Study On Mechanism Of Complex Fracture Expansion Of Tight Sandstone Reservoir

The reserves of unconventional oil and gas resources in China are very abundant,and the tight sandstone oil and gas reserves account for a large proportion.Due to the characteristics of ultra-low porosity and low permeability in tight sandstone reservoirs,it is difficult to achieve a good yield increase effect by conventional fracturing to form a single crack.Therefore,in order to effectively exploit tight sandstone reservoirs,reservoirs must be volume-fractured to form complex meshes.In order to solve this problem,this paper takes the tight sandstone reservoir in the A area of Daqing as the research object,and uses the internal physical model experiment method to study the mechanism of the complex fracture network of tight sandstone reservoirs.The specific research content is as follows:(1)The rock samples of 6 wells in the A area of Daqing were tested for rock mechanics,in-situ stress tests and evaluation of reservoir brittleness.The depth of tight sandstone reservoirs has a significant effect on the mechanical properties of rocks.With the increase of confining pressure,the elastic modulus and compressive strength become larger,and the elastic modulus changes more obviously with the confining pressure,and the Poisson’s ratio varies.The increase of confining pressure became smaller.The evaluation results of reservoir brittleness indicated that the brittleness index of reservoir rocks in the A area of Daqing was between 40.9 and 46.5,and the brittleness was good.The 1582.23~2097.35 m reservoir was a fracturing dessert zone.The spatial pattern of internal cracks in rock specimens was obtained by CT scan technique and image three-dimensional reconstruction.Combining with the results of geostress direction test,the natural fractures in the A area of Daqing are relatively developed,and the angle between the maximum horizontal principal stress and the horizontal principal stress is between 0° and 50°.(2)Tight sandstone hydraulic fracturing embedding experiments were carried out.The effects of stress difference,displacement,brittleness,and natural fractures on the formation of the seam network were experimentally studied.The specific conclusions were: the smaller the stress difference,the smaller the angle between the direction of the maximum horizontal principal stress and the natural cracks,the greater the brittleness of rock,the greater the displacement,the greater the range of fracturing fluid in the rock sample during hydraulic fracturing,and the more complex the seam network formed by hydraulic fracturing;It will increase the fracture pressure of the rock and reduce the rock initiation pressure.It will also reduce the extension time of the fracturing fluid in natural fractures.(3)As a result of physical model experiments,when the natural fracture and the maximum horizontal principal stress direction are 90°,when the artificial fracture encounters a natural fracture,it will directly pass through the natural fracture,and it is difficult to form a complex fracture network in the hydraulic fracturing process;When the angle between the crack and the direction of the maximum horizontal principal stress is less than 50°,the artificial fracture will be extended along the natural fracture,and the hydraulic fracturing will form a complex fracture network.(4)The experimental test results show that the rock brittleness index in Daqing A region is greater than 40,the horizontal stress difference is mostly less than 5 MPa,and the angle between natural fracture and the maximum horizontal principal stress is between 0° and 50°.The tight sandstone reservoir in Daqing A region can be subjected to volume pressure fracturing,reservoir reconstruction will obtain effective economic exploitation value.The research results of this paper can also be applied to evaluate the volume fracturing of tight sandstone reservoirs in other blocks.