Basal Combustion-A Recovery Technology for Heavy Oil Reservoirs Underlain by Bottom Water

Abstract

Abstract In Western Canada, many heavy oil reservoirs are underlain by thick bottom water. Very few of these reservoirs are commercially exploitable by conventional cold production or thermal technologies because of the high water mobility. The water zone could act as a strong water source that causes production wells to water-out quickly, or as strong energy sink that takes away most of the injected heat energy. As a result, these reservoirs are generally not attractive to produce, despite the enormous oil resources contained in them. Can the high mobility of water be beneficial to oil recovery? Can the known properties of bottom water (e.g., the oil-water contact, viscosity, density, and thermodynamic properties of water, etc.) be used as process tools to implement a successful recovery process? This paper attempts to address the above issues by introducing a new "Basal Combustion" concept that is built on the energy and mass transport characteristics of bottom water. Numerical simulation results indicated that, with the help of an effective transport and gathering system, a near level combustion surface could be initiated at the bottom of a heavy oil zone. The gas and water flows could serve as underground "conveyor belts" to deliver heat to the cold oil zone and, at the same time, remove mobilized and upgraded oils from the high temperature regions. High recovery and high energy efficiency can be expected. Introduction The Basal Combustion Process was conceived by not equating "bottom water" to "problems." Rather, the predictable features of bottom water were being sought after as tools for developing a new enhanced oil recovery technology. Specifically, the well known transport and thermodynamic properties of water were considered as potential assets for solving the mechanistic problems of in situ combustion. If the high predictability and reliability of heat and mass flows in bottom water could be translated into low operating risk, the injection of combustion air through bottom water would mean energy efficiency and profitability for in situ combustion. Observations from field operations and numerical simulation studies indicated that, when gas is injected into a bottom water heavy oil reservoir, most of it will flow along the upper part of the water zone and form a "basal gas layer" underneath the heavy oil zone. If oxygen is contained in the gas, this "basal gas layer" could become an extremely good in situ combustion site. The reasons are:the oxygen rich gas is in direct contact with hydrocarbon fuels, andonce ignited, this combustion layer will utilize a "heating from below" (or hot plate heating) mechanism to mobilize and upgrade the hydrocarbons above it. The potential benefits of basal combustion to heavy oil development in Alberta can be very significant. However, it would be risky to conduct field tests without a more concrete understanding of how this concept could be applied. Numerical simulation is therefore an effective method to obtain preliminary information for assessing the process feasibility, and for identifying needs for laboratory research work.