Abstract

Steam assisted gravity drainage (SAGD) is one of the more popular enhanced oil recovery method of producing heavy oil and bitumen. In conventional SAGD approach, steam is injected into a horizontal well located above a horizontal producer. A steam chamber grows around the injection well and displaces heated oil toward the production well. There are several variations of this process: vertical injector-horizontal producer and singlewell (SW) SAGD where only one horizontal well is used by injecting steam from the toe of the horizontal well with production at the heel. Some advantages of technically challenging process include cost savings and utility in relatively thin reservoirs. To improve early-time response of SW-SAGD, it is necessary to heat the near-wellbore area to reduce oil viscosity and allow gravity drainage to take place.

This paper investigates the optimization of the startup procedure for SW-SAGD as the project economics are influenced significantly by the early production response. An experimental investigation of two early-time processes namely cyclic steam injection and extreme pressure differential between injector and producer to improve reservoir heating, is discussed and compared to other well configurations. Crushed limestone saturated with heavyoil 12.8 °API) and water that was packed in a semi-scaled laboratory model is used for the experiments.

The effectiveness of the methods are compared within themselves and to conventional SAGD by measuring the size of the steam chamber as a function of time. It is found that the steam chamber area for cyclic steam injection is slightly greater than that of extreme pressure differential case. Furthermore, numerical simulation studies of different early time processes are performed and compared to experimental data using a commercial simulator. It was observed that the numerical model results underestimated the cumulative oil recovery and the steam chamber size. Results from this study, including cumulative recoveries, temperature distributions, and production rates display the differences among the methods.

Introduction

Gravity drainage of heavy oils in situ recovery processes is of considerable interest in oil industry. Since these oils are very viscous and almost immobile, a recovery mechanism is required by which the viscosity of the material is lowered to the point where it can flow to a production well. Conventional thermal processes, such as cyclic steam and steam assisted gravity drainage (SAGD) (1,2,3,4) are based on thermal viscosity reduction. Cyclic steam incorporates a drive enhancement from thermal expansion. On the other hand, SAGD is a process based on gravity drainage and horizontal wells. In this process, a growing steam chamber forms as steam is injected from a horizontal injection well into the reservoir and steam flows continuously to the perimeter of the chamber where it condenses and heats the surrounding oil(1). Heat is transferred by conduction, convection, and by latent heat of steam. The heated oil drains, driven by gravity, to a horizontal production well located at the base of the reservoir just below the injection well (1) as shown in Figure 1. This process is effective and can be economic if the steam requirements are too high (5).

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