Hydraulic-Fracture Predictions With a Fully Coupled Geomechanical Reservoir Simulator

Abstract

Summary A new geomechanical reservoir simulator has been developed that combines hydraulic fracture growth, multiphase/multicomponent Darcy/nonDarcy porous flow, heat convection and conduction, solids deposition, and poroelastic/poroplastic deformation in a single application. The equations for the different mechanisms such as fracture-width changes, laminar-channel flow in the fracture, porous flow in the reservoir, heat convection and conduction, and poroelastic/poroplastic deformations are combined to produce an implicit fully coupled formulation. The nonlinear system of equations is solved through use of a full Newton-Raphson expansion of all solution variables, which enhances solution stability and allows second-order convergence rates for the nonlinear iterations. The program contains two separate criteria that one can use to model fracture propagation. Fracture-growth computations can be based on critical stress-intensity factors or can use cohesive elements that exhibit strain-softening behavior. The critical stress-intensity factor is based on the asymptotic stress/strain state at the tip of a fracture and is limited to linear poroelastic applications or applications in which the plastic zone is small relative to the fracture length; while cohesive elements are based on energy-release rates and cohesive stresses and can be used for both poroelastic and poro-plastic applications. In addition to the fracture-propagation logic, the program allows a dry zone to develop at the fracture tip as a natural part of the solution process. It is shown in sample simulation that a dry zone may develop naturally at the tip of a propagating fracture when there is a large pressure drop down the fracture. The new geomechanical simulator is described and several examples are included to demonstrate the predictive capability of the application. Examples are also included to highlight the differences between the two fracture-propagation models and to illustrate when a dry zone may be expected to develop at the fracture tip. The examples also allow one to compare the program's predictions with analytic solutions validating the fracture propagation algorithms used in the application.