Coupling Of Fluid Flow And Soil Behaviour To Model Injection Into Uncemented Oil Sands
Authors: Settari A. , Kry P.R. , Yee C.T. 
Published: March 2010

Injection into unstimulated heavy oil reservoirs generally results in disturbance to the soil matrix. To correctly model the fluid distribution, the fluid flow behaviour must be coupled to the mechanical behaviour of the sands. A model has been developed representing the conditions present during isothermal leak-off from fracture face, accounting for the physics of this coupling.

The model assumes that the minimum effective stress controls the soil behaviour and that the fluids are linearly compressible. The model satisfactorily represents fracture growth inferred from pressure observation wells and measured mean times for communication between cyclic steam stimulation wells.

Several new features generated by the model can be correlated with field behaviour. Nonlinear compressibility of the oil sand at low effective confining stresses causes increases in porosity which explain observed injectivity with zero initial mobility. Shear failure occurs around a fracture primarily due :0 decreasing effective stresses as the local pore pressure increases: Dilatant failure behaviour increases porosity and permeability, producing plateaus of increased water saturation. The coupling of the fluid and solid mechanics in the new model represents a significant advance in the realism of modelling of fluid flow and fracturing in oil sands.

Introduction

Injection of hot water or steam in oil sands results in complex interactions of the soil mechanics of the uncemented matrix and fluid flow and heat transfer in porous media. In the past, the recovery mechanisms were studied mostly using multiphase flow models developed for consolidated porous media(1–3). Such models are not adequate for modelling oil sands with low initialobility and one has to resort to artificial means of providing injectivity. In 1980s the importance of the geotechnical aspects was recognized, but the applications were limited mostly to surface mining problems. In most in situ projects, the formation is fractured during the first, or several of the injection cycles. Early work on fracture modelling(4, 5) showed that the proper representation of the fracturing and soil mechanics will be the key to any realistic modelling effort in oil sands. To date, all models including the fracture have been severely compromised by poor treatment of fracture mechanics or assumptions of linear elasticity(6, 7).

This work describes part of the results of an ongoing research effort for the development of a practical, but realistic model for thermal processes in oil sands. In this paper we will restrict the discussion to isothermal behaviour for the following reasons:It is necessary to understand isothermal behaviour before studying more complex thermal process.Fracture is established on the first cycle of steam injection during which the temperatures will be lowest. Due to heat transfer in the fracture, the injected fluid at the tip will have a temperature close to that of the reservoir.

Even for the isothermal case, one has to deal with several facts observed in the field, which contradict classical reservoir and fracture mechanics:Observed fracture dimensions are relatively small and fracture widths are large(8).Injectivity is larger than what would correspond to in situ mobility of fluids(5).