Production Performance Evaluation of Wells Completed in Unconventional Reservoirs Using Capillary Pressure Data and Relative Permeability Effects

Abstract Techniques that have been developed and used for evaluation of the production performance of horizontal wells completed in low-permeability reservoirs that are intersected by multiple transverse fractures are presented in this paper. Specifically, the effects of capillary pressure and relative permeabilities are included in the production analyses in a practical way using appropriately defined multiphase pseudopressure and pseudotime integral transformations and laboratory-determined capillary pressure measurements of low-permeability formation cores. The principal type of low-permeability, unconventional resources that have been of greatest interest for exploration and development in North America recently are liquids-rich unconventional resources, due primarily to economic considerations. The principal focus of the analysis techniques reported in this paper therefore pertain primarily to liquids-rich unconventional resources, but the same general multiphase analysis approach may also be utilized in unconventional gas reservoir analyses as well. Multiphase expressions have been developed and used in this study for the pseudopressure and pseudotime integral transformations in order to effectively linearize the multiphase diffusivity equations governing the flow of multiple fluids in the reservoir. The multiphase analysis examples reported in this paper utilize Mercury Injection Capillary Pressure (MICP) data for computation of multiphase flow relative permeability relationships. This type of capillary pressure measurement is likely only directly applicable for low (microDarcy) or higher permeability formations, as MICP measurements may tend to destroy the pore structure of extremely low-permeability (nanoDarcy) cores due to very high injection pressures. In extremely low-permeability formations, other capillary pressure measurement methods may be employed with the multiphase reservoir analysis techniques reported in this paper, without any loss of overall generality. Capillary pressure measurements can be used to evaluate relative permeabilities for multiphase flow analyses. The Burdine relative permeability relationships have been used in this study. However, the multiphase analyses developed in this work are readily amenable for the use of other relative permeability correlations as well. The production analyses and examples presented in this paper demonstrate that capillary pressure and relative permeability effects may not be assumed as negligible in low-permeability, unconventional reservoir analyses in general. Omission of these important effects in unconventional reservoir analyses may lead to significant errors in the estimates of the reservoir intrinsic properties and well completion effectiveness. The multiphase production performance analysis techniques introduced in this study are demonstrated with an application to a representative example of the production performance of a liquids-rich unconventional resource. The example demonstrates the importance that capillary pressure and relative permeability effects can have on the production performance of unconventional reservoirs.