ABSTRACT: Hydraulic fracturing is one of the most effective conventional reservoir-stimulation techniques currently utilized in the petroleum industry. Hydraulic fracture (HF) – Natural fracture (NF) interactions during the hydrofracturing process are complex in space and time. Although in the past five decades, several numerical, analytical and experimental studies have explored and characterized the interactions between natural fractures and hydraulic fractures, there remains differing opinions on the fundamental behaviors of hydraulic fractures when and where they interact with natural fractures. Here, we present a comprehensive review of the propositions and limitations of these studies in order to emphasize the need for further studies on NF-HF interactions and highlight the important fundamental questions that are yet to be answered. Our review focuses on previous experimental, analytical and numerical studies that investigated the impact of geomechanical and geometrical properties, and operational parameters on the result of NF-HF intersections during fracture initiation and propagation. We envisage that advancement in our understanding of NF-HF interactions will help in optimizing the fracture-stimulation processes during hydrofracturing operations.

ABSTRACT: During production from gas reservoirs the formation pressure decreases, leading to increased levels of effective stress. As a result, the reservoir rock is compressed and deformed, leading to displacement of overlaying formations, which can propagate all the way to the ground surface. In unconventional reservoirs such as coalbeds, the production of gas is associated with desorption of gas from the coal matrix, which induces relatively large volumetric changes in addition to the pressure decrease. While reservoir models consider this phenomenon when investigating the variation of the fracture aperture and permeability, mechanical models usually either neglect or oversimplify its influence on the bulk volumetric response of the coal and consequently the displacement of the ground surface. In order to address this issue, a finite element model has been constructed, in which a horizontal poro-elastic hydrogeological profile is subjected to depressurisation in combination with a time-dependent Richard’s equation and linear elasticity. The effect of sorption-induced shrinkage is included in the model using the definition of and the concurrent effects of shrinkage on both permeability and bulk deformation. The results presented in this paper show that implementation of sorption-induced shrinkage can have a significant impact on the modelled geomechanical response of the coalbed and the estimated ground surface subsidence.

ABSTRACT: A laboratory hydraulic fracture experiment was conducted on a Montney shale specimen under the conventional triaxial compressional stress condition. Acoustic emissions (AEs) were recorded using 7 piezoelectric transducers throughout the duration of the test to investigate the hydraulic fracture propagation processes. Distilled water was injected at an average rate of 2.5 mL/min until the sample failed. As expected, a hydraulic fracture formed perpendicular to the externally applied minimum principal stress. Due to the nature of tensile fractures, the relative plasticity of the Montney shale lithology compared to more brittle granitic specimens used in many previous studies, and the reverberatory character of the recorded waveforms, AE seismicity was relatively sparse (600 events) and only a small fraction (about 6%) of AE events could be located. Nevertheless, the hydraulic fracture plane in the middle of the specimen was clearly evident through the hypocenter location plot. In general, locating the AE hypocenters is very challenging compared with granitic materials due to the low signal-to-noise ratio of the waveforms emitted from shale during the hydraulic fracturing process. Moreover, event amplitudes recorded in this experiment are significantly lower than in a previously reported AE experiment where failure occurred under compression.

ABSTRACT: Collision avoidance technology is one of the key technologies in the cluster well drilling process. In order to improve the measurement accuracy and reduce the collision risk, according to the principle that the alternating magnetic field generated by the current accumulated on the casing has a gradient in the radial direction, a new method of anti-collision while drilling based on radial gradient measurement is proposed. In addition, a probe that can measure the magnetic field gradient in the radial direction is designed. A calculation model for the spatial positional relationship of adjacent wells is established. The effects of well spacing, sensor spacing, corner angle and well angle on the magnetic field gradient were analyzed by numerical simulation. The research shows that: (a) The method of anti-collision while drilling with radial gradient measurement is suitable for close-range measurement. The smaller the well spacing, the higher the measurement accuracy. (b) The sensitivity of the probe and the sensor spacing have a certain influence on the measurement results. The higher the sensitivity, the larger the spacing, the higher the measurement accuracy. (c) The magnetic field gradient periodically changes with the change of the rotation angle.

