Practical Application of Coalbed Methane Material Balance Technique to Produce Well by Well Production Forecast in Undersaturated Queensland Surat Basin Coals

Abstract

Use of p/Z versus cumulative produced gas plots is an industry standard practice to estimate gas initially in place (GIIP) and expected ultimate recovery (EUR) for conventional volumetric gas reservoirs that behave as a "tank". In this paper applicability of coal seam gas (CSG) material balance technique as proposed by King (ref 1) on Surat Basin undersaturated coals was investigated. A modified King's material balance technique, as well as the Jensen and Smith approach, were successfully applied to production data from undersaturated coals, producing robust dynamic GIIP and EUR estimates based on observed reservoir depletion trends. Stability of produced GIIP estimates was tested by means of sensitivity analysis and robustness of the technique demonstrated by comparison with results of numerical reservoir simulation. Finally, reservoir tank models were linked with the well inflow relationship to produce well by well production forecasts. The key difference of CSG wells, compared to conventional gas wells is that well productivity considerably changes during the life of a well. This gas productivity increase phenomenon was captured in the forecast by means of well PI versus recovery factor relationship. Thus, a practical CSG well by well production forecasting approach is proposed.