Abstract

Both foamy oil flow and sand production are believed to be closely associated to production in the heavy-oil reservoirs in Northwest of Canada. Reservoir depletion after a period of production induces stress concentration which may destroy or weaken the formation cohesion and then sand flow becomes possible. At the same time a large drawdown during the production creates a foamy oil zone, in which a larger pressure gradient and seepage force, which can drive a large amount of sand into a well, are created. Studying sand production for management and exclusion purpose, which may also enhance reservoir performance, a geomechanical model is coupled to a transient foamy oil flow, and by this model the accumulated sand production can be evaluated. Considering the heavy-oil reservoirs in Frog Lake and those in Lloydminster, both experimental and field results are studied and used to validate the model proposed. It is concluded that the simple model proposed can be used as an efficient tool to evaluate the field performance, but a fully coupled multiphase numerical simulator should be developed for more realistic field simulations.

Introduction

Field results from many heavy oil reservoirs in northwest of Canada such as Lindbergh, Frog Lake, and Lloydminster fields suggest that primary recovery is mainly governed by the processes of sand production and foamy oil flow. To operate in such reservoirs, the challenge we are facing is how to produce oil with a maximum capacity while keep sand production under control. Ideally we hope to keep as much oil produced, while the sand production minimum. Unfortunately controlling the sand often results in reducing oil flow. For example it has been reported that an average oil production of only 0.5–3 m3/days can be achieved for a well in which no sand production is allowed, while 7–15 m3/day oil may be produced with sand production. It seems that sand production either increases the reservoir mobility or allows the development of high permeable zones such as channels (wormhole) as postulated. Encouraging sand production to enhance oil production, on the other hand, poses environmental problems. Consequently neither trying to eliminate the sand production completely nor letting sand produced freely, engineers attempt to optimize the field operations by developing a criterion to balance the oil and sand productions.

Investigation on sand production has been extensive, but primarily limited to the areas of incipience of sand production and control. Bratli and Risines studied sand arching and production initiation from a cavity. A critical flow rate before sanding was developed for single phase steady-state flow. Weingarten and Perkins extended such a study to gas reservoirs, in which the gas density is characterized as a function of pressure. By a similar approach, Wang and Dusseault analyzed the risk of sand production near an inclined wellbore for both oil and gases reservoirs subject to three different in-situ stresses. Both the thermal effect and the impact due to a nonlinear yielding behavior on the near-well stress have been also studied. By these studies, both the critical drawdown and wellbore flow rate before massive sand occurs can be calculated, and only a steady-state and single-phase fluid flow are considered. In practice, however, a transient flow process can be maintained either due to foamy oil behaviors and permeability mobilization even after a long period of time. This may be confirmed by the fact of that sand production often correlates to a time-dependent and multiphase flow process. Another important issue is that, in addition to estimating sand production onset, we also hope to be able to calculate the accumulated sand production.

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