Investigation Of Leakoff Characteristic And Model For Fractured Tight Gas Reservoirs

Tight gas is the focus of unconventional gas exploration and development. Hydraulic fracturing is the key technology to develop these reservoirs. The fractured tight gas reservoir is an important type of tight gas reservoir. The presence of natural fractures in the reservoir is able to improve the flow capacity. However, natural fractures will also cause excessive fracturing fluid leakoff which result in narrower hydraulic fracture width and even screen out. The fluid leakoff in natural fractures has not been studied enough. Therefore, this paper carried out research on fracturing fluid leakoff in fractured tight gas sands, especially on leakoff in natural fractures.Reservoir characteristics of fractured tight gas sands were firstly analyzed. Then the features of leakoff were analyzed by combining reservoir characteristics and fracturing pressure response. After that a classification of the leakoff media was proposed:Before opening of natural fractures there are two leakoff media, unopened natural fractures and matrix; after the opening of natural fractures there are three leakoff media, opened natural fractures, unopened natural fractures and matrix. The opened natural fracture is the main path which causes most of the fluid leakoff.The opening of natural fractures was studied. Firstly a model describing hydraulic fracture induced stress field was established based on the displacement discontinuity theory. Then mechanical discriminant model for the opening of natural fractures was developed by considering fracture induced stress and in-situ stress. Factors affecting natural fracture opening, such as net pressure and Poisson's ratio were analyzed.In order to describe fluid flow in opened natural fracture, factors affecting natural fracture width and fracturing fluid viscosity were firstly studied. The effects of fracture roughness, filter-cake and fracture deformation on natural fracture width were investigated. Relationships between natural fracture width and related parameters, such as fractal dimension, were established; the influences of shear rate and temperature on the apparent viscosity were investigated. The relationships between apparent viscosity and the flow exponent, temperature and activation energy were established.The fluid flow within opened natural fractures cannot be considered as the seepage process, therefore a model for fluid leakoff in opened natural fractures was developed based on the basic fluid flow equations in fracture media. Besides, the equations of natural fracture width and fracture fluid viscosity proposed in previous chapters are also included in the model. Then dual media seepage model was utilized to describe the leakoff in unopened natural fractures and matrix. Through the combination of different models, the fracture fluid leakoff could be simulated before or after the opening of natural fractures. The impacts of natural fracture properties, fracturing fluid properties and pressure on the leakoff were discussed. Factors controlling leakoff were also identified based on the simulation.In order to investigate the influence of hydraulic fracture propagation on natural fracture leakoff, a novel model was established. It takes the fluid exchange between opened natural fractures and the hydraulic fracture into consideration. The model is developed by coupling of two-dimensional hydraulic fracture propagation model, mechanical discriminant model for the opening of natural fractures and leakoff model of opened natural fractures. To solve the multi-domain and nonlinear coupling model, a semi-analytical solution was proposed. The impacts of in situ horizontal stress difference and pumping rate on fracturing fluid leakoff were analyzed.The results of this thesis could enrich and improve the leakoff theory for hydraulic fracturing in the fractured reservoirs and provide a reference to fracturing optimization.