Geomechanical Problems Research Articles

Subsidence and Effective Stresses Distribution Using Finite Element Techniques for an Iraqi Oilfield

Geomechanical problems are the most important problems that happen in the Zubair oil field; treating these problems requires a lot of time (Non-Productive Time) and thus increases the cost of drilling the well. Wellbore instability, subsidence, reservoir compaction, casing smash, pipe damage, and well kick is the main geomechanical problems facing the drilling process in the Mishrif formation, Zubair oilfield. The main goals of this study are to estimate the changes in stresses and subsequent subsidence values for this field during the production and injection periods. These estimation values, many problems can be avoided, thus increasing the drilling efficiency. This study is to introduce the one way coupling between the reservoir model and geomechanical model using the finite element method. The finite element technique in CMG 2018 program was used to estimate the stress states during the production or injection operations in this field of interest. The results of the 3D finite element model showed that the effective vertical stress rises by 32 psi during production while the effective horizontal stress increases by 16 psi. This may be explained by the fact that variations in pore pressure have little or no impact on the total vertical stress generated by weight. The results of this study demonstrated that the finite element method is a conservative method for coupling reservoir geomechanics and fluid flow. Subsidence values were 6.096 mm in the north part of the Al-Hammar dome, while at the center the subsidence was -5.1816 mm. Shuaiba dome has negative subsidence which is about -9.75 mm. It is important to note that the positive results subsidence signify the pore volume compacting, which may have an impact on the permeability and porosity of the reservoir petrophysics. As a result of a negative subsidence deformation, different failures including well casing damage, wellbore failure, and pipe smashing, are expected. Based on these results, production can cause an increase in the differential stress, which leads to rock shear failure and in injection cases, increasing pore pressure can cause a tensile rock failure.

Special issue on Recent Advances in Solving Coupled Problems in Geomechanics, Geophysics, and Geotechnical Engineering

Special issue on Recent Advances in Solving Coupled Problems in Geomechanics, Geophysics, and Geotechnical Engineering

A three-phase two-point MPM for large deformation analysis of unsaturated soils

This paper presents a three-phase two-point formulation of the material point method (MPM) for modeling unsaturated soils involving large deformation. In the formulation, solid phase material is represented by one layer of material points while liquid and gas phases are modeled by another layer of material points. The formulation is a u-U formulation. It eliminates advection term between solid and liquid phases and is capable to address large relative deformations between the two phases. The advection term between liquid and gas phases are assumed to be small and not considered. The proposed formulation is validated with numerical models for small and finite deformation problems. The capacity of the method for study of geomechanics problems is demonstrated with simulation of seismic-induced ground liquefaction with an unsaturated embankment. The method is able to simulate large ground deformation due to soil liquefaction and generation of excess pore pressure, and captures pore pressure dissipation through the rapid water drainage through high-permeable soils.

A Fully Coupled Discontinuous Deformation Analysis Model for Simulating Hydromechanical Processes in Fractured Porous Media

Numerical simulations play a key role in the optimization of fracturing operation designs for unconventional reservoirs. Because of the presence of numerous natural discontinuities and pores, the rock masses of reservoirs can be regarded as fractured porous media. In this paper, a fully coupled discontinuous deformation analysis model is newly developed to simulate the hydromechanical processes in fractured and porous media. The coupling of fracture seepage, pore seepage, and fracture network propagation is realized under the framework of DDA. The developed model is verified with several examples. Then, the developed DDA model is applied to simulate the hydraulic fracturing processes in fractured porous rock masses, and the effects of rock mass permeability on fracturing are investigated. Our findings suggest that high rock permeability may inhibit the stimulation of fracture networks, while increasing the viscosity of fracturing fluids can enhance the fracturing efficiency. This study provides a valuable numerical tool for simulating hydromechanical processes in fractured and porous media and can be used to analyze various geo-mechanical problems related to fluid interactions.

