Factors That Control Condensate Production From Shales: Surrogate Reservoir Models and Uncertainty Analysis

Abstract

SummaryRapid development of shales for the production of oils and condensates may not be permitting adequate analysis of the important factors governing recovery. Understanding the performance of shales or tight oil reservoirs producing condensates requires numerically extensive compositional simulations. The purpose of this study is to identify important factors that control production of condensates from low-permeability plays and to develop analytical “surrogate” models suitable for Monte Carlo analysis. In this study, the surrogate reservoir models were second-order response surfaces functionally dependent on the nine main factors that most affect condensate recovery in ultralow-permeability reservoirs. The models were developed by regressing the results of experimentally designed compositional simulations. The Box-Behnken (Box and Behnken 1960) technique, a partial-factorial method, was used for design of these experiments or simulations. The main factors that controlled condensate recovery from ultralow-permeability reservoirs were reservoir permeability, rock compressibility, initial condensate/gas ratio (CGR), initial reservoir pressure, and fracture spacing