Measurement of Inflow Performance Parameters for Coal Seam and Devonian Shale Gas Reservoirs and Its Implication in Numerical Reservoir Simulation

Abstract

SPE Member Abstract This paper describes potential inconsistencies between the traditional methods used to estimate inflow performance parameters for conventional gas reservoirs and the way these parameters are used in analysing coal seam and Devonian shale gas reservoirs, particularly in numerical reservoir studies. These inconsistencies arise because of the unique reservoir characteristics and mechanisms associated with unconventional gas reservoirs. For unconventional gas reservoirs undergoing non-equilibrium (dual-porosity) desorption, inflow performance parameters measured from deliverability tests may not be appropriate for numerical reservoir simulation. This is because the static pressures used in deliverability testing are influenced by the effects of pressure depletion in the fractures (during flowing conditions) and blow-down from the rock matrix (during shut-in). Under flowing conditions, the inflow performance in numerical reservoir simulators is governed solely by the cleat (fracture) pressure of the grid block. Consequently, if a non-equilibrium desorption formulation is used in the simulation study, then the use of inflow performance parameters derived by deliverability testing may yield erroneous results. The inconsistencies and analysis techniques discussed in this paper are applicable for all reservoir studies involving the inflow performance from unconventional gas reservoirs (analytical or numerical), however, they are more critical for numerical studies. These techniques are believed to be useful for understanding the need for representative inflow performance parameters for unconventional gas reservoirs. Introduction The inflow performance parameters of wells in conventional gas reservoirs are generally derived from two sources, transient pressure data and stabilized deliverability data. Traditionally, unconventional gas reservoirs, particularly coal seams, have been evaluated early in the field life when water saturations are high (in many cases Sw = 100) using transient tests. This is because any analysis technique involving water injection can be treated as single-phase flow. Consequently, slug tests and injectivity/falloff tests are generally used to evaluate wells at virgin conditions. P. 233^