Coupling Of A Fracture Mechanics Model And A Thermal Reservoir Simulator For Tar Sands

Abstract

Abstract This paper describes a new technique for dynamic coupling of a fracture mechanics model which predicts the growth of a fracture from a steam-injection well, with a general reservoir simulator which predicts the well injection/production behaviour. The approach of partial coupling of the fracture model with a reservoir thermal simulator is described. This approach provides computational efficiency and great flexibility. The method accounts for the dynamic fracture behaviour both during injection fracture growth) and production and can be used in conjuction with any conventional simulator. The key elements of the method are the dynamic enhancement of transmissibilities in the fracture plane, and a concept of pseudoized relative permeability. Verification of the technique and examples of field-scale thermal simulations using two different simulators are given. Introduction Injection of hot water or steam in oil sands results in complex interactions of several mechanisms, notably the fracture initiation and growth, the soil mechanics of the uncemented porous matrix and the resulting poroelastic and thermoelastic stress-strain behaviour, and fluid now and heat transfer in porous media. In the past, these major components were considered independently, and the attention was focussed in petroleum engineering mostly on the thermal multiphase flow models. For the study of the recovery mechanisms, the now models for consolidated media were Used(1–9) and when found inadequate, they were modified to account in some way for the fracture and soil mechanics aspects. Currently, the awareness of the importance of geotechnical aspects in reservoir engineering is glowing. On the other hand, geotechnical engineers are more interested in situ problems(10). A practical approach of partially decoupled modelling, which combines reservoir, fracture and stress models, was conceptually described(11). The details of the coupling of soil mechanics with reservoir flow were described for the isothermal case(12) and for the thermal case(13). This paper describes a method of coupling a fracture mechanics model with a thermal simulator. The coupled model retains the dynamic character of the fracture and at the same time results in a flexible and practical simulation system which can use existing thermal models. The work reported here is part of the over-all research of developing a comprehensive, modular modelling system outlined(14). Modelling of Fractures in Oil Sands Fracturing is one of the essential requirements to achieve a reasonable rate of steam injection into unconsolidated tar sand formations. Thus, rigorous representation of a fracture plays an important role in reservoir simulation. To date, a fracture in oil sands was represented in reservoir simulators in a simplified fashion using stationary or pressure-dependent change of transmissibililies. Fracture propagation and fluid flow in the fracture cannot be simulated accurately in this fashion. Coupling with Fracture Mechanics Realistic simulation of fractures requires a model of fracture extension, which is either fully coupled or partially coupled with reservoir flow model. As discussed(10), the partially coupled system a number of advantages. In particular, it allows fracture mechanics and reservoir flow problems to be treated on different scales which are natural to the problem.