Inflow Performance Relationship correlation of 2 Phase CBM Reservoir

AbstractThe inflow performance relationship (IPR) for a coalbed methane (CBM) reservoir is relatively complex as the production behaviour is different from conventional gas wells, as gas in coal is stored by means of adsorption. Most often coal reservoirs are relatively under saturated, meaning the pressure in the reservoir needs to be substantially depleted prior to gas desorption. Gas from coal is produced at relatively low reservoir pressures, thus requiring the pseudo pressure function to be included in any deliverability calculations. A two phase flow system in a coal reservoir adds to that complexity as relative permeability, reservoir geo-mechanics and saturation needs to be taken into account within the IPR correlation. The IPR correlation for coalbed methane wells is an important part of any well performance optimisation and nodal analysis.A simplistic IPR correlation for multi-phase coalbed methane wells was generated based on the history matching of field production data. The correlation has been tested both for a multi seam completion in the Surat Basin and for horizontal wells in low permeability coals within the Bowen Basin. Furthermore the empirical CBM IPR correlation for single and multi-phase reservoir was compared. A sorption isotherm was used to limit the maximum recovery and generate recovery efficiency based on the IPR correlation.One of the potential risks to a CBM development is the likely loss in recovery efficiency due to higher backpressures on wells as a result of suboptimal gathering network sizing due to a chosen surface concept. This risk needed to be further evaluated and addressed for potential outcomes when the field starts production. The study was initiated by means of using the CBM field IPR's to evaluate this risk in more detail, identify mitigation measures and capture any associated opportunities that may be available.