Theoretical and statistical methods of determining recovery factors ofsolution, associated and non-associated natural gas, including gas condensate, are presented as a set of practical tools.
The effects of recovery mechanisms and pool and gas-well performance on gasrecovery are examined for conditions applying to constant volume, water drive, gravity segregation and secondary recovery. Ways of recognizing such cases areindicated.
The supporting techniques comprise material balance calculations, performance predictions and statistical analyses of depleted pool and predictedrecovery data. In material balance calculations, the gas recovery is consideredin terms of either:
initial gas in place multiplied by a recovery factor or
initial gas in place less the gas remaining at abandonment.
Performance predictions are based on geological, engineering and economic factors. The background study dealt with explicit correlations between:
recovery factor, abandonment pressure or related expressions and pool depth, pay thickness, initial reservoir pressure, flow capacity and size of gas accumulation; and
equivalent residual gas saturation and formation water saturation, porosity, permeability and specific surface area.
The format for several correlationspresented is predetermined by theoretical considerations. Existing short-cutmethods are also evaluated. Easy-to-use approximations readily yield reliableestimations despite their simplicity and can be of considerable significance;however, the practicing engineer is urged to recognize and understand their limitations.
An extensive review of methods of determining recovery and an analysis ofmany evaluations by industry form the foundation for the recommended guidelinesto serve the engineer. The methods of determining recovery factors areconsidered according to their practicality; i.e. the availability ofdata and the complexity of implementation. Useful relationships are classified, important steps of the proposed procedures are outlined and severalapplications are illustrated.