ABSTRACT: The fracture energy of the shale possesses a significant anisotropy due to the shale bedding and has great effects on the fracturing path. To investigate these effects, three types of the shale specimens are applied in fracturing test with three-point bending. The Digital Imaging Correlation (DIC) and Acoustic Emission (AE) methods are also integrated to analyze the damage processes. According to the AE energy, the damage process of the shale can be divided into three parts using the proportion between the loading value and the peak value of the specimen. In Part I, the sample does not damage. In Part II, the damage begins to appear in the sample, and the work of the external forces is mainly contributed to create the new cracks in the rock layers and to promote the opening and dislocation of the layer interfaces. In Part III, the cracks develop because the damages in the shale are accumulated to an extreme value. Fracture energy anisotropy extent of the shale has been obtained from these partitions, which can be further applied to optimization of the perforation and hydraulic fracturing direction.

ABSTRACT: Hydraulic fracturing is the best method to improve production in deep shale, and the complex natural fracture affects the fracture propagation. In this study, numerous true tri-axial experiments were conducted on hydrostone to study the effect of the natural fracture complexity. The position and the relative angle of natural fracture were mainly studied. The acoustic emission was used to detect the fracture propagation and the pumping curve was analyzed. The results showed that the three-dimensional fracture morphologies can be classified into five ways: arresting, bypassing, crossing, vertical extension and slipping. Symmetrical natural fracture around the wellbore helped to increase the fracture height extension. When natural fractures distributed asymmetrically, the hydraulic fracture showed obvious single wing crack. The hydraulic fracture tended to activate the natural fracture and changed direction along the natural fracture with a small approaching angle. The pumping curve rising in the propagation stage showed the fracture arresting the natural fracture. More complex fracture network corresponded to more fluctuated curve. The results give a deep understanding of the effect of the natural fracture complexity in 3D and help to predict the fracture intersection morphology in the field.

ABSTRACT: Biogenic coalbed methane (BCBM) reservoirs aim to convert the carbon in u coal to methane using microbial techniques. Given the nascency of the technology, there is a dearth of reservoir scale investigations of the effects of bioconversion. This paper presents the results of high resolution microscopy, aimed at capturing the changes in pore structure as a result of bioconversion. The images were processed to measure pore-strains due to bioconversion, which was used as an input parameter for a new poro-mechanical model, developed to describe the stress and permeability behavior of BCBM reservoirs. Imaging results showed that the fracture porosity varies significantly post-bioconversion. Fractures wider than 5 microns, widened post-bioconversion, whereas fractures narrower than 5 microns reduced in width. Reservoirs with wide fractures exhibited significant enhancement of the state of effective stress and modeled permeability with depletion. Reservoirs with narrower cleats indicated the exact opposite. These results clearly demonstrate the importance of geologic/reservoir assessment prior to selecting a reservoir for biogenic treatment. Reservoirs with a large distribution of wide cleats would not only enhance gas production rates by providing the microbes with more surface areas, but also boost subsequent flowrates of methane.

ABSTRACT: This work offers a predictive model as a success-of-completion strategy (treatment) for a major successful shale play (Wolfcamp). The predictive model may be used to evaluate spacing between fracture clusters and the number of clusters and perforations, and to guide future selective optimum completion for the shale play. Many important parameters that control behavior of producing wells have been analyzed, including number of days on production; depth; fluids in bbl; horizontal well completion configurations; stages per well; fracture type; average water requirement; proppant type; fluid type; hydraulic horsepower (HHP) per stage; lb/ft 2 of proppants per stage; number of stages, and lateral length (completed interval) of horizontal wells,.

ABSTRACT: Prediction of coal permeability is very important as it is directly related with coal bed methane (CBM) production, underground coal mining, and CO 2 sequestration in deep coal seams. However, the anisotropy of both internal fractures of coal and outer stress environment brings difficulty in replicating the gas transport characteristics of coal at the laboratory scale. There is limited research in this area. Thus, in this paper, we performed a series of permeability measurements on cubic high-rank coal samples using the true-triaxial apparatus. The principal stresses and flow directions were systematically varied during the test. The results show that differential stress of 20 MPa compressed to coal resulted in an order of magnitude drop in permeability. For the pre-existing natural fractures in coal, the butt cleat plane was more sensitive to variations in the principal stress. By using an exponential equation containing different mean cleat compressibility and stress terms, the anisotropic coal permeability data obtained under true-triaxial stress conditions can be accurately predicted. The experimental and theoretical results of this study can help better understanding gas transport in coal seams and optimizing the layout of the gas drainage boreholes and well design during CBM production or CO 2 sequestration.

