Study On Stress Sensitivity Of Tight Sandstone Reservoir Rock In Central Junggar Basin

The development of tight sandstone reservoirs in C hina has great potential.In the process of development and exploitation,with the continuous production of oil and gas fluids,the reservoir rock pressure changes,which will affect the effective stress and permeability of rocks.The selection of effective stress theory and the influence rule and evaluation of stress sensitivity are the main problems in seepage research.Terzaghi effective stress theory and laboratory experiments are usually used to study seepage.However,their application scope is limited,and conventional methods lack of seepage simulation under real reduction of reservoir rock pore structure.Therefore,starting with the study of micro-pore characteristics of rocks by capillary pressure method,casting slice and micro-CT scanning,this paper combines stress sensitivity experiment with permeability effective stress theory,reconstructs and simulates three-dimensional core of compact sandstone samples based on experimental results and micro-CT scanning images,and is particularly sensitive to micro-pore structure and stress of tight sandstone reservoir rocks.The stress sensitivity and productivity of rocks are studied and evaluated by various methods.The main work and conclusions are as follows:(1)The micro-pore characteristics of core were studied by mercury injection test,casting thin section observation and micro-CT scanning,which laid a foundation for further analysis of stress sensitivity of rock.Image sharpening,optimum threshold selection,binary image segmentation and image filtering are applied to reconstruct the three-dimensional core model.(2)Stress sensitivity experiments are carried out.Based on experimental data and effective stress theory,the effective stress functio n of permeability and the law of permeability changing with effective stress are analyzed.The effective stress coefficient is calculated by response surface method,and the law of effective stress coefficient and permeability changing according to differe nt pressure conditions is studied.The results show that the effective stress coefficient obtained by response surface method is more in line with the actual situation,and Terzaghi effective stress theory is not applicable to tight sandstone reservoir rocks.Through the analysis of various relationship models,it is concluded that the best fitting effect of the relationship between permeability and effective stress is the power relationship,which is consistent with the characteristics of micro-cracks in rocks.(3)Based on the three-dimensional reconstruction of core,the stress sensitivity of core is simulated numerically,and the permeability trend under different pressures is calculated according to the simulation results.The results show that the fluid velocity is very small,the fracture effectiveness is not high,and the permeability of each cross section is different,which indicates that the core has strong heterogeneity.The trend of average simulated permeability changing with pore fluid pressure is the same as that obtained from stress sensitivity experiment,but the numerical value is slightly larger,which is due to the identification error of pore and fracture in the modeling process.(4)According to the industry standard method,stress sensitive coefficient method and stress sensitive index method,the stress sensitivity strength of tight sandstone reservoir rocks in Junzhong area is slightly different.The industry standard method exaggerates the stress sensitivity of rocks to a certain exte nt.The stress sensitive index method is different according to the different effective stress theory adopted.Comprehensive comparison shows that the tight sandstone in Junzhong area is different.The stress sensitivity is medium strength.The productivity formula considering stress sensitivity is deduced,which shows that the stress sensitivity of reservoir rock has a great influence on productivity.