Abstract

A series of flow visualization experiments was carried out to examine the pore scale behaviour of solution gas drive process in heavy oil reservoirs. The main objective was to test several speculative theories that have been put forward to explain anomalous production behaviour of heavy oil reservoirs producing under solution gas drive process. Contrary to previous postulations, the asphaltene constituents did not appear to play a significant role in nucleation and stabilisation of the gas bubbles evolved during the solution gas drive process. Experimental evidences also suggest that the production of heavy oil is not accompanied by a large population of micro-bubbles. These observations suggest that the production enhancement in solution gas process in heavy oil reservoirs is related to other mechanisms such as viscous coupling effects, sand production, wormhole effects, etc.

Introduction

Primary production of heavy oil reservoirs operating under solution gas drive mechanism exhibits unexpectedly higher primary recovery with slower pressure decline rate, lower than expected gas oil ratios and higher oil production rates. Some of these reservoirs which are prolific during the primary production phase have shown very poor response to secondary recovery techniques, such as thermal recovery. The ongoing observations in the fields and preliminary observations in the laboratories strongly suggest that the cold production process of heavy oil reservoirs by solution gas drive process involves a multitude of effects. A detailed analysis of such unusual production behaviour was first provided by Smith. He suggested that solution gas drive in heavy oil reservoirs involves simultaneous flow of oil and gas in the form of micro-bubbles. Following this, the flow behaviour of such gas-oil dispersions has been subject of several investigations and considerable speculation. However, solution gas mechanism in heavy oil reservoirs remains controversial and poorly understood.

Background

In solution gas drive process, the main source of energy driving the oil towards the wellbore is the evolution and expansion of the gas bubbles initially dissolved in the oil. The role of the gas bubbles in the oil displacement process has been studied for a long time. The first visual study of the behaviour of solution gas process at microscopic level was performed by Chatenever et al. using thin glass bead packings and thin sections of natural sandstone and limestone. With the advent of glass micromodels, flow visualization studies were conducted to examine the microscopic behaviour of solution gas drive process. All these studies provided a direct observation of pore level events. However, a comprehensive understanding of the pore scale physics in solution gas drive process is not yet attained.

Moreover, recent observations in the field led to a revised thinking of the mechanisms involved in solution gas drive process in heavy oil reservoirs. The flow of heavy oil under solution gas drive process appears to be more complex than what is expected from conventional solution gas drive theories. None of the previous studies focused on the behaviour of solution gas process in heavy oil reservoirs. To acquire an improved understanding of the solution gas drive mechanisms, it is necessary to consider the pore scale physics. Most of the questions concerning nucleation, growth, coalescence and flow of the gas bubbles dispersed in oil can be answered only by direct examination of individual pore scale events. Although it is not possible to visually examine the processes occurring at pore level in actual reservoir rocks, a very close approximation can be achieved in micromodel.

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