An advanced numerical model is developed to investigate the different aspects of stimulation in naturally-fractured rocks. A fully-coupled 3D HF–NF “GeoFrac-3D” model is developed and used to study ...hydraulic and natural fracture(s) interactions in 3D without any major assumptions concerning the intersection and coalescence. Rock deformation and stresses are modeled using three-dimensional displacement discontinuity (DD) method. Contact elements are used to represent the closed natural fractures along with the Mohr–Coulomb criterion to determine the contact status of the fractures. Fracture propagation is modeled using a mixed-mode propagation scheme. A novel fracture coalescence scheme is integrated in the 3D HF–NF model to investigate intersection problems for a wide range of NF dip angles and strikes. The simulation results indicate that hydraulic fractures experience pressure drop upon intersection with permeable natural fractures. The pressure drop is followed by an increase in the injection pressure as the hydraulic fracture pressurizes the natural fracture. Moreover, the results show that the HF may propagate in other directions away from the NF when it is partially arrested by the natural fractures. Simultaneous interaction with multiple NFs and/or stress barriers was found to result in complex HF geometries with non-uniform fracture aperture distributions that could, in turn, affect proppant placement.
Highlights
A fully-coupled 3D HF–NF “GeoFrac-3D” model is developed and used to study hydraulic and natural fracture(s) interactions in 3D without any major assumptions concerning the intersection and coalescence.
Rock deformation and stresses are modeled using three-dimensional displacement discontinuity (DD) method. Contact elements are used to represent the closed natural fractures along with the Mohr-Coulomb criterion to determine the contact status of the fractures.
A novel fracture coalescence scheme is integrated in the 3D HF–NF model to investigate intersection problems for a wide range of NF dip angles and strikes.
The model allows for different HF propagation geometries, mechanical and hydraulic interaction with the natural fractures, intersection with natural fractures with any dip angle and strike, different modes of intersection such as arrest, cross, and the engulfing pattern.
A combined boundary element/finite element numerical method is employed to investigate the response of a hydraulic fracture/natural fracture network to injection. The model is three-dimensional and ...poroelastic. It considers the effects of fracture interactions, fluid diffusion into the matrix and the natural fractures, fracture opening and shear dilation. The natural fractures behavior is modeled using non-linear deformation in both shear and normal directions. For the shear mode, the slip-weakening model is used to simulate the post-shear failure behavior of the fractures. An example simulation is carried out to study the response of a fracture network to injection. The calculated injection pressure profile is used to ascertain the deformation response of the fracture network and its permeability enhancement. The simulation results clearly illustrate the potential for induced microseismicity due to permanent shear slip on natural fractures surrounding a hydraulically driven major fracture. Although the slip on the natural fractures can potentially contribute to seismic activity, its contribution to permeability enhancement depends on whether or not they propagate to connect to form a “hydraulically” connected network with the main fracture. When the natural fracture system is not connected to the main flow path, the MEQ information may overestimate the so-called stimulated reservoir volume.
•A combined boundary element/finite element numerical method is developed.•It considers the effects of stress shadow, fluid diffusion and fracture opening and ride.•The response of a hydraulic fracture/natural fracture network to injection is investigated.•Simulations illustrate injection pressure variation with network deformation and permeability change.•The simulations illustrate the potential for induced micro-seismicity due to slip on natural fractures.