ABSTRACT: This paper presents the results of a 2018 NIOSH (National Institute for Occupational Safety and Health) study to evaluate longwall-induced subsurface deformations and permeability changes and their effects on shale gas well integrity and safety under shallow cover. A study site was selected over a southwestern Pennsylvania coal mine, which extracts 457-meter-wide longwall faces under 147 meters of cover. One in-place inclinometer (IPI) well and three permeability monitoring wells were drilled and installed over a 38-m by 84-m centers abutment pillar. In addition to the inclinometer well and monitoring wells, surface subsidence measurements and underground coal pillar pressure measurements were conducted as the first 457-meter-wide longwall panel on the south side of the abutment pillar mined by. Comparisons of a series of FLAC3D finite difference simulation results and the surface, subsurface, and underground instrumentation results show that the measured IPI casing deformations are in reasonable agreement with those predicted by the FLAC3D models, and that the measured surface subsidence and pillar pressure are in excellent agreement with those predicted by the 3D models. Measured permeability changes were incorporated into a preliminary calculation to evaluate the effect of a hypothetical shale gas well leak on longwall ventilation, which indicates negligible impact after completion of first panel mining. Results from this 2018 research clearly indicate that under shallow cover, the measured horizontal displacements within the abutment pillar are at least one order of magnitude higher than those measured in a 2017 study under deep cover and slightly higher than those measured in a 2014 study under medium cover. INTRODUCTION Due to a recent shale gas boom, approximately 1,500 unconventional shale gas wells have been drilled through active and future coal reserves in Pennsylvania, West Virginia, and Ohio over the past 15 years. These shale gas wells have penetrated many mineable coal seams, in particular, the Pittsburgh Seam. These unconventional gas wells, whether tapped into the Marcellus or Utica formations, contain very high gas pressure. Strata deformations associated with longwall extractions could induce high stresses and deformations in the shale gas well casings, which may compromise the mechanical integrity of the production, intermediate, and coal protection casings (Figure 1). Damaged well casings could potentially introduce high-pressure, high volume explosive gas into underground mine workings, particularly during an extended well shut-in period, which could seriously jeopardize underground miners’ safety and health.

ABSTRACT: The natural gas reservoirs in Linxing, Shanxi Province are in the state of sand, coal and mud interbed, containing multiple sets of gas layers, mainly distributing tight gas, shale gas and coalbed gas, in order to maximize the economic effect of “multi-gas co-exploitation”, Different exploitation schemes are proposed for each type of layer which has different reservoir physical properties. T-shaped well drilling technology is proposed based on the coal seam distribution characteristics and physical parameters of the reservoir. The T-shaped well was drilled by flexible drilling tools, which can drill three horizontal branches sideways in each coal seam to achieve efficient exploitation of coalbed gas. In this paper, the extended finite element method is used to optimize the wellbore orientation and wellbore length of the T-shaped well, and finally determine the optimal well track. The optimization results can effectively guide the on-site construction and enhance and maintain the coalbed gas production.

ABSTRACT: Casing deformation in shale gas wells during multi-fracturing has seriously affected the efficiency and safety of shale gas production in China. Firstly, the characteristics of casing deformation in Weiyuan shale gas field were statistically analyzed, and main influence factors were obtained. Second, the influence of the induced stress from crack is analyzed, and a stress analysis model for casing-cement-formation system stress field nearby wellbore during fracturing is established. Finally, the loads near wellbore in shale gas wells is studied, incorporating variation fracturing methods and operation parameters. The results show that the casing deformation in shale gas wells mainly occurs near the heel of the horizontal section, and shear failure is the main type of casing deformation; The initiation and expansion of crack can lead to the accumulation of non-uniform stress nearby wellbore during fracturing. Further, this will induce formation slip and cause the shear and collapse failure of casing; Accurate identification of faults, natural cracks and improved fracturing technology can alleviate the stress concentration of the casing and reduce the risk of casing deformation failure. This study further reveals the mechanism of casing deformation during multi-fracturing in shale gas wells, and proposes fracturing measures for casing integrity. The results demonstrated in this paper can provide the theoretical basis for preventing casing deformation during fracturing in shale gas wells. 1. INTRODUCTION With the development of unconventional gas reservoirs for shale gas in the world, Significant progress in the exploration and development of shale gas has been made in China recent years, including the construction of the Changning-Weiyuan shale gas field of CNPC and the Chongqing Fuling shale gas field of Sinopec. However, in the national shale gas demonstration area of the Changning-Weiyuan, southwestern Sichuan Basin, since the exploration and development in 2013, some shale gas wells have had some problems, such as poor cementing seal integrity and serious casing deformation. Difficulties in the entry of casings in horizontal well sections, especially in the process of fracturing stimulation and transformation, the impact of casing deformation on the smooth entry of bridge plugs has caused some reservoir sections to be forced to abandon, and even the bridge plugs cannot be drilled smoothly. The emergence of these problems has affected the improvement of single well production in shale gas horizontal wells.

