar Field in India’s Cambay Basin for CO_(2) Storage

Our research is centered on the Gandhar oil field, which was discovered in 1983, where daily oil production has declined significantly over the years. The primary objective was to evaluate the feasibility of carbon dioxide(CO_(2)) storage through its injection into the siliciclastic reservoirs of Ankleshwar Formation. We aimed to obtain high-resolution acoustic impedance data to estimate porosity employing model-based poststack seismic inversion. We conducted an analysis of the density and effective porosity in the target zone through geostatistical techniques and probabilistic neural networks. Simultaneously, the work also involved geomechanical analysis through the computation of pore pressure and fracture gradient using well-log data, geological information, and drilling events in the Gandhar field. Our investigation unveiled spatial variations in effective porosity within the Hazad Member of the Ankleshwar Formation, with an effective porosity exceeding 25% observed in several areas, which indicates the presence of well-connected pore spaces conducive to efficient CO_(2) migration. Geomechanical analysis showed that the vertical stress(Sv) ranged from 55 MPa to 57 MPa in Telwa and from 63.7 MPa to 67.7 MPa in Hazad Member. The pore pressure profile displayed variations along the stratigraphic sequence, with the shale zone, particularly in the Kanwa Formation, attaining the maximum pressure gradient(approximately 36 MPa). However, consistently low pore pressure values(30-34 MPa) considerably below the fracture gradient curves were observed in Hazad Member due to depletion. The results from our analysis provide valuable insights into shaping future field development strategies and exploration of the feasibility of CO_(2) sequestration in Gandhar Field.

Geothermo-mechanical alterations due to heat energy extraction in enhanced geothermal systems: Overview and prospective directions

Geothermal energy from deep underground (or geological) formations,with or without its combination with carbon capture and storage (CCS),can be a key technology to mitigate anthropogenic greenhouse gas emissions and meet the 2050 net‐zero carbon emission target.Geothermal resources in low‐permeability and medium‐and high‐temperature reservoirs in sedimentary sequence require hydraulic stimulation for enhanced geothermal systems (EGS).However,fluid migration for geothermal energy in EGS or with potential CO_(2) storage in a CO_(2)‐EGS are both dependent on the in situ flow pathway network created by induced fluid injection.These thermo‐mechanical interactions can be complex and induce varying alterations in the mechanical response when the working fluid is water (in EGS) or supercritical CO_(2)(in CO_(2)‐EGS),which could impact the geothermal energy recovery from geological formations.Therefore,there is a need for a deeper understanding of the heat extraction process in EGS and CO_(2)‐EGS.This study presents a systematic review of the effects of changes in mechanical properties and behavior of deep underground rocks on the induced flow pathway and heat recovery in EGS reservoirs with or without CO_(2) storage in CO_(2) ‐EGS.Further,we proposed waterless‐stimulated EGS as an alternative approach to improve heat energy extraction in EGS.Lastly,based on the results of our literature review and proposed ideas,we recommend promising areas of investigation that may provide more insights into understanding geothermo‐mechanics to further stimulate new research studies and accelerate the development of geothermal energy as a viable clean energy technology.

A gated recurrent unit model to predict Poisson’s ratio using deep learning

Static Poisson’s ratio(vs)is crucial for determining geomechanical properties in petroleum applications,namely sand production.Some models have been used to predict vs;however,the published models were limited to specific data ranges with an average absolute percentage relative error(AAPRE)of more than 10%.The published gated recurrent unit(GRU)models do not consider trend analysis to show physical behaviors.In this study,we aim to develop a GRU model using trend analysis and three inputs for predicting n s based on a broad range of data,n s(value of 0.1627-0.4492),bulk formation density(RHOB)(0.315-2.994 g/mL),compressional time(DTc)(44.43-186.9 μs/ft),and shear time(DTs)(72.9-341.2μ s/ft).The GRU model was evaluated using different approaches,including statistical error an-alyses.The GRU model showed the proper trends,and the model data ranges were wider than previous ones.The GRU model has the largest correlation coefficient(R)of 0.967 and the lowest AAPRE,average percent relative error(APRE),root mean square error(RMSE),and standard deviation(SD)of 3.228%,1.054%,4.389,and 0.013,respectively,compared to other models.The GRU model has a high accuracy for the different datasets:training,validation,testing,and the whole datasets with R and AAPRE values were 0.981 and 2.601%,0.966 and 3.274%,0.967 and 3.228%,and 0.977 and 2.861%,respectively.The group error analyses of all inputs show that the GRU model has less than 5% AAPRE for all input ranges,which is superior to other models that have different AAPRE values of more than 10% at various ranges of inputs.

Design,Development and Testing of the JTech Bolt for Use in Static,Quasi-Static and Dynamic Domains

Designing a rock reinforcement element requires knowledge of:geomechanical behaviour,interaction of the reinforcement element with rock mass and the element’s mechanistic response in static and dynamic environments.Using this knowledge the JTech bolt was developed and subjected to a thorough program to test,gather data and validate the bolt performance in varying domains.By conducting FE(finite element)modeling,the simulation reviews the JTech bolt design evaluating the effects of threadbar geometric variation,threadbar and nut engagement results under high stress,coating friction response and effects of thread tolerance extremes on the failure mode.These results determine safety factors,tolerances and quality management criteria.Once manufactured,in-situ system testing,laboratory and underground short encapsulation testing,resin mixing testing,double shear testing and dynamic testing at varying velocity and mass,determine the system’s capacity and effectiveness in static,quasi-static and dynamic mining environments.In this paper,the process and results are described.

