| Rich oil and gas resources are contained in Carbonate reservoirs.Wherein,60% of the proved oil-gas reserves in the world concentrates in marine carbonate reservoir,which has become the key field of oil and gas exploration and development and has attracted the attention of all countries in the world.Due to the various types of carbonate reservoirs,complex reservoir space and characteristics of deep burial,high temperature,high pressure,strong heterogeneity and crack development,the cost for oil and gas development is high,and the production increasing improvement for reservoir is difficult.So,it needs to step up the research on the key fracturing technology for the stimulation of the carbonate reservoir.This paper has taken carbonate reservoir of block Shunbei in Xinjiang as the research object,and the mechanical deterioration mechanism and acidizing fracturing technology on carbonate rocks has been deeply studied by micro analysis of reservior core,laboratory physical and mechanical tests,laboratory fracturing physical simulation experiments and theoretical analysis.The main results obtained are as follows:(1)The mineral component and micro-structure of Shunbei carbonate rock have been analyzed by X-ray diffraction and electron microscope scanning.The results show that this reservoir mainly contains calcite and its content accounts for more than 90%.This reservoir has showed features of high carbonate,low quartz and less impurity.Besides,it has higher brittleness index and better reservoir fracturing material foundation;A flake-like sedimentary structure is presented inside the mineral.The calcite enrichment area has a tight muti-layer structure.And the mineral particles are cemented by calcareous,in which filling residual fractures,corrosion holes and inter-crystal micro-fractures are existde.So,it is difficult to reconstruct the reservoir volume.(2)The basic mechanical characteristics and deformation features of Shunbei carbonate rocks have been systematically studied through uniaxial and triaxial compression tests,Brazilian tests and direct shear tests.And we have set a compressibility prediction model and evaluation standard based on rock elastic index,comprehensive brittleness index,fracture toughness index and shear strength index.The results show that the compressibility index of Shunbei carbonate rocks is 0.777,which indicates better fracturing property and complex fracture morphology can be formed by fracturing reconstruction.(3)The physical and mechanical damage deterioration characteristics and micro interaction mechanism of Shunbei carbonate rocks have been revealed by physical and mechanical tests after fracturing fluid treatment.The results show that the physical parameters,micro-properties and mechanical features of Shunbei carbonate rocks obviously deteriorate with the erosion time under gelling acid treatment,and the optimum deterioration time is 30 min according to the evolution equation of deterioration degree and damage variables.Based on the comparison of physicalmechanical features,their deterioration effects are as: gelled acid > cross-linked acid > slick water.(4)An experimental acid fracturing simulation system for different fracturing fluids has been built,and a method of characterization and effect evaluation for the post-fracturing cracks has been proposed,which is based on pump pressure curve,acoustic emission characteristics,sample deformation features and fracture extension morphology.The fracturing process of carbonate rocks mainly includes three stages: initial pump pressure accumulation and hole filling stage,micro-fracture and crack extension stage,fracturing damage and flow channel forming stage.And there are mainly four types of cracks: the principal fracture along the axis,the secondary fracture derived from the fracture extension,the naturally opened fracture and the shear slip fracture.(5)Fracturing physical simulation experiments with different parameters under controlled conditions have been carried out.The effects of fracturing fluid medium,pump-discharge capacity,fracturing fluid viscosity,confining pressure and combination fracturing technology on fracturing process and fracture characteristics have also analyzed.The results indicate that the initial fracturing pressure is mainly affected by the fracturing fluid viscosity,pump displacement capacity and confining pressure,and the fracturing fluid medium and its combination fracturing technology mainly affect the fracture formation and morphology and then further affect the fracture complexity and flow conductivity.Slick water is a non-reactive fracturing fluid,which has strong fracture-making ability,many types of fractures tend to form,but the opening degree of the fracture is low.While,gelled acid is a reactive fracturing fluid which can create fracture channels with high roughness and larger width by deterioration on rock properties and corrosion on fracture walls.A combination of low-flow slick water and high-flow gelling acid is helpful for reducing pressure,and extending cracks and enhancing flow conductivity. |
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