Matrix shrinkage in coal-seam gas reservoirs as a result of production, and the resultant impact on absolute permeability, is a current area of intense interest. A number of publications have documented the existence of this phenomenon. The exponential growth behaviour of coal seam permeability with reservoir pressure depletion has been observed previously at the Fairway wells in the San Juan basin. More recently, an exponential trend has also been confirmed for a group of 10 wells in a region northeast of Fairway. An increase in the absolute permeability of coal seam is a result of matrix shrinkage caused by gas desorption, which becomes a dominant factor on cleat permeability over the effective horizontal stress change during production. Bottom hole pressure recovery during shut-in periods can provide valuable data concerning the impact of coal seam gas production on reservoir permeability. Arrow Energy's historical production data, from the MGP field in the Bowen Basin, contains over 60 instances of data on pressure recovery during shut-in. These Time lapse Build-up periods have been analysed using the pseudo-pressure method (Kamal and Six, 1991) in an effort to gain an understanding of the extent and effect of this phenomenon within the company's existing assets. The method is based on a pseudo pressure function that uses the adsorption isotherm and the relative permeability relations of the reservoir. The infinite acting radial flow regime was identified and the gradient was recorded on a semi log plot of the pseudo pressure versus dimensionless time. Results from these tests show multi fold increase in absolute permeability as a function of the estimated reservoir pressure.
The advancement in Production Data Analysis (PDA) techniques allows the determination of useful reservoir properties such as permeability, drainage radius and original gas in place (OGIP). Recent publications (Clarkson 8 et al 2007; Clarkson 9 et al 2008) demonstrate the modification of production type-curves and material balance methods for Coalbed Methane (CBM) reservoirs.One of Arrow Energy's projects is located in the Surat basin in Queensland. The coals intersected in the Surat Basin are members of the Walloon Coal Measures (Walloon Subgroup) which are divided into the Juandah Coal Measure and the Taroom Coal Measure. The Walloon Coal Measure architecture is an inter-bedded coal, siltstone and sandstone sequence. The coals are generally thin with permeability ranging from 5-5000 mD. The Walloon subgroup coals are classified as high volatile bituminous. The Surat Basin has been developed using vertical and deviated wells penetrating the whole sequence. The Surat Basin coal reservoirs can be undersaturated or saturated. In the undersaturated case, single phase flow of water occurs during the early dewatering period. In the saturated case, two-phase flow of gas and water occurs at initial production.Arrow Energy has produced from the Surat Basin since 2006. The large amount of production data serves as a valuable source for PDA. Results indicate that the flow regime is dominated by linear flow during the transient period followed by boundary-dominated flow. A flowing material balance method is used to validate reservoir parameters from well simulation history matching and type-curves methods. The reservoir parameters such as permeability, drainage radius and OGIP obtained from all the methods are compared and quality checked. The decline curve analysis (DCA) method is performed on wells which have entered boundary-dominated flow to obtain the estimated ultimate recovery (EUR) and recovery factor.The PDA study is carried out for approximately 250 existing production wells in the Surat Basin. The reservoir parameters obtained from the various methods agree with one another, indicating reliable results. The permeability values obtained from this study are within the range of the Drill Stem Test (DST) results in the Surat Basin.
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