ABSTRACT

Experimental studies present the effect of horizontal and vertical fractures and well configurations on the SAGD process in a three-dimensional model using 12.4 ° and 18 °API gravity crude oils. A total of eleven runs were conducted, using a 30 cm x 30 cm x 10 cm rectangular-shaped box model. Temperature distributions, the rise and growth of the initial steam chamber were observed by using 25 thermocouples. Three different well configurations were investigated – a horizontal injection and production well pair, a vertical injection – vertical production well pair and a vertical injection – horizontal production well pair with and without fractures that provided a vertical path through the horizontal producer for 12.4 °API gravity crude oil. The effect of fracture orientation (vertical or horizontal) on steam-oil ratio (SOR) and oil recovery was studied using horizontal well pair scheme.

The experimental results indicated that vertical fractures improved SAGD. Maximum oil recovery was observed during the horizontal injection – horizontal production well scheme with a fractured model, because of the favourable steam-chamber geometry. Runs showed that the location of the fractures affects the performance of the process. During the early stages of the runs, the fractured model gave significantly higher SORs than those observed in the uniform permeability reservoir. The fractures were successful in shortening the time to generate near breakthrough condition between the two wells.

In this study, numerical simulation of the SAGD process is performed. The CMG-STARS thermal simulator was used to simulate the data from the present SAGD experiments for fractured reservoirs. The simulation uses a two-component (water and heavy oil) black oil, three-phase (water, heavy oil and steam) and three-dimensional numerical model. The results from the history-matched numerical simulation are found to be in reasonable agreement with those of the experiment for oil production rate, cumulative oil production, steam chamber and temperature profiles in the model. The numerical simulation using STARS has provided relatively good result for the history matching of the experimental SAGD process using with scaled reservoir model.

INTRODUCTION

Steam-assisted gravity drainage (SAGD) is a promising recovery process for producing heavy oils and bitumen resources. The method ensures both a stable displacement of steam and economical rates by using gravity as the driving force and a pair of horizontal wells for injection/production. In the SAGD process, this is achieved by drilling a pair of horizontal wells located at a short distance one above the other. Steam is injected into the upper well and hot fluids are produced from the lower well. This progressively creates a steam chamber, which develops by condensing steam at the chamber boundary and gives latent energy to the surrounding reservoir. Heated oil and water are drained by gravity along the chamber walls of the production well (Butler and Stephens, 1981; Joshi and Threlkeld, 1985; Joshi, 1987; Butler, 1987).

     Figure 1 shows a vertical section through a rising steam chamber. During the rise period, the oil production rate increases steadily until the steam chamber reaches the top of the reservoir. SAGD with horizontal wells not only offset the effect of very high viscosity by providing extended contact or by heating but also maintain the necessary drive needed to move the oil, as the reservoir becomes depleted. A steam-assisted gravity drainage process also maintains reservoir drive and allows high recoveries. However, because of their considerable heat requirements, these processes are limited in their economic use to higher quality reservoirs (Joshi, 1991). In SAGD, horizontal wells are usually employed as injectors as well as for producers although it is possible to use multiple vertical injectors (Butler, 1994). The SAGD process is characterized mainly by gravity drainage. The higher steam pressure allows shorter breakthrough time from the injection well to the production well, and higher spread rate of the steam chamber because higher-pressure drop between two wells may cause driving force for moving oil. Thus pushing effect or moving oil caused by pressure difference between two wells should be suppressed as little as possible especially for laboratory experiments with scaled model.