Horizontal well fracturing is most commonly used to improve well productivity from the lower quality formations that could not have been economically developed using the conventional hydraulic ...fracturing approach. These horizontal wells can be stimulated to increase reservoir permeability via multiple hydraulic fractures using different design concepts. In this paper, we present a 3D fully coupled numerical model with capabilities to simulate multiple fracture clusters propagation from horizontal wells. The numerical model is developed using a combination of the boundary element method, the finite element method, and the linear elastic fracture mechanics for the fracture propagation. The fluid flow inside the fracture is assumed to be laminar flow and the fluid follows Newtonian behavior. The Galerkin’s finite element approach is used for fluid flow modeling, the rock mass deformation is simulated using elastic displacement discontinuity method, and crack tip displacement approach is used for the mixed-mode fracture propagation. The brief descriptions of the governing equations and their numerical implementation are presented first. Then, the fracture propagation model is verified using semi-analytical solution from the Khristianovic-Geertsma-de Klerk (KGD) model for a circular planar fracture. A sensitivity analysis of various parameters effecting the mechanical interaction among multiple propagating fractures is conducted. Finally, numerical examples of multiple fracture propagation for sequential and simultaneous fracturing procedures from a single horizontal well and multiple horizontal wells are presented. In simultaneous fracturing case, the fluid injection rate is dynamically partitioned among the fractures depending upon wellbore frictional losses, perforation frictional losses, and fluid pressure drop in the fractures. Results of a sensitivity analysis demonstrate the effects of the in-situ stresses, rock and fluid properties, and “stress shadowing”. The simultaneous and sequential fracturing results show that the generated fracture network geometries are strongly influenced by the mechanical interactions among the fractures or fracturing stages. The conventional zipper fracturing tends to generate nearly straight fractures with potential for tip coalescence and thus wellbore communications. The modified zipper fracturing tends to yield potentially more complex fracture geometries due to fracture turning.
•A fully coupled three-dimensional hydraulic fracture simulator model is developed.•The model uses the displacement discontinuity method.•The model can simulate simultaneous or sequential multiple fractures from a single or multiple well.•Example simulations illustrate that mechanical interaction or “stress shadowing” effect between fractures.•Simulations indicate zipper fracturing can be used in close horizontal wells with potential for more complex fracturing.
Analysis of fluid flow, permeability, and micro-seismicity in large fracture networks are essential for economical production from engineered geothermal and petroleum reservoirs. In this work, the ...fluid pressure change, as a result of injection and production inside a stochastic, deformable, non-propagating fracture network is modeled using the fast multipole displacement discontinuity method combined with the finite difference method. The former is employed to simulate the response of the fracture network to changes in the pressure due to fluid injection. In addition to pressure changes inside the fracture network, changes in fracture status with respect to normal (“joint” versus hydraulic fracture) and shear (stick versus slip) deformation, dilation, and friction coefficient are considered. Rate-and-state friction model and the Mohr-Coulomb criterion are used to model slip and potential micro-earthquakes and their seismic moments. Simulation results show that increase in pressure inside a fracture network leads to permanent slip and nucleation of micro-earthquakes which are mostly influenced by the orientation of the fractures and the magnitude of fluid pressure and the injection rate. Injecting the same volume of water over a longer time period decreases the number and magnitude of seismic events.
Summary
The hydraulic fracturing technique has been widely applied in many fields, such as the enhanced geothermal systems (EGS), the improvement of injection rates for geologic sequestration of CO2, ...and for the stimulations of oil and gas reservoirs. The key points for the success of hydraulic fracturing operations in unconventional resources are to accurately estimate the redistribution of pore pressure and stresses around the induced fracture and predict the reactivations of preexisting natural fractures. The pore pressure and stress regime around hydraulic fracture are affected by poroelastic and thermoelastic phenomena as well as by fracture opening compression. In this work, a comprehensive semi‐analytical model is used to estimate the stress and pore pressure distribution around an injection‐induced fracture from a single well in an infinite reservoir. The model allows the leak‐off distribution in the formation to be three‐dimensional with the pressure transient moving ellipsoidically outward into the reservoir from the fracture surface. The pore pressure and the stress changes in three dimensions at any point around the fracture caused by poroelasticity, thermoelasticity, and fracture compression are investigated. With Mohr‐Coulomb failure criterion, we calculate the natural fracture reactivations in the reservoir. Then, two case studies of constant water injection into a hydraulic fracture are presented. This work is of interest in the interpretation of microseismicity in hydraulic fracturing and in the estimation of the fracture spacing for hydraulic fracturing operations. In addition, the results from this study can be very helpful for the selection of stimulated wells and further design of the refracturing operations.
