Methods For Modeling Dynamic Fractures In Coupled Reservoir And Geomechanics Simulation

Abstract

Abstract Proper representation of dynamic (propagating) fractures is important for modeling applications in which the fracture is directly coupled to a reservoir simulator. Such a model, while once applied only to unconventional fracturing applications is now being developed for conventional fracturing techniques (such as fracturing in low permeability reservoirs) as a more realistic tool for modeling both fracture growth and post fracture production responses. In an uncoupled model (such as conventional fracturing software) the reservoir coupling is usually simplified to a 1-dimensional leak-off model which lends itself easily to construct dynamic grids for the fracture. However, coupled models will generally require a dynamic fracture propagating through a stationary reservoir/stress grid. This creates a well-known grid effect resulting in oscillation of fracture growth with time, and limiting the stability of the model. In most unconventional fracturing applications, the fracture volume is small compared to injected fluid volume due to high leak-off, which causes a singularity of mass balance constraint or fluid volume in facture. If one were to use a conventional fracturing model with a dynamic grid, this would result in convergence problems due to the high leak off of injected fluid. Also, what is not represented in conventional models is the influence that high leak-off has on the far-field stresses and pressures, which in turn influences the fracturing mechanics. It is believed that the use of a fully coupled dynamic model such as the one being developed here will generate more realistic representions of the fracture/reservoir response. This paper presents numerical techniques necessary for the successful development of such a fully coupled model. Four different methods for representing dynamic fracture propagation were formulated. These four methods take into consideration the mutual influence between dynamic fracture propagation and reservoir flow, treat the fracture as a highly permeable part of the reservoir, and use one (common) grid system to model both dynamic fracture propagation and reservoir flow in a fully coupled manner. The individual methods differ by the algorithms, by which the dynamic modification of the transmissibility for fractured grid(s) is derived, and range from an empirical approach to the use of analytical fracture models.