Abstract

This project establishes a new workflow to improve the dewatering efficiency of under-saturated CSG (Coal Seam Gas) reservoirs, while minimising reservoir damage related to coal compressibility. The over-arching goal is to maximise long-term productivity of wells to improve the lifecycle economics, not simply to target short-term success. By linking the strategy for well operations to productivity metrics it will also account for reservoir quality differences between wells, an important factor given the highly heterogeneous nature of CSG reservoirs.

The basis of this workflow is to use metrics that characterise the productivity of individual wells to optimise the rate at which the fluid level in the wellbore is reduced from initial conditions. This would address two conflicting qualities: operating wells too aggressively (fluid level reduction too fast) increases the risk of reservoir damage and productivity reduction, whereas operating wells too conservatively (fluid level reduction too slow) erodes the economic value of the asset. Diagnosing this correctly to achieve optimised well performance is challenged without sufficient production monitoring and reservoir surveillance. This is a consequence of the non-linear production response with pressure due to the pressure-dependant compressibility of coal. The workflow addresses this through production analysis of individual wells over time with the aid of diagnostic plots for well productivity.

The initial application of this workflow has demonstrated an incremental benefit to the dewatering efficiency of CSG wells. This is achieved in two ways: lower productivity wells are saved from additional reservoir damage and productivity degradation, while improved performance and accelerated dewatering is achieved from wells that are more productive. The scope and results of this trial are discussed in the second part of this paper.

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