Production induced fracture closure of deep shale gas well under thermo-hydro-mechanical conditions

Deep shale gas reservoirs have geological characteristics of high temperature,high pressure,high stress,and inferior ability to pass through fluids.The multi-stage fractured horizontal well is the key to exploiting the deep shale gas reservoir.However,during the production process,the effectiveness of the hydraulic fracture network decreases with the closure of fractures,which accelerates the decline of shale gas production.In this paper,we addressed the problems of unclear fracture closure mechanisms and low accuracy of shale gas production prediction during deep shale gas production.Then we established the fluid—solid—heat coupled model coupling the deformation and fluid flow among the fracture surface,proppant and the shale matrix.When the fluid—solid—heat coupled model was applied to the fracture network,it was well solved by our numerical method named discontinuous discrete fracture method.Compared with the conventional discrete fracture method,the discontinuous discrete fracture method can describe the three-dimensional morphology of the fracture while considering the effect of the change of fracture surface permeation coefficient on the coupled fracture—matrix flow and describing the displacement discontinuity across the fracture.Numerical simulations revealed that the degree of fracture closure increases as the production time proceeds,and the degree of closure of the secondary fractures is higher than that of the primary fractures.Shale creep and proppant embedment both increase the degree of fracture closure.The reduction in fracture surface permeability due to proppant embedment reduces the rate of fluid transfer between matrix and fracture,which has often been overlooked in the past.However,it significantly impacts shale gas production,with calculations showing a 24.7%cumulative three-year yield reduction.This study is helpful to understand the mechanism of hydraulic fracture closure.Therefore,it provides the theoretical guidance for maintaining the long-term effectiveness of hydraulic fractures.

A REVISED SOLUTION OF EQUIVALENT PERMEABILITY TENSOR FOR DISCONTINUOUS FRACTURES

The equivalent permeability tensor is essential to the application of the equivalent porous media model in the numerical seepage simulation for fractured rock masses. In this paper, a revised solution of the equivalent permeability tensor is proposed to represent the influence of the fracture connectivity in discontinuous fractures. A correction coefficient is involved to reflect the com- plex seepage flow type through the rock bridge. This correction coefficient is back analyzed from single-hole packer tests, based on the Artificial Neural Network （ANN） back analysis and the Finite Element Method （FEM） seepage simulation. The limitation of this back analysis algorithm is that the number of single-hole packer tests should be equal or greater than the number of fracture sets, and three is the maximum number of the fracture sets. The proposed solution and the back analysis algorithm are applied in the permea- bility measurement and the seepage simulation for the Xiaowan arch dam foundation.

Numerical analysis of the hydraulic fracture communication modes in fracture-cavity reservoirs

The fracture-cavity carbonate reservoirs in the Tahe Oilfield in China are mainly exploited by fracturing.We need the hydraulic fractures to communicate with caves to create a flow channel.However,due to the existence of the fracture-cavity systems,the hydraulic fracture propagation morphology is complicated,while the propagation characteristics are not clear.To analyze the hydraulic fracture propagation in fracture-cavity carbonate formations,based on the discontinuous discrete fracture model,we developed a solid-seepage-freeflow coupled fracturing model for fracture-cavity formations,which can simulate the complex interaction behavior of fractures and caves.Based on the simulation results,we found the interaction rule between hydraulic fractures and fracture-cavity systems:the stress concentration around caves is the main factor that determines the fracture propagation path.Deflection due to stress concentration is usually not conducive to communication,while natural fractures distributed around caves could break the rejection action.Increasing the hydraulic energy in the hydraulic fracture can make fracture propagate directly and reduce the influence of deflection.The steering fracture formed by temporary plugging is beneficial to the communication of fracture-cavity systems in the non-principal stress direction.According to the simulation results of different fracture-cavity characteristics,we raised four optimization communication modes for fracture-cavity carbonate formation to provide references for fracturing optimization design and parameter optimization.

Modeling rock failure using the numerical manifold method followed bythe discontinuous deformation analysis

A complete rock failure process usually involves opening/sliding of preexisting discontinuities as well as frac- turing in intact rock bridges to form persistent failure sur- faces and subsequent motions of the generated rock blocks. The recently developed numerical manifold method （NMM） has potential for modelling such a complete failure process. However, the NMM suffers one limitation, i.e., unexpected material domain area change occurs in rotation modelling. This problem can not be easily solved because the rigid body rotation is not represented explicitly in the NMM. The discontinuous deformation analysis （DDA） is specially de- veloped for modelling discrete block systems. The rotation- induced material area change in the DDA modelling can be avoided conveniently because the rigid body rotation is represented in an explicit form. In this paper, a transition technique is proposed and implemented to convert a NMMmodelling to a DDA modelling so as to simulate a complete rock failure process entirely by means of the two methods, in which the NMM is adopted to model the early fracturing as well as the transition from continua to discontinua, while the DDA is adopted to model the subsequent motion of the generated rock blocks. Such a numerical approach also im- proves the simulation efficiency greatly as compared with a complete NMM modelling approach. The fracturing of a rock slab with pre-existing non-persistent joints located on a slope crest and the induced rockfall process are simulated. The validity of the modelling transition from the NMM to the DDA is verified and the applicability of the proposed nu- merical approach is investigated.

Experimental and Numerical Study of Shear Fracture in Brittle Materials with Interference of Initial Double Cracks

A simultaneous experimental and numerical study of shear fracture of concrete-like materials is carried out using Brazilian disc specimens with initial double edge cracks and fourpoint bending beam specimens with double edge-notches.The interference effects of two cracks/notches are investigated through varied ligament angles and crack lengths.It is shown that shear fracturing paths change remarkably with the initial ligament angles and crack lengths.The cracked specimens are numerically simulated by an indirect boundary element method.A comparison between the numerical results and the experimental ones shows good agreement.