Using Pressure Transient Analysis to Validate the Initial Hydrocarbon Composition and Improve Hydrocarbon Production Forecasting, Shown at an Example Rich Gas Condensate Field

Abstract

In addition to the challenges of production forecasting of gas fields, gas condensate production forecasting needs to address effects related to condensate drop out in the reservoir. To forecast hydrocarbon production from gas condensate fields, Drill Stem Tests (DST) and fluid sampling are required. For the example reservoir, an Equation Of State (EOS) model was generated using laboratory data. DSTs were performed and analysed for reservoir permeability, liquid drop out and velocity stripping near-wellbore. In addition to pressure transient analysis, numerical simulations were performed using the EOS. The simulations indicated that the initial fluid composition determined from fluid sampling leads to a good match of the drawdown and build-up pressures, however, the pressure derivative could not be matched satisfactory for a reasonable set of relative permeabilities, velocity stripping effects and reservoir permeabilities. Using the initial fluid composition as additional matching parameter lead to an acceptable match of the pressure derivative as well. This indicates that despite careful fluid sampling, the composition of the sampled fluid was not representative for the initial reservoir fluid. In addition, simulating the well test revealed that the local equilibrium assumption in the numerical simulation might not always hold in the reservoir. Gas and condensate production forecasts using the initial fluid composition of the sampling and the modified initial fluid composition after history matching were performed. The results indicate that for the example reservoir, the sampling derived initial fluid composition would overestimate the cumulative gas production by 4 %, underestimate condensate production by 9 %, overestimate condensate recovery factor by 17 %, overestimate plateau gas production duration by a factor of 9 and underestimate condensate production rate by 35 %. These differences might lead to different economics and development strategies (e.g. depletion versus gas recycling).