A Coupled Geomechanic-Multiphase Flow Model For Analysis Of In Situ Recovery In Cohesionless Oil Sands
Authors: Fung L.S.K. 
Published: March 2010

It is now generally recognized that reservoir operations in oil sands promote fabric disturbance in the sand matrix resulting in large changes in pore volume and permeability. These disturbances depend on the stress stales which are continuously altered by temperature and pressure changes due to fluid and heat diffusion and convection processes. Thus, a proper analysis of fluid flow behaviours also requires the solution oj the stress-deformational changes in the reservoir.

This investigation is concerned with the numerical solution of the coupled geomechanic-mu!tiphase flow model suitable for the analysis of oil-sand reservoir behaviour. The coupled theory for the isothermal situation is described in detail. This is the first part of an ongoing research effort to develop a realistic simulation tool for this class of problem. The system a/nonlinear partial differential equations is discretized using a control-volume finite-element method (CVFEj which is found to be well-suited for the analysis. The resulting equations are then solved using a fully-coupled, fully implicit approach. A hyperbolic dilatant stress-strain model is incorporated to represent the mechanical behaviour of uncemented sands. As far as we know, this is the first successful attempt in solving the coupled system using the method. Some results from the initial verification and testing are included. Areas of ongoing research are also palmed out.

Introduction

The exploitation of the vast tar-sand deposits of Peace River, Cold Lake, and Athabasca in Alberta remains one of the most challenging problems in the oil industry. The majority of these deposits are overlain by over 500 m of overburden and thus necessitates the use of in situ recovery methods. The bitumen viscosities at original reservoir temperature are high (&gt; 100 000 cp in Cold Lake and &gt; 1 000 000 cp in Athabasca). and as such, bitumen at native condition is immobile. However, the viscosities decline exponentially with an increase in temperature. This makes thermal recovery methods particularly attractive. Tar sands at initial conditions have no injectivity unless there exist communication channels intersecting the injectors. Therefore, methods of creating hot communication paths are required. These include the circulation of steam in a horizontal well as a means of heating an annulus of sand around the well in the HASD (Heated Annulus Steam Drive) or the injection of steam at a pressure large enough to overcome the minimum effective stress, thus inducing formation failure.

The study of thermal recovery processes in oil sands involves the analysis of complex interactions of geomechanics and multiphase thermal flow in cohesionless porous media. The oil-sand matrix has no cohesion because of the absence of mineral cement. The sand matrix is further characterized by an interpenetrative grain structure with large grain-to-grain contact areas. This peculiar fabric is the result of the predominant quartz dissolution and recrystallization diagenetic processes. It was suggested that oil sands constitute a distinct class of frictional materials known as locked sands(1). The native porosity is lower than what can be achieved in the laboratory after the fabric has been disturbed.