Study On Compressibility Evaluation Of Tight Sandstone Reservoirs In Kong 2 Member Of Dagang Oilfield

Volume fracturing technology, which requires forming complex fracture network after fracturing, is an important stimulation method for tight oil and gas reservoirs. The fracability fracture index is a key parameter to evaluate the complexity of fracture network, which is significant for fracturing candidate selection.This paper focus on the fracability evaluation method of Dagang Kong 2 tight sandstone reservoir on the basis of experimental and theoretical analysis. Firstly, the basic geologic feature of Kong 2 tight reservoir is analyzed, a new method of rock crack morphology characterization and its influence factors are studied. Then the energy characteristics of rock deformation are studied and the brittleness evaluation model of tight sandstone reservoir is built, the fracability index considering natural fracture openness is established. Finally, fracability evaluation method is proposed by combination of brittleness and natural fracture openness. The overall research results are as follows:(1) A series of experiments such as rock mineral composition, triaxial compression and in situ stress test were carried out to analyze the basic geologic feature of Kong 2 tight reservoir. The results show that the quartz content of tight sandstone is the highest, which is 27.14% averagely, the second and third are feldspar and clay minerals which is 17.57% and 16.85% averagely. The distribution of mineral composition is quite different; the average value of Young's modulus is 17743MPa, Poisson's is 0.267 and lithology has little effect on the mechanical characteristics; the horizontal principal stress variation difference coefficient is between 0.2-0.3; natural fractures are well developed and the average dip angle is 24 degrees, the average strike angle is 34.8 degree. The results show that the reservoir has the capability of forming fracture network;(2) The rock deformation and failure law are studied, a new method of rock crack morphology characterization is proposed by using fractal geometry theory to analyze the relation between rock mechanics parameters and rock crack morphology. The results show that the expansion of rock is an important factor to characterize the fracture morphology and the fracture morphology, which is not related to the mineral component characteristics, peak strength and residual strength, has good agreement with Young's modulus, Poisson's ratio and peak strain.(3) The crack energy was calculated by establishing stress-strain relationship considering expansion. The fracture energy dissipation characteristics of the tensile and shear failure and the relationship between the parameters and fracture energy are analyzed. The brittleness evaluation model of tight sandstone reservoir was built by combination crack morphology and energy. And the brittleness method for shale and tight sandstone is compared. The results show that the correlation between crack energy and fracture complexity coefficient is more than 0.7, other effective factors are Young's modulus, dilation angle and peak strain, while Poisson's ratio, mineral composition and peak stress have not good correlation with crack energy; Shale mechanics properties and mineral compositions are closely related, while the correlation between tight sandstone brittleness and mineral contents is small.Besides, micro fractures and bedding is not well developed in tight sandstone, therefore, natural fracture is the key factor of forming fracture network.(4) The two-dimensional crack propagation model and displacement discontinuity method are employed to study the influence factors of natural fracture openness. The fracability index considering natural fracture openness is established. The results show that the smaller the tensile strength of natural fracture, the greater the opening angle range. The smaller the approaching angle and the smaller the coefficient of stress difference, the greater the length of the openness.(5) The fracability evaluation method is proposed by combination of brittleness and natural fracture. The results show that the weight coefficients between brittleness index and Young's modulus, shear dilatancy angle and peak strain were 0.262,0.353, and 0.385, respectively. The approaching angle and coefficient of stress difference are the key factors which influence the activation of natural fracture and the weight coefficients are 0.52 and 0.48, respectively.The studing results show that good brittleness and degree of natural fracture development are necessary for good formation fracability.The results are significant for fracturing design of tight sandstone reservoir in the Dagang Oilfield.