A Model of the Steam Drag Effect In Oil Sands
Authors: Edmunds N.R. 
Published: March 2010

Steam flooding oil sands requires some fluid communication path between injector and producer. This may be developed by exploiting reservoir zones with natural mobility, due to low bitumen saturation; or by artificial techniques such as fracturing. At the time of heat breakthrough to the producers, this communication path will generally be of limited volumetric extent, and have a fluid mobility thousands of times that of the virgin reservoir. A steam flood will there/ore fend to conform closely for this path, leading to early breakthrough and poor recovery.

Bitumen recovery after breakthrough requires the steam front to advance into the reservoir in a direction essentially normal to the flow. One mechanism by which this can occur is steam drag, V1!here a thin layer of oil sand adjacent to the channel is heated, allowing the bitumen to be dragged away by the flowing steam. Although steam drag operates at low oil steam ratios, high bitumen production rates and recovery are possible.

In this paper a simple analytical model of the steam drag effect is derived. A principal finding is that the rate of bitumen production should be proportional to the square root of the reservoir pressure gradient. Analytical predictions are found to be in reasonable agreement with numerical simulation results.

The influences of reservoir quality and well geometry are discussed.

Some practical recovery processes based on the steam drag concept are steam foam flooding, and bitumen recirculation. These appear to have the potential to recover a large fraction of the bitumen at acceptable oil/steam ratios.

Introduction

In a linear flood steam is well known to efficiently displace even extremely heavy bitumens from oil sand. In addition, much of Alberta's oil sand resource has excellent porosity, oil saturation and pay thickness, factors which are conducive to profitable steam flooding.

The problem, of course, is that native high-grade reservoir generally has negligible mobility, making injection and production impossible at practical rates. To carry out a core flood in the laboratory, For example, it is usually necessary to preheat the core and/or to employ extremely high pressure gradients.

In the field, it is necessary to create and maintain continuous heated channels between injectors and producers, before steam can be injected and bitumen produced on a continuous basis. This in itself is difficult enough, and is the subject of much current research. Some operators can exploit natural zones of low bitumen saturation, such as water sands(l), while others must resort to artificial means for example Fructuringt (2,3), or horizontal wells(4).

A dilemma arises when the operator must now recover bitumen from the unheated remainder of the reservoir. After having deliberately created a communication channel having fluid mobilities thousands of times greater than the virgin reservoir. Bitumen behaves more like a solid than a fluid, with typical viscosities of 100,000 to 10,000,000 mPa's at reservoir conditions. Any practical pressure gradient is incapable of displacing it at a significant rate.