TESTING OF ROCK SAMPLES FROM OIL AND GAS FIELDS TO DETERMINE THEIR PHYSICAL AND MECHANICAL CHARACTERISTICS

Determination of the physical and mechanical properties of anisotropic limestone rocks is an important and urgent task, since carbonate deposits are very often found in oil and gas fields of the Mangistau region. The data show that limestone creates from 70% to 90% of all problems related to the stability of wells, which is due to the peculiarities of their physical and mechanical properties. Experimental studies of the physical and mechanical properties of limestone have features both in terms of monolith selection and in terms of preparation of samples for research, and in the methods of conducting and processing experiments. These features are due to extremely low permeability, anisotropy of elastic and strength properties due to their layered structure. The experience of testing core samples of layered limestone is considered, tests are carried out on equipment under the program of consolidated-undrained loading in accordance with the standards. As a result of experiments on core samples cut at an angle, along and across the layering, the anisotropic elastic properties of rocks, as well as their dependencies on geophysical parameters, were studied. The anisotropy of elastic properties has a significant effect both on the stress-strain state when solving specific problems of continuum mechanics and on the design values of the initial stress field as a whole. Taking into account the anisotropy parameters obtained in this paper will make it possible to solve Geomechanics problems in relation to layered limestones of the Sarmatian stage.

Novel coupled hydromechanical model considering multiple flow mechanisms for simulating underground hydrogen storage in depleted low-permeability gas reservoir

Depleted low-permeability gas reservoirs are the primary storage media for underground hydrogen storage (UHS). An accurate assessment of the geomechanics and complex flow mechanisms in a depleted low-permeability gas reservoir is crucial for predicting the injection and production capabilities of UHS facilities. This paper proposes a novel hydromechanical multicomponent model that couples non-Darcy flow, relative permeability hysteresis (RPH), and geomechanics to evaluate the hydrogen storage capacity of UHS facilities in a depleted low-permeability gas reservoir. The coupled model was solved using a fully coupled strategy, employing the finite volume method to solve non-Darcy flow equations and the virtual element method to solve geomechanical problems. The results indicate that the high-velocity non-Darcy (hvnD) and geomechanical effects reduce the working gas volume by 12.82% and 10.56%, respectively. Additionally, the low-velocity non-Darcy (lvnD) effect causes the third stress/strain distribution to exhibit a “focusing” phenomenon, resulting in a 13.19% reduction in the working gas volume. This paper also describes a case where residual gas is captured by mechanisms such as “water lock” and “capillary capture” through the RPH effect. Considering the RPH effect, the gas-water transition zone expanded by 30%, and the peak volume of gas-containing pores in the water zone increased by 1.48 times. Ignoring the RPH effect led to a 19.73% overestimation of the utilization rate of the gas-containing pore space. Hence, this study achieves safe and successful seasonal hydrogen storage and provides theoretical support for designing multicycle operational plans.

Development of Mathematical Modeling Methods and Solution of Present-Day Problems in Geomechanics at the Institute of Mining SB RAS

Development of Mathematical Modeling Methods and Solution of Present-Day Problems in Geomechanics at the Institute of Mining SB RAS

Research into mechanical properties of ore and rocks in the ore deposits with assessment of the mass stress state natural field

Purpose. The research aims to conduct a comprehensive study of the mechanical properties of ores and rocks within the Zhilandy Group field, as well as to assess the natural field of mass stress state to solve geomechanical problems in optimizing mining operations. Methods. To determine the boundary conditions when measuring the stress-strain state of the mass, a complex methodology is proposed, including stress measurements using hydraulic fracturing methods of the wells and determining the physical-mechanical properties of rocks. Five rock probes have been tested. Twelve tests (six tests in natural and six in water-saturated states) have been conducted for each probe. Findings. Hydraulic fracturing tests at the metering stations show significant tectonic stress due to the shape of structural folds and the mass fracturing. It has been revealed that the hard rock mass is characterized by non-uniform fracturing. It is of tectonic origin and averages between 10-15 and 15-25 fractures per meter for different lithological varieties. The maximum horizontal stress at the stations is oriented along an azimuth of 70° ± 10. Originality. The influence of water saturation on the reduction of strength and deformation characteristics of rocks has been determined for the conditions of the Zhilandy Group field, which shows significant variations depending on the rock type. Especially important is the revealed fact of a significant decrease in uniaxial compression and uniaxial tensile strength, as well as a decrease in Young’s modulus and cohesion factor. Linear dependences of stresses occurring with depth have been obtained from the measurement results. Practical implications. The results obtained are of significant importance for the mining industry. Understanding the extent of reduction in strength characteristics during water saturation and assessing the natural field of the stress mass state allows more accurate prediction of rock behavior.

