Multi-physics Coupling In Carbonate Rock Fracture Deformation Mechanism

Unconventional oil and gas resources has played an important role in China's oil and gas exploration and development,especially fractured carbonate reservoir,for its very rich reserves,has been becoming the focus of research.Development,opening and closing degree of carbonate rock fractures are not only critical for accumulation,flow capacity and productivity construction of fractured reservoir,but also roots of working fluid leakage,fluid replacement,pressure control and the difficulty to predict productivity performance during drilling,completion and production process.How to reflect the real formation fracture morphology,seepage characteristics and deformation mechanism is the key to solving these problems.However,it is rather difficult to quantitate the real underground fracture space,resulting in problems of well hole-formation pressure prediction and control,flow capacity prediction and fracture width prediction,which differ greatly with practical engineering.Fracture deformation regularity under fluid-solid coupling is vital to determine a reasonable method of drilling and development,work system,recovery rate,and to optimize late-stage stimulation.In this paper,a systematic study of carbonate rock fracture deformation mechanism for multi-field coupling is conducted.Specific research contents and major achievements are listed as follows:(1)Based on the theory of fracture surface morphology,general fracture feature recognition technology was analyzed.With photoelectric core Eascan-D measurement system,experiments of carbonate rock fracture surface parameter acquisition and analysis,and morphology characterization of asperity on fracture surface were launched to establish surface and spatial three-dimensional model of fractures.(2)Fracture closure deformation mechanism was studied to understand three stages of fracture closure process;macro-mechanics triaxial experiments for fractured carbonate rock samples were conducted to obtain macro-mechanical parameters;By using typical elastic-plastic contact model of asperity,micro-mechanics experiments involving different diameters were designed to investigate asperity micro-mechanics properties,also,structural changes and mechanical mechanisms of asperity under load were revealed by verifying experimental results with the finite element simulation method.(3)Cubic law and the modified model were analyzed,indicating that a more precise solving method must be applied to fluid flow within natural fractures;By comparison between N-S equation,Stokes equations and the Reynolds equation,it is pointed out that selecting N-S equation method would be more accordant with flow pattern of natural fracture space,while solving by Reynolds equation would be easier to get numerical solution.(4)According to the established theoretical models of fluid flow in fractures and particle migration coupling,fracture spatial deformation regularities were discovered,by the application of multi-physics field coupling simulation software,to obtain flow field velocity,streamline distribution characteristics within fractures,and to investigate the relationships between pressure difference,fracture width,speed and volume,as well as the effect of particle size,fracture width on particle migration.(5)Combined spatial structure parameters of the rock fracture with asperity micro-mechanics properties of fracture surface and macro-mechanics properties of rock matrix,a research method for spatial structure multi-physics coupling deformation of carbonate rock fracture was proposed,providing theoretical basis for a more sophisticated fracture multi-field coupling deformation.(6)Calculated for multi-physics coupling under drilling shaft operating conditions,changing rules of fracture flow,stress around the borehole and fracture spatial structure were analyzed,which would serve as theoretical support for quantitative description of fracture spatial structure and flow ability under the complex down-hole mechanical environment.