Maintaining wellbore stability is a critical problem when drilling for petroleum production in high pressure and temperature shales that tend to swell and deteriorate in the presence of the water ...phase of a drilling fluid. In this paper, a finite element model is developed for coupled linear and non-linear chemo-poro-thermoelasticity. The model is first verified through comparisons with the analytical results for the cases of linear poroelasticity, poro-thermoelasticity, and chemo-poroelasticity. Then, the non-linear finite element model is used to analyze the problem of a wellbore in a chemo-poro-thermoelastic formation with non-hydrostatic in situ stress field. The results indicate that the coupling between temperature and salinity has an important impact on wellbore stability, namely heating and a lower solute mass fraction in mud increase pore pressures, the radial and tangential total stresses in the formation, and vice versa. Finally, the prediction of the non-linear chemo-poroelastic model are compared to those obtained using a linear model. Results show that for mud properties within the range of interest, the linear model provides good results for the purpose of assessing shale instability.
Investigation of shear slip in hot fractured rock Huang, Kai; Cheng, Qinglu; Ghassemi, Ahmad ...
International journal of rock mechanics and mining sciences,
August 2019, 2019-08-00, 20190801, 2019-08-01, Letnik:
120, Številka:
C
Journal Article
Recenzirano
Odprti dostop
This paper presents the results of a shear slip experiment in hot rock and its simulation and analysis using 3D coupled thermo-poromechanical model with the aim of assessing the role of pore pressure ...and temperature change on shear slip in granitic rock. The test is carried out in a triaxial system at representative reservoir pressure and temperature using right circular cylindrical specimens of Westerly granite with a 30° precut and ground surface. After the pore pressure and temperature were stabilized, the differential stress and absolute pore pressure were systematically varied to produce different effective stress states to evaluate the shearing response of the saw-cut fracture. The key objective of the experiment is to test the cooling effect on the fracture slip. The experimental results were then analyzed using a newly developed 3D thermo-poromechanical finite element model which uses traditional 4 node tetrahedron elements for intact rock deformation and transport processes. Zero thickness contact interface element is employed to capture the closure and slip on the fracture surface. The stick/slip state of fracture is determined by the Mohr-Coulomb criterion. Simulation results show that the convective cooling effects on rock/fracture surface promote deformation and slip of the fracture. The cold fluid cools the surrounding rock material and results in normal stress reduction that causes portions of the fracture surface to undergo stick/slip state changes, and induces irreversible slip. The experimental work along with modeling provides a physics-based understanding of the role of coupled processes on shear stimulation phenomenon.
Almost all reservoir rocks contain natural fractures. The presence of such discontinuities affects reservoir stimulation to a certain degree. The interaction between the natural and hydraulic ...fractures determines the overall fracture trajectory and the stimulated reservoir volume. In contrast to conventional hydraulic fracturing, natural fractures do not propagate in a tensile-dominated regime. Propagation of mechanically closed fractures lies in the mixed-mode propagation regime which differs from that of open fractures. Therefore, the response of natural fractures to pore pressure and stress perturbations should be carefully addressed within the context of reservoir stimulation. This study aims at investigating the response of closed natural fractures to water injection in poroelastic rocks. A poroelastic displacement discontinuity (DD) model is developed and used to shed light on the propagation of closed natural fractures. Mohr–Coulomb contact elements are used in this study to identify the contact status of the elements in the transverse direction. Moreover, fracture propagation is effectively accounted for based on linear elastic fracture mechanics. Hydro-mechanical coupling is achieved by combining an implicit finite difference scheme with the poroelastic DD model. Our simulation results indicate that natural fracture shear slip and propagation are likely to occur when the closed natural fractures are subject to direct water injection. The injection pressure required to maintain the propagation in poroelastic rocks was found to be consistently higher than the pressure in its elastic counterpart. The propagation trajectory of the
wing cracks
was, on the other hand, found to be similar to that in elastic rocks. It was found in our results that the formation of wing cracks and shear (secondary) cracks is an integral part of the reservoir stimulation in geothermal systems where natural fractures are in direct contact with the injection wells. This point is often overlooked or poorly treated and has led to erroneous interpretations from the reservoir stimulation standpoint.
PDF