Fracture characterization of Asmari Formation carbonate reservoirs in G Oilfield, Zagros Basin, Middle East

The Asmari Formation in the G oilfield on the Iran-Iraq border is a fractured-porous multi-lithology mixed reservoir, for which fracture is an important factor affecting oil productivity and water cut. The characterization and modeling of fractures in the carbonate reservoir of G oilfield are challenging due to weak conventional well log responses of fractures and a lack of specific logs, such as image logs. This study proposes an integrated approach for characterizing and modeling fractures in the carbonate reservoir. The features, formation mechanism, influencing factors, and prediction methods of fractures in the Asmari Formation carbonate reservoirs of G oilfield were studied using core observation, thin section, image log, cross-dipole acoustic log (CDAL), geomechanics numerical simulation (GNS), and production data. According to CDAL-based fracture density interpretation, GNS-based fracture intensity prediction between wells, and DFN-based rock fracture properties modeling, the quantitative fracture characterization for G oilfield was realized. This research shows that the fractures in the Asamri Formation are mainly medium-to high-angle shear fractures. The substantial compression stress during the Miocene played a major role in the formation of the prominent fractures and determined their trend in the region, with primary trends of NNW-SSE and NNE-SSW. The fracture distribution has regularity, and the fractures in zone A dolomites are more highly developed than that in zone B limestones vertically. Horizontally, fractures intensity is mainly controlled by faults and structural location. The results of this study may benefit the optimization of well design during field development. From 2019 to 2021, three horizontal wells pilot tests were deployed in the fractures belt in zone A, and these fractures prominently increased the permeability of tight dolomite reservoirs. The initial production of the wells is four to five times the average production of other wells in the area, showing a good development effect. Meanwhile, the updated numerical simulation validates that the history match accuracy of water cut based on the dual-porosity model is significantly improved, proving the fracture evaluation and prediction results to be relatively reliable and applicable.

Deep Learning Applied to Computational Mechanics:A Comprehensive Review,State of the Art,and the Classics

Three recent breakthroughs due to AI in arts and science serve as motivation:An award winning digital image,protein folding,fast matrix multiplication.Many recent developments in artificial neural networks,particularly deep learning(DL),applied and relevant to computational mechanics(solid,fluids,finite-element technology)are reviewed in detail.Both hybrid and pure machine learning(ML)methods are discussed.Hybrid methods combine traditional PDE discretizations with ML methods either(1)to help model complex nonlinear constitutive relations,(2)to nonlinearly reduce the model order for efficient simulation(turbulence),or(3)to accelerate the simulation by predicting certain components in the traditional integration methods.Here,methods(1)and(2)relied on Long-Short-Term Memory(LSTM)architecture,with method(3)relying on convolutional neural networks.Pure ML methods to solve(nonlinear)PDEs are represented by Physics-Informed Neural network(PINN)methods,which could be combined with attention mechanism to address discontinuous solutions.Both LSTM and attention architectures,together with modern and generalized classic optimizers to include stochasticity for DL networks,are extensively reviewed.Kernel machines,including Gaussian processes,are provided to sufficient depth for more advanced works such as shallow networks with infinite width.Not only addressing experts,readers are assumed familiar with computational mechanics,but not with DL,whose concepts and applications are built up from the basics,aiming at bringing first-time learners quickly to the forefront of research.History and limitations of AI are recounted and discussed,with particular attention at pointing out misstatements or misconceptions of the classics,even in well-known references.Positioning and pointing control of a large-deformable beam is given as an example.

Displacement-based back analysis of mitigating the effects of displacement loss in underground engineering

Displacement-monitoring-based back analysis is a popular method for geomechanical parameter estimation.However,due to the delayed installation of multi-point extensometers,the monitoring curve is only a part of the overall one,leading to displacement loss.Besides,the monitoring and construction time on the monitoring curve is difficult to determine.In the literature,the final displacement was selected for the back analysis,which could induce unreliable results.In this paper,a displacement-based back analysis method to mitigate the influence of displacement loss is developed.A robust hybrid optimization algorithm is proposed as a substitute for time-consuming numerical simulation.It integrates the strengths of the nonlinear mapping and prediction capability of the support vector machine(SVM)algorithm,the global searching and optimization characteristics of the optimized particle swarm optimization(OPSO)algorithm,and the nonlinear numerical simulation capability of ABAQUS.To avoid being trapped in the local optimum and to improve the efficiency of optimization,the standard PSO algorithm is improved and is compared with other three algorithms(genetic algorithm(GA),simulated annealing(SA),and standard PSO).The results indicate the superiority of OPSO algorithm.Finally,the hybrid optimization algorithm is applied to an engineering project.The back-analyzed parameters are submitted to numerical analysis,and comparison between the calculated and monitoring displacement curve shows that this hybrid algorithm can offer a reasonable reference for geomechanical parameters estimation.