Modelling Of Cavity Stability And Sand Production In Heavy Oil Reservoirs
Authors: Fung L.S.K. , Wong R.C.K. 
Published: March 2010

Abstract
Cold flow production of unconsolidated bituminous reservoirs reduces the in situ effective stress state to a very low value. The unloading coupled with viscous gradient leads to the yielding and progressive failure of the perforation cavity. The stability of the cavity is primarily influenced by the pressure gradient at the cavity boundary and the cohesion of oil sands.
In this paper, a three-dimensional cavity stability criterion is derived based on the equilibrium considerations at the cavity boundary. It is found that a perforation cavity will remain stable even with substantial yielding so long as the cohesive strength remains intact. Local collapse and cavity propagation occur when the critical seepage force is achieved. The direction of cavity growth is therefore a function of the local flow regime, fluid and rock properties, boundary conditions and cavity geometries. For a longitudinal perforation cavity, streamlines converge to the tip where the exit pressure gradient is the largest. High-resolution simulation of near-cavity flow indicates that substantial pressure gradient can be developed in the transient period to cause cavity collapse. Based on the comparisons of simulation results and the cavity stability criterion, the possible mechanisms for cavity growth under field conditions are evaluated.

Introduction
The primary production of heavy oil is emerging as a viable recovery technique. This method involves the solution gas drive mechanisms as well as sand co-production. The solution gas drive and foamy oil mechanisms have been discussed by a number of recent papers(l-3). Field production data appear to suggest that these production mechanisms are rather complex and are quite different from those encountered in the conventional solution gas drive process in structurally rigid reservoirs. One of the unique features is that a good oil production rate is generally accompanied by sand production. Sand production is believed to increase the effective area of inflow near the wellbore. One of the possible scenarios for the enhanced permeability zone is the formation of long horizontal channels. Wong et al.(4) found that a circular channel of less than 150 mm in diameter in oil sands retains its integrity even under an overburden effective stress of 6 MPa, provided that there is no inflow into the cavity. The seepage force as a result of the exit pressure gradient is the key factor causing the instability of sand cavity and subsequent sand production.
In this work, the investigation of Wong et al. is extended to consider the stability of a general three-dimensional cavity. Using an equilibrium stability analysis, together with the Mohr-Coulomb failure criterion, and the kinematic condition for shear stress mobilization, the critical condition of cavity stability under the influence of seepage forces is derived. This condition is cast in the form of a dimensionless stability number. The influence of exit pressure gradient on cavity stability is investigated using a decoupled approach where the flow around the cavity is modelled with high-resolution flexible-grid reservoir simulation utilizing the recently developed control-volume finite-element (CVFE) technique. In this work, the simulation is restricted to single phase.