Abstract

The primary recovery of a medium-heavy oil reservoir with a strong bottom aquifer is generally poor. The introduction of horizontal wells that are drilled at the top of the oil column has improved the oil recovery. However, even horizontal wells suffer from fast water breakthrough that leads to oil production at a high water cut. Given the low primary recoveries, such fields are attractive EOR targets.

In situ combustion (ISC) is a displacement process generally applied to medium-heavy oil reservoirs in order to increase oil production by reducing the oil viscosity. In thick reservoirs (thicker than 10 meters), oil recovery could be severely challenged by gravity override of the injected gasses. In reservoirs without active aquifers, a significant part of the incremental oil is produced by gravity drainage after breakthrough.

We propose an ISC strategy where an infill producer is drilled close to the oil-water contact so that a significant amount of heat can be rapidly deployed in the middle and upper sections of the reservoir. Subsequently, the aquifer is used to sweep the warm oil through the heated zone towards the producers. The ISC process is compared with steam injection that also employs an additional infill producer. ISC and steam injection are used to deploy heat in the reservoir.

Numerical simulations show that the oil is produced at much lower water cut compared to the cold case (50–60 % versus 95%). Simulated oil recoveries increase significantly for both ISC and steam injection. A detailed comparison of these two processes is presented in this paper.

Introduction

The primary recovery of medium viscosity oils (10-5000 cp) from reservoirs with strong bottom aquifers is generally poor due to the unfavorable mobility ratio between the oil and the displacing aquifer water. Typically, the production is associated with an early and high water cut. Usually, developments employ horizontal producers drilled at the top of the oil column in order to delay, as much as possible, the arrival of water cones. Fundamentally, it appears that there are two ways to increase the recovery from such fields. The first is to improve the mobility ratio between the oil and the displacing water while maintaining the aquifer drive and the second is to introduce a different drive mechanism.

The mobility ratio could potentially be improved by increasing the viscosity of the aquifer water with polymer injection or by reducing the oil viscosity with the introduction of electrical heaters. However, these options do not appear to be very practical. Therefore, different options that could be considered are injection of solvent and thermal methods such as steam drive or In-Situ Combustion (ISC). In this case, thermal methods are preferred, given the range of the oil viscosities considered (10–5000cp).

Steam drive is one of the most successful EOR methods and it has been widely applied since the 1960's 1. However, it might not be at its optimum conditions for reservoirs with a strong bottom aquifer because the pressure during the application of the process has to be maintained at or above the aquifer pressure. This condition is important because when the aquifer pressure is high, little latent heat is available to be transferred to the reservoir once the steam condenses.

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