A STUDY OF FLUID INFILTRATION AND PROPAGATION OF VERTICAL HYDRAULIC FRACTURE IN A POROUS MEDIUM (DARCY'S LAW, FINITE-ELEMENT METHOD, DISPLACEMENT DISCONTINUITY, FLUID LEAK-OFF)

The infiltration of fracturing fluid into a rock medium and its effect on the size of the hydraulically induced fracture were examined in this study. In formulating the problem, the equation for the fluid motion inside the fracture was derived by considering the conservation of the fluid mass, and the equation which relates the fluid pressure and the fracture opening was derived by considering the over-all deformation of the porous rock medium. The finite element method and the displacement discontinuity method were employed to obtain the solutions for the problem.;The results from the study have indicated that: (1) The poro-elastic model predicts a shorter fracture length and a wider fracture width in comparison with those predicted from the Geertsma model in which the rock is modeled as an elastic medium. The effect of rock porosity becomes significant when the porosity and the over-all modulus of the medium are high. (2) For a given pumping rate, the dimension of the fracture is controlled by the viscosity of the frac-fluid, the mechanical properties of the rock medium, and the mobility of the leak-off fluid in the rock medium. (3) A higher leak-off fluid mobility will produce a smaller fracture, a larger wet zone, and a higher frac-fluid pressure inside the fracture. (4) The empirical leak-off coefficient is related to the permeability and the thickness of the wet rock region and the filter-cake on the fracture surface. The effect of filter-cake is an important factor in the design of a hydraulically induced fracture.