This paper describes some of the theoretical aspects which have been considered in developing a coupled flow/deformation finite element package capable of modelling the nonlinear oil sand characteristics. gas evolution, and the nonlinear fluid and flow behavior under the boundary conditions imposed during the recovery process. It is shown that the sand production is governed by the changes in permeability and soil properties which develop around the wellbore. The scheme employed in forming the wellbore controls the fluid pressure gradient close to the well and hence the reservoir stability. It is shown that the volume of evolved gases is responsible for most of the fluid flow under primary conditions. The model provides means of testing the influence of various recovery schemes which can be employed in controlling the sand and fluid production.
Formation of wellbores into the deep seated oil sand reservoirs leads to significant changes in the stress state and pore pressure profile around the cavity. The shear stresses developed and the pore pressure gradients established are sufficient to cause instability over an appreciable distance beyond the stress relieved zone. The instability gives rise to sand production which is believed to be the major reason for (a) poor performance in the subsequent recovery projects (e.g. steam injection), (b) severe channelling in water floods, and (c) direct communication between offset wells. Furthermore, sand production alters the geometry around the hole which affects the fluid production during the primary stage. Recovery predictions require a knowledge of soil movements, changes in material properties and geometry. An understanding of the problem would assist in making operations changes, or employing new schemes, which could improve oil production.
Analytical solutions to analyze sand stresses around a wellbore have been attempted but under Some simplifying assumptions. Risnes et al. (1) have presented elasto-plastic solutions and proposed a stability criterion for the problem by assuming steady state flow conditions, isotropic and homogeneous soil, fully saturated fluid, constant material properties and geometry. The assumptions employed, however, conceal features which may play an important role in the overall performance. For instance, the fluid pressure gradient near the wellbore would initially be well in excess of the long term steady state gradient which may result in non-predictable early instability. Eventually steady state conditions will be approached but by this time the effective geometry of the well and the properties of the formation will be considerably different from the original. Moreover, in the solutions provided by Risnes et al. no allowance is made for any gas exsolution. This is a serious limitation since the main cause of fluid flow under transient conditions is attributed to the volume of gas which evolves when the confining pressure is reduced below the bubble pressure (Byrne and Vaziri(2))
This paper describes some of the theoretical aspects which have been considered in developing a finite element package capable of modelling the nonlinear soil and fluid characteristics under realistic boundary conditions.
