Abstract

The experiments on initial stages of steam assisted gravity drainage(SAGD) process have been carried out using two-dimensional scaled reservoir models to investigate its production process and performance. Rising or growing process of the initial steam chamber, its shape and area, and temperature distributions have been visualized by using video and thermal-video pictures. As a drainage mechanism, the relationship between isothermal lines and chamber interface have been presented. The temperature on the interface, where the chamber was expanding, was maintained at almost constant temperature of 80 C. Furthermore, the effect of vertical well spacing between two horizontal wells on oil recovery has been investigated. For the case of usual SAGD, oil production rate increases with increasing vertical well spacing, however the leading time to start oil production by gravity drainage becomes longer. The results show that the well spacing may be a representative length for initial stage of the process. Based on these experimental results, a modified SAGD process by adding intermittent steam injection from the lower production well have been proposed. By applying the modified process, the time to generate near break-through condition between two wells was relatively shorten, and oil production was enhanced at the stage of rising chamber compared with that of usual SAGD process.

Introduction

There are vast heavy oil and oilsands reserves not fully exploited, because it is not easy to produce heavy oil efficiently and economically. However, steam-assisted gravity drainage (SAGD) process has been successfully applied to oilsands fields. The process has been developed by Butler and his co-workers. Their ideas was to overcome the problems associated with the highly viscous bitumen by gravity drainage in steam chambers generated by displacement of heavy oil. As shown in the reports on the UTF projects (phase A and B) in Canada, the SAGD process (see Fig. 1) has proven to be very superior process for the recovery of the bitumen due to its high recovery factor. Chung and Chung & Butler have reported experimental results for SAGD process with scaled and visual reservoir models. Furthermore, Chow & Butler have reported the numerical simulation results matching the Chung's experimental results with the STARSTM. Recently, Mukherjee have reported the successful forecast of the performance for the phase B of the UTF project. Butler gives an excellent review of the SAGD process.

In this paper, the SAGD process operated by steam injection from upper well and production from lower well like that of UTF project, is hereinafter referred as "usual SAGD." A problem of the usual SAGD process for oilsands reservoirs is leading time to generate a steam chamber in near break-through condition between two horizontal-wells before production stage by rising and expanding chamber. The more economical SAGD production should be achieved by a modified process to shorten the period of initial stage and enhanced steam injection for effective usage of steam generation and production facilities.

First, we have investigated characteristics of the usual SAGD process, especially expanding rate of steam chamber by gravity drainage and effects of vertical well spacing on it. It was found that by using shorter vertical spacing between two wells, leading time is reduced while production rate after break-through becomes lower. Based on experimental results, a modified process has been proposed to start oil production earlier and enhance oil production at higher rate after break-through. In this process, steam is injected from both of upper and lower wells. Then, the lower well has both functions of production and steam injection, which is similar to the single SAGD well developed and reported by Liderth. The steam is injected intermittently from the lower well, because steam injection holes and oil production holes at the well are quite close to prevent steam break-through.

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