ABSTRACT: Characterization of rock permeability change during the fracture network evolution is critical to the design and assessment of geological storage facilities. Under geo-storage conditions, rock fractures propagate during damage processes and re-bond during healing. However, natural heterogeneities in rocks significantly reduce experimental repeatability. Moreover, healing is a time-consuming process and requires high pressure or temperature to achieve. In this study, we resort to the 3D printing technology for specimen preparation so that complex rock microstructure changes are precisely controlled. We validate the specimen preparation by comparing experimental permeability test results with analytical solutions. Channel networks with different tortuosity, porosity, fracture branch, and structural anisotropy are designed to mimic various microstructure changes in rocks. To fabricate fractured rocks, we also embed heterogeneous fracture networks into 3D printed specimen and modify fracture geometry to account for anisotropic microstructural changes. Experimental findings revealed the evolution trend of permeability with these structural characterization parameters. They also match well with those obtained from computational fractured rock models under identical hydro-mechanically coupled conditions. This study is expected to provide new insights into the dependence of rock permeability on its microstructures under complex geo-storage conditions.

ABSTRACT: The purpose of this paper is to present a numerical code for rock mechanics modelling of discontinuities such as fractures, joints and faults. We implement this in an open-source Matlab Reservoir Simulation Toolbox designated for rapid prototyping of geomechanics, reservoir modelling and simulation problems. Appropriate and explicit modelling of such discontinuities become a central element of the scientific discussions. In particular, we contemplate processes when fractures open, close and slip due to stress changes in the subsurface. Existing numerical codes use either long-syntax programming languages where details of implementation are not explicitly shown or commercial simulators that often lack the flexibility for implementing new ideas. We use the recently developed Virtual Element Method (VEM) and solve contact problems that allow fractures to dynamically interact with the surrounding rock mass and each other, transferring contacting stress across faces due to closure and friction. This method has advantages over standard Finite Elements or Finite Volumes in solving the contact problem because it handles complex, not necessarily convex grid geometries and hanging nodes. For the contact problems, we implement contact detection, normal and tangential contact interaction algorithms. We provide numerical examples and match to analytical solutions in order to validate employed algorithms.

ABSTRACT: Ordovician carbonate rocks make up the main reservoir stratum in the Tuofutai region of the Tahe Oilfield. The rocks exhibit extensive formations of the faults, fractures and caves which results in a rather complex stress distribution. To determine the variation in the stress field and the rotation of the principal stress, fracture distribution was obtained by logging data and FRACMAN numerical analysis was conducted. The results showed that: 1) two conjugate faults and fractures were recognized by FMI images and logging data, and the trends of the fractures were about NNW343° and NNE28°, which means the trend of maximum horizontal principle stress is about NNE10. 2) The vertical stress is the maximum principle stress at about154-170MPa, the maximum horizontal principle stress is about 122-135MPa and the minimum horizontal principle stress is about 89-95MPa when the depth is 6200-6700 m. 3) The value of each principle stress decreased about 5-12MPa in the fault zone and the degradation is about 8-10%. The trend of maximum horizontal principle stress varied from NNE5° to NNE30° at the ends of the fault. 4) The value of each principle stress decreased about 3-5MPa with fracture development and the degradation is about 3-5%. The trends usually held steady; 5) The value of maximum horizontal principle stress decreased about 20-30MPa around the caves and the minimum principle stress decreased about 10-20MPa; and the maximum horizontal principle stress varied from NNW282° to NNE70°. 6) The stress field in the surrounding rock was mainly concentrated within the distance of 0.5 times of the cave radius. The rotation of the principal stress mainly occurred at the end of faults and around the caves.

ABSTRACT: It is important to determine well spacing for maximizing oil recovery in shale reservoirs with complex hydraulic and natural fracture geometries. The optimal number of well placement should be achieved to minimize well interference. Although there are many simulation studies to optimize well spacing, very few studies have been performed to combine the fracture propagation model and reservoir simulation considering complex fracture configurations. The goal of this study is to fill this gap by integrating a complex fracture model and reservoir model through a non-intrusive embedded discrete fracture model (EDFM) method. The fracture model was applied to predict complex fracture geometries. After that, such fractures were swiftly transferred to traditional reservoir simulators based on the non-intrusive EDFM technology. We applied the workflow to examine the effect of well number with varying well spacing on well performance in the Permian basin with and without considering natural fractures. Finally, the optimal well spacing was discussed. This study can provide key insights into optimization strategies for well spacing in shale reservoirs.

