A simple spreadsheet model has been developed to estimate Original Gas In Place (OGIP), layer productivity and recoverable reserves for wells with commingled production, completed in multi-layered tight gas reservoirs. Differentiating the productivity between multiple layers of contrasting permeability is old technology. This model, however, replicates the observed material balance trend while also honouring total well production data by varying layer properties. The P/Z trend of the higher permeability layers and lower permeability layers is mapped to "envelope" the productivity index (PI) weighted P/Z curve that is used to match historical data.

This technique has been made applicable to the multi-layered reservoir environment by grouping the various kh terms, from all "high permeability" layers, into one model layer and all "low permeability" kh values into the "tighter" model layer. Published literature1 has already shown that the generation of the layer P/Z curves is applicable for reservoirs with permeability in the range of 0.1 to 10 md. The model has been successfully applied to match and predict the productivity for various wells in Cooper Basin fields, with permeability in this range, and P/Z plots that exhibit curvature. Case studies show that any change in bottom hole pressure conditions (eg. compression) or skin (eg. stimulation) can also be accounted for in the model. Various simulation models have been generated to confirm this technique's applicability to wells in Australia's Cooper Basin, and to establish the PI weighting method.

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