Production data analysis using type curves in tight gas condensate reservoirs – Impact of pressure-dependent permeability

Pressure-dependent matrix properties are known to be particularly important in tight reservoirs. A significant amount of literature examine the impact of such effects on pressure transient analysis, and often under single-phase conditions; very few consider similar effects on decline type curve analysis of long-term production data, and under multiphase conditions. This paper examines production decline in tight gas and gas condensate reservoirs in the presence of pressure-dependent permeability (p-k) and porosity (p-ϕ), highlighting the impact of such effects on the decline behaviour and on the quality of reservoir and well productivity parameter estimates obtained using decline type curves, under both single-phase and multiphase conditions. Production data was generated using one-dimensional radial homogeneous reservoir simulation models with permeabilities of 0.1 md and 1 md, and with p-k and p-ϕ effects implemented using drawdown-dependent transmissibility and pore volume multipliers. P-k relations with concave-upward exponential behavior and a less common concave-downward exponential behaviour were considered. The data were first analysed using conventional (i.e. constant property) modern decline type curve analysis techniques, and then using pseudovariables modified to account for p-k effects (and material balance pseudotime integrals modified here for p-ϕ effects for the first time). Single-phase cases were first considered, and then the simultaneous impact of multiphase and stress-dependent matrix effects was examined using gas condensate reservoir models operated at pressures lower than dewpoint pressure, the analysis of which relied on our earlier-proposed equivalent-phase methods. The study shows that strong p-k effects result in reductions in permeability estimates and underestimations in drainage area obtained using conventional decline type curve analysis - both observations being consistent with existing literature. In addition, the results reveal how high degrees of pk effects can produce responses in the production decline data plot, during boundary dominated flow, which mimic those associated with pressure support. The results also demonstrate that the use of pseudovariables modified to account for p-k/p-ϕ effects, effectively linearize the flow equations, allowing for accurate parameter estimation using conventional modern decline type curves in stress-sensitive tight gas and gas condensate reservoirs.