ABSTRACT: Previous studies on the strength and deformability of pillars in hard-rock mines have demonstrated that the influence of natural fractures diminishes with increasing pillar width. Jointing conditions and fracture intensity/intersections play a critical role in the behaviour of slender pillars (width to height ratio less than 0.5). An important challenge is the characterization of the intact rock damage as a function of the in-situ fracture network and loading conditions. In this paper, we present advanced pre-processing tools to measure fracture intensity and fracture density prior to loading and relate these measures to the evolution of damage (intensity and density) with increasing pillar load. A hybrid finite-discrete element method with fracture mechanics capabilities is used to simulate intact rock failure and allows to incorporate discrete fracture systems. This method compared to continuum modelling has the advantage of fully considering both brittle failure and kinematic mechanisms. This research considers four synthetic pillar models with different embedded discrete fracture traces of similar total length. Newly developed post-processing tools are able capture the location of damage events track the intensity/density of rock damage as the simulation progresses and relate these changes to the modelled pillar strength. The results of this synthetic rock mass pillar study provide useful insights on damage evolution and rock bridge strength characterisation in pillars. Our results show that pillars with different initial jointing conditions may yield similar strength but undergo different stress paths. Detailed interpretation of the results has allowed the contribution of damage to block creation with increasing load to be highlighted. The roles of structurally-controlled failure and brittle mechanisms are discussed and implications on pillar design and potential future trends in pillar damage characterization presented.

ABSTRACT: With the shale gas boom over the past decade, many shale gas wells have been drilled through the Pittsburgh coal seam to the Marcellus shale formation. The wells located in longwall barrier pillars are influenced by longwall mining, and their stability has been a safety concern for both the mining and gas industries. This study presented a case in which two longwall panels in the Pittsburgh seam were mined near two shale gas wells in a barrier pillar between the two longwall bleeders. The first longwall panel was mined before the gas wells were drilled and installed in the barrier pillar adjacent to the longwall setup bleeders. The second longwall setup bleeders were developed later on the other side of the barrier pillar, and the second longwall panel was subsequently mined. The stability of the gas wells was evaluated using a numerical modeling approach. The model was set up based on site-specific mining conditions and overburden geology and included sufficient details to simulate the gas well casings, mining sequence, and longwall retreating. The modeling procedures produced realistic results of stresses and deformations around the barrier pillar and in gas well casings, which were verified by in-mine observations.

ABSTRACT: Reservoir compaction and surface subsidence are significant issues for shallow coal seam gas plays, and regulatory authorities can mandate subsidence predictions as a License-To-Operate requirement. It is therefore important to accurately determine the compaction behavior of the coal under the expected in-situ conditions as the reservoir depletes. This paper reports the results of methane pore pressure depletion tests under approximate uniaxial-strain and in-situ stress conditions on whole core coal samples from a coal seam gas reservoir. Test results showed that there is a substantial increase in the compressibility of the core material when the effects of methane desorption are included. The effective uniaxial-strain compaction coefficient C m show a clear increase in compressibility with decreasing pore pressure, and an apparent acceleration of the increase below 1.2 MPa pore pressure. For the tested samples the Cm values began at 1.5 to 4.4x10 -4 /MPa and increased to peak values of 5.1 to 11x10 -4 /MPa. The compressibility excluding the desorption effects was generally below 2x10 -4 /MPa and constant or decreasing with decreasing pore pressure. These data agree with previously reported results on coal from other locations and clearly show the importance of including methane desorption effects in reservoir compaction and subsidence calculations for coal seam gas fields.

ABSTRACT: Acidizing and wormholing in carbonate reservoirs is studied through the analysis of the linear acidizing problem theoretically and experimentally. Experimentally, linear acidizing tests were performed in Mons chalk, a high porosity analogue of North Sea reservoir chalk. In the experiments the critical injection velocity for wormhole formation at minimum acid injection was obtained. Theoretically acidizing was analyzed by formulating the problem as reactive moving boundary Stefan type problem. Wormholing was analyzed through a linear stability analysis from the equilibrium state of uniform dissolution. The results of the stability analysis determined the critical injection velocity for wormholing and size effect with respect to the specimen diameter.