The Mechanism Of Self-supporting Fracturing In Daqing's Peripheral Low Permeability Reservoir

Currently, the producing reserves of the low permeable and extra-low permeable reservoirs are less than 40% proved by Daqing's peripheral oilfield accumulatively. The water-fracs is the key technology to develop this kind of the economic marginal reservoirs,which can effectively improve the flow condition near the bottom of the well and improve the oil well productivity. Due to the shortcomings of the high costs and second damage, the traditional gelled fracturing cannot reach the expected results. Then the water-fracs treatment technology has shown up, which works as the fracturing fluid by adding the friction reducer agent, active agent, antiswelling agent or the linear gel into the water to carry out fracturing operations. It has achieved some good results, while it is not suitable for all of the low permeable formations, and that some well fracturing operations fail or have little effect.According to the particularity of the water-fracs technology, in the aspect of the fracture initiation, extension and closure mechanisms it has a number of differences from the conventional sand fracturing. So to understand the water-fracs mechanism deeply and to establish the corresponding reservoir screening method, it is the premise for promoting the application of water-fracs. Based on this problem, the following works are carried out.(1) Selecting the well Gu708-68-70, Zhao103 and Xing 11-51- Bing242 of Daqing's peripheral low permeable layers as the study object, three axis stress tests, Brazilian spilt tests and fracture toughness tests are carried out. Then the rock mechanical indices are obtained including elastic modulus, Poisson's ratio, compressive strength, tensile strength and fracture toughness. The Moore strength envelope line is also drawn and the cohesion and internal friction angle are calculated.(2) Using the paleomagnetic experiments, differential strain experiments, acoustic emission experiments and field water-fracs experiments, the ground stress magnitude and orientation of Daqing's peripheral low permeable reservoirs such as the southern GuLong are evaluated comprehensively.(3) The mathematical model of water-fracs stress and seepage field coupling and the criterion of fracture initiation and propagation are established. Applying the finite element method, the processes of the fracture crack, steering and extension are simulated. There are two kinds of fracture morphology existing in the water-fracs' s supporting fractures, which is tensile and shear. The steering angle and slip distance are influenced by many factors such as the reservoir heterogeneity, the ground stress deviation and the angle between principle stress,and the formation of the joint network is mainly controlled by the natural fracture system and ground stress difference.(4) The closure mechanism of self-supporting fracture in the process of water-fracs' s pressure relief and backflow is revealed. By the fractal interpolation method, the fracture wallsurface was constructed. Combined with the numerical simulation, under the meshed and faulted state the contact stress nephogram, strain nephogram, and closed residual width of the self-supporting fracture wall with different roughness are obtained. And the formation mechanism of the flow conductivity of water-fracs self-supporting fractures is determined.(5) The method about the flow conductivity tests and reservoir screening of the self-supporting fracture are built. Through splitting and slipping the rock sample, the specimen in line with the self-supporting fracture closure form was prepared and the method to testing the flow conductivity is developed. And by taking the stimulation ratio and dimensionless flow conductivity as evaluation indexes, the water-fracs reservoir screening methods and evaluation standards are set up.According to the above research results, a series of problems for Daqing's peripheral low permeable reservoirs are further revealed, such as the basic physical and mechanical characteristics, ground stress characteristics, propagation mechanism of the water-fracs crack,closure mechanism, flow capacity and reservoir screening method, which can provide the technical support for the application of water-fracs.