Sequential formulation of all‐way coupled finite strain thermoporomechanics for largely deformable gas hydrate deposits

AbstractWe develop a numerically stable sequential formulation of thermoporomechanics for largely deformable gas hydrate deposits, extended from the fixed stress split of infinitesimal transformation. Constitutive equations are based on the total Lagrangian approach for both flow and geomechanics, including dynamic full tensor permeability and thermal conductivity updated from the deformation gradient. For space discretization, we take the cell‐centered finite volume and node‐based finite element method for flow and geomechanics, respectively. Then, we propose a sequential implicit method for all‐way coupled thermoporomechanics, where the nonisothermal multiphase flow problem of gas hydrates is solved implicitly first and then the geomechanics problem is solved implicitly at the next step. During solution of the flow problem, we fix the rate of first Pioal total stress for numerical stability as well as apply porosity correction and entropy correction to account for geomechanical effects. We test numerical examples where flow and geomechanics parameters are based on deep oceanic gas hydrate deposits. When applying depressurization, even though the results between the infinitesimal transformation and finite strain geomechanics are similar in the early stages due to small deformation, we find differences between them in the late times as deformation becomes large. Accordingly, permeability and thermal conductivity tensors become nonisotropic full tensors although they are initially isotropic. We identify numerical stability of the developed sequential method from the test cases that exhibit the highly complex coupled gas hydrate systems with large deformation. Thus, the proposed sequential formulation can be applied in largely deformable gas hydrate systems.

Study on the Effect of Cyclic Loading on the Geomechanics of Sandstone Reservoirs in Gas Storage

In the implementation process of carbon capture and storage (CCS), the geomechanical problems arising from the cyclic injection of carbon dioxide into the formation cannot be ignored. To clarify the influence of cyclic loading on the geomechanics properties of reservoir rocks, cyclic loading tests were carried out on rocks in sandstone reservoirs with the RTR‐2000 Rock Mechanics Test System, and the evolution of compressive strength, elastic modulus, and Poisson’s ratio parameters was analyzed, which revealed the law of the geomechanical deterioration and damage evolution of sandstone reservoirs under cyclic loading. The results show that under the same cyclic load, when the number of cycles is increased to a certain degree, the influence on the peak strength is small. As the number of cycles increases, the modulus of elasticity of rock appears to be increasing and then decreasing, whereas a rapid increase in Poisson’s ratio and a slow increase in Poisson’s ratio occur with the increase in the number of cycles. Under the same number of cycles, the peak strength of the core gradually decreased with the increase of the maximum cyclic stress. When the cyclic load was smaller than the yield stress of the core, the effect of cyclic loading on the strength of the core was small, and when the cyclic load was larger than the yield stress, the effect of cyclic loading on the strength increased significantly. With the increase of cyclic load, the modulus of elasticity showed a tendency to increase and then decrease, and Poisson’s ratio had a tendency of decreasing and the overall degree of change was small.

Geomechanics applied to the petroleum industry: Wellbore stability, sand production, and hydraulic fracturing

This article introduces three applications of geomechanics in oil and gas industry, encompassing wellbore stability analysis, hydraulic fracturing, and sand production. In this paper, we reviewed three commonly used applications involving transforming stress values from the in situ coordinate system to the wellbore centric coordinate system, which have been published in the previous studies. Subsequently, various failure criteria are applied to these three geomechanical problems. First, wellbore stability analysis involves six distinct scenarios across different oil reservoirs. The results obtained enable the selection of appropriate drilling mud densities to prevent collapses and instability of wellbore. Second, regarding sand production modeling, three oil fields are presented as examples. The results consistently indicate instances of sand production under various well production conditions. Finally, the application of geomechanics in hydraulic fracturing is illustrated. The findings distinctly illustrate the evolutionary pattern of fracture dimensions, highlighting a consistent trend in fracture length development. Notably, the expansion phase of the fracture exhibits a rapid onset during the initial stages, followed by a transition into an exceedingly gradual propagation state.

Technology for studying the electrophysical characteristics of sedimentary rocks of South Yakutia at the railway station Kyurgellyakh using the method of remote inductive probing

The possibilities of a new technology for applying the method of remote inductive sensing at a frequency of 1,125 and 0,281 MHz in the separation interval of the emitting and receiving antennas in terms of quantitative assessment of electrophysical characteristics in the rock foundation of engineering structures at the railway station Kyurgellyakh in South Yakutia are studied. In the strata of icy and clay deluvial-eluvial formations and sedimentary rocks (dolomites, limestones), a general increase in depth of the effective values of the electrical resistance and the real part of the complex relative permittivity, which is atypical in AC geoelectrics, has been established. The range of variability of resistivity and permeability is 600–1600 Ohm·m and 3–7 arb. units. According to a sharp change (from a smaller to a larger one) in the growth rate of resistance and permeability, two boundaries were found. The first structural boundary lies at a depth of about 10 m and corresponds to the lower boundary of the area of intense weathering. The second petrophysical boundary lies at a depth of about 20 m and characterizes the transition of the highly weathered and highly fractured upper part of the sedimentary rock mass into the lower consolidated solid part located in the area of thermal rest. The obtained results give grounds to recommend a proven technology for solving the problems of geomechanics and thermophysics of frozen soils in terms of determining their strength and depth of the lower boundary of the layer of annual heat exchanges.

