3-D Fracture Simulation Research For Low Permeability Reservoirs Hydraulic Fracturing

Putaohua formations and Fuyang formations of Daqing peripheral oilfields are a low-permeability reservoir, naturally low productivity. Hydraulic fracturing technology is potential to increasing per-well production and can increase oil reserves. However, because of the complex lithology and structure of the low-permeability reservoir, the traditional fracturing design method is't suitable for construction requirements, and is difficult to predict and control the fracture. Further the poor effect and success low. Fracture breakdown mechanism and extension law of Daqing peripheral low-permeability reservoir is researched using damage mechanics and fracture mechanics, which is of great significance and benefit for the development of Daqing peripheral oilfields.Reservoir geological features and rock mechanical properties have to be taken into consideration in the process of fracturing design. In this paper, sedimentary, structure, lithology and reservoir characteristics of Putaohua reservoir in Aonan oilfield are researched, and the mechanical properties parameters and determination of geostress field are tested which is basis for three-dimensional hydraulic fracturing numerical simulation.The perforation geostress mechanical model of low permeability reservoir is built according to the rock mechanics, seepage mechanics, elastic-plastic mechanics, considering solid-fluid coupling and rock material nonlinearity. Adopting the transient analysis, low-permeability reservoir geostress distribution of different stages is obtained using the finite element, such as drilling - cementing - perforation -fracturing. By Calculating the actual well, the error rate of breakdown pressure between test and calculation is 3.5 percent. It is proved that the model is reasonable. On this basis, analyzed the perforation parameters and the perforation eye pollution effect on breakdown pressure and location. Proposing reasonable perforation scheme, provide prophase guarantee for hydraulic fracturing effect to low-permeability reservoir.The features of hydraulic fracturing have inertia and speed, the rock mass failure and development, fluid-solid coupling, the process is a couple problems of material non-linear, geometry non-linear and structural dynamics. Through mechanical analysis, the 3-D fracture mechanical model of low permeability reservoir is established, considering solid-fluid coupling, material non-linear, dynamic effect of fracture propagation. The stress equilibrium equations of rock masses and the fluid seepage equilibrium equations under condition of non-darcy flow is established according to rock mechanics, seepage mechanics. Using finite element theory, Finite Element Scheme of fluid-solid coupling is obtained. Damage criterion and evolution equation of fracture extension is established on the preset fracture surface of the reservoir rock using the combination damage mechanics with fracture mechanics, which is embedded into the fluid-solid coupling equation to solve using the Newton-Raphson method, load incremental method and the linear search method, finally realize the dynamic description of 3-D fracture of the low-permeability reservoir. In order to test the correctness of low-permeability reservoir 3-D hydraulic fracture numerical simulation method and theory, calculation is carried on Zhao 38-271 Well.The mean error rate of fracture size is 10.7 percent, meets the requirements of engineering precision, which proves the correctness of the simulation.In order to make a reasonable process of the hydraulic fracturing for low-permeability reservoir, qualitatively analyze some factors influence on fracture propagation form using low-permeability reservoir 3-D hydraulic fracture numerical simulation method, such as mechanical properties of the rock, perforation parameters, construction parameters ,the fracturing fluid viscosity, filtration coefficient and so on.Interlayer is main parameter of controlling fracture height and affecting fracture form in low-permeability reservoir, as well as the main basis of making hydraulic fracturing measures. Therefore, emphatically analyze some factors influence on fracture propagation form using low-permeability reservoir 3-D hydraulic fracture numerical simulation method, such as the geostress difference, modulus of elasticity difference and interlayer height, which make the theoretical guidance for isolation effect of the low-permeability reservoir to effectively improve producing degree of oil reservoir and fracturing effect.With the great development of low-permeability oil and gas, the prediction software that suitable for using to 3-D hydraulic fracture geometry form is much needed and exigent. Using the finite element method, the low-permeability reservoir 3-D hydraulic fracture numerical simulation method and techniques can provide more reliable and accurate predicted means for hydraulic fracturing design, which improve hydraulic fracturing success rate to the low-permeability reservoir. As a result, this technology has very important significance and wide application prospect.