Abstract

In an earlier paper, it has been shown that both the GOR and the composition of produced gases in a SAGD project may be calculated from first principles by considering the solubility of gases in the produced fluids.

A simulated production forecast from an Athabasca property is used to calculate the gas production characteristics for three cases; (a) original cap gas at virgin pressure: (b) cap gas replaced at the same pressure with carbon dioxide; and (c) cap gas replaced with flue gas from the steam generator. Comparison of the three cases shows that case (b) and (c) are accompanied with gas production containing significantly higher fractions of the re-pressurization gases, carbon dioxide and/or nitrogen, than case (a).

The presence of the non-combustible gases introduces a thermal inefficiency if the normal practice of directing produced gases to the steam generator is followed. The effect of this inefficiency is calculated by estimating flame temperature for combustion of gas from case (a), and comparing with the other two cases. The inefficiency is then corrected by adding make-up gas to the gas from case (b) and (c), until the flame temperature is identical to that of case (a).

The potential for sequestration of the greenhouse gas CO2 is then considered in terms of initial CO2 injected, cumulative CO2 produced back and CO2 formed by combustion of make-up gas. The predicted overall utility of depressurized zones for sequestration purposes is discussed.

Introduction

In Alberta, the Energy and Utilities Board has found that a resource conflict exists between production of bitumen and overlying gas in the McMurray formation. For the Athabasca oil sands deposit the Board has already ordered the cessation of drilling for gas reserves, and shut in existing gas production in the Surmont area. At the time of writing, further resource conflict hearings are in progress.

Accordingly, there has been interest in the possibility of repressurization of depleted gas caps, such that bitumen recovery could still follow gas production. In view of recent concerns about reductions of emissions of the so-called "greenhouse gases", attention has turned to the potential for sequestration of locally available carbon dioxide or flue gases for the purpose of achieving the repressurization.

In a recent paper, a predictive capability of gas production from production projects using Steam Assisted Gravity Drainage (SAGD) technology was Described1. The concept employed in such predictions is that produced gases from SAGD are not normally produced by free gas entering the producing wellbore, but by gases dissolved in the produced fluids. The Henry's Law constants for the important gases in water, and to a lesser extent bitumen, are known under the prevailing conditions of temperature and pressure, and are given in reference 1.

If one assumes that either carbon dioxide or flue gas is injected into the gas zone after 90% depletion of the gas, and since SAGD always produces methane, carbon dioxide and smaller amounts of hydrogen sulphide, it is possible, with the aid of a bitumen production forecast from a simulator, to predict the composition and gas: oil ratio (GOR) of the produced gases and so obtain a carbon dioxide balance for such a repressurization scheme.

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