Numerical modeling of problems of geomechanics in the study of an inhomogeneous stress field in the near-wellbore zone

Some of the main actual problems of geomechanics related to the numerical simulation of an inhomogeneous stress-strain state in the near-wellbore zone during the development of oil and gas fields are considered. Methods for analyzing the stress field are presented to identify possible zones of destruction of the main structural elements of the well and perforation channels. For slotted perforation, an example of calculating the changes in reservoir permeability during the transformation of effective stresses near the well is given.

Uncertainty quantification of inverse analysis for geomaterials using probabilistic programming

Uncertainty is an essentially challenging for safe construction and long-term stability of geotechnical engineering. The inverse analysis is commonly utilized to determine the physico-mechanical parameters. However, conventional inverse analysis cannot deal with uncertainty in geotechnical and geological systems. In this study, a framework was developed to evaluate and quantify uncertainty in inverse analysis based on the reduced-order model (ROM) and probabilistic programming. The ROM was utilized to capture the mechanical and deformation properties of surrounding rock mass in geomechanical problems. Probabilistic programming was employed to evaluate uncertainty during construction in geotechnical engineering. A circular tunnel was then used to illustrate the proposed framework using analytical and numerical solution. The results show that the geomechanical parameters and associated uncertainty can be properly obtained and the proposed framework can capture the mechanical behaviors under uncertainty. Then, a slope case was employed to demonstrate the performance of the developed framework. The results prove that the proposed framework provides a scientific, feasible, and effective tool to characterize the properties and physical mechanism of geomaterials under uncertainty in geotechnical engineering problems.

Circumventing volumetric locking in stabilized smoothed particle finite element method and its application to dynamic large deformation problems

AbstractThe smoothed particle finite element method (SPFEM) is an effective framework for large deformation analysis. The original SPFEM possesses the rank deficiency issue due to the direct nodal integration technique, which can be overcome by incorporating the strain gradient stabilization method. However, an extra volumetric locking due to the strain gradient stabilization term has been found. In this study, we propose a simple and efficient approach, the B‐dev approach, which only considers the deviatoric part of the smoothed strain gradient for stabilization to overcome the volumetric locking issue. First, the correctness of the stabilized SPFEM method is verified by an elastic cantilever beam vibration problem without considering incompressibility. Then, through the example of the strip footing penetration problem, the volumetric locking in the stabilized SPFEM is demonstrated and the capability of the proposed B‐dev approach in terms of overcoming the volumetric locking is verified. Furthermore, the stabilized SPFEM with B‐dev approach is applied to two types of elastoplastic problems in geotechnical engineering. All numerical results illustrate that the proposed approach can improve the performance of the stabilized SPFEM in dealing with incompressible and large deformation problems in geomechanics.

Finite elements limit analysis formulation using the power type yield criterion for plane strain and axisymmetric stability problems

The current research employs a power type yield criterion which accounts for the dependency of the shear strength parameters, namely, cohesion and internal friction, on stress level which becomes quite effective in obtaining very accurately the solutions of different stability problems in geomechanics involving soil and rock media. The present manuscript introduces plane strain and axisymmetric formulations which deal with the power type yield criterion and are based on the lower and upper bound finite elements limit analysis in conjunction with the power cone programming (PCP). To demonstrate the efficacy of the proposed formulations, the problems of the determination of the bearing capacity of strip and circular foundations in a cohesive-frictional soil medium have been addressed. In the case of a strip footing, the earthquake inertial forces in a horizontal direction have also been incorporated. The adaptive mesh pattern has been employed to improve the accuracy of the solution at the expense of minimal computational effort. The results have been thoroughly validated by making the comparison of the obtained solutions with that reported in literature. The proposed formulations are expected to be quite useful in solving different stability problems in geomechanics.

An h-adaptive edge-based smoothed point interpolation method for elasto-plastic analysis of saturated porous media

This study presents the first application of the edge-based smoothed point interpolation method (ESPIM) for adaptive analysis of coupled elasto-plastic problems in saturated porous media. The original ESPIM is enhanced with an adaptive strategy involving an automated h-refinement scheme, a simple, yet effective, recovery-based error indicator, and an efficient data transfer scheme to analyse coupled elasto-plastic problems in geomechanics. The governing equations are presented and discretised in space and time to yield the fully coupled system of equations. The solution strategy is then discussed in terms of handling the material non-linearity, and accommodating the proposed adaptivity procedure in the numerical algorithm. Four numerical examples comprising two different bearing capacity problems, a two-dimensional consolidation problem, and a slope stability analysis are thoroughly investigated. Different adaptive analyses with a range of adaptivity control parameters are investigated in each example and the performances of different techniques are compared in terms of accuracy and the time of the analysis. The results show the ability of the proposed adaptive analyses in yielding accurate and efficient results. Conclusions are drawn in terms of the most efficient numerical set-up to solve non-linear problems in porous media using the proposed numerical scheme.

Neutral plane of single pile in clay subjected to surcharge loading using the “critical state soil mechanics” (CSSM)

ABSTRACT Piles are designed to support direct loads, however, floating piles in clay are often subjected to indirect loading (surcharge). In this case, the pile’s shaft will be subjected to negative skin friction on part or on its full length, depending on the value of the surcharge, soil condition and pile’s length. The total negative skin friction acting on the pile’s shaft is known as the drag-load, which will reduce the pile capacity and perhaps pull the pile out of its cap, leading to catastrophic failure of the structure. A numerical model was developed using the finite element technique to simulate the case of a single pile floating in clay subjected to surcharge. The model utilizes the concept of the Critical State Soil Mechanics (CSSM) and the constitutive law of the Modified Cam Clay (MCC). The MCC is capable of incorporating the effect of soil deformation and the stress history into the stresses acting on the pile’s shaft. It is widely used in modelling geomechanics problems. The objective of this study is to determine the location of the neutral plane on the pile’s shaft for a given surcharge, soil condition and pile’s length. Thus, the positive and negative skin frictions acting on the pile’s shaft can be determined and accordingly the pile capacity. After validating the numerical models with the data available in the literature, the model was used to generate a wide range of data, which commonly used in practice, to examine the effect of the parameters which govern the location of the neutral plane for these piles. Design charts and design example are presented.

Nonlinear finite volume discretization of geomechanical problem

AbstractElliptic differential operators describe a wide range of processes in mechanics relevant to geo‐energy applications. Extensively used in reservoir modeling, the Finite Volume Method with TPFA can be consistently applied to discretize only a specific type of application under severe assumptions. In this paper, we introduce a positivity preserving Nonlinear Two Point Stress Approximation (NTPSA) based on the recently developed collocated Finite Volume scheme for linear elastic mechanics. The gradient reconstruction is different from the one used in Nonlinear TPFA, but a similar form of weighting scheme is employed to reconstruct the traction vector at each interface. The convergence of the scheme is tested with a homogeneous anisotropic stiffness tensor. The motivation behind the implementation of a new discretization framework in mechanics is to develop a uniform discretization technique preserving monotonicity for generic poromechanics applications.

A coupled FEM-DEM study on mechanical behaviors of granular soils considering particle breakage

Particle breakage often occurs in the fields of geological, geotechnical and hydraulic engineering like rock avalanche, landslides and high earth-rock dam, which affects the design and countermeasures of relevant engineering structures. Investigation of particle breakage’s effect on mechanical behaviors of granular soils has always been the hotspot within the community of particulate materials. In this study, a coupled finite element method (FEM) and discrete element method (DEM) modeling has been developed to tackle boundary value problems (BVPs) related with particle breakage. In the method, the particle breakage simulation is implemented in DEM through the conventional bonded particle modeling (BPM), while the stress–strain characteristics of representative volume element (RVE) captured by DEM is directly transmitted into the FEM solver to avoid the complicated and phenomenological constitutive equations incorporating into particle breakage. As an illustration, we apply the proposed multiscale modeling to investigating mechanical behaviors of quartz sands subjected to particle breakage through the high confining pressure biaxial compression simulation. And effects of particle strength, particle shape and confining pressure on particle breakage characteristics and further mechanical behaviors of quartz sands are also discussed accompanied with the underlying micromechanisms. Results demonstrate that the coupled FEM-DEM modeling can be able to deal with the complex geomechanics problems involving particle breakage.