MODELING RESERVOIR STIMULATION INDUCED BY WELLBORE FLUID INJECTION

A novel numerical software: “Hydro-PED”, is presented for modeling hydraulic stimulation by wellbore fluid injection. The coupled equations of poro-elastic deformation with damage evolution, and groundwater flow are solved using the Explicit Finite Difference Lagrangian Method for solid deformation and the Finite Element Method for fluid mass conservation. Rock properties are coupled with the state of damage, and seismic events are nucleated as damage grows to a value (~1) that causes the loss of convexity of the elastic strain energy function. The seismic event is simulated by a stress drop using the Drucker-Prager model and produces a rapid release of the elastic energy and accumulation of a permanent plastic strain. Elastic and hydrological properties are coupled with the damage of the rock and as a result may vary by nine orders of magnitude as the rock is damaged and healed. This numerical implementation takes advantage of the ability of the explicit scheme to account for large variations in the stiffness values in neighboring elements with no numerical instabilities. Results show that the propagation of the enhanced damaged reservoirs could be divided into three stages. 1) Fluid flow into the rock with no seismic events. 2) Seismic events begin and accelerate. Pore pressure at the tip of the reservoirs is above 90 % of the pressure in the injection well. The velocity of the advancing reservoirs is limited only by the rate of damage accumulation. 3) Reservoir growth decelerate (>30 hours). Fluid transport becomes a limiting factor as the reservoirs are too long to efficiently transfer the pressure from the well to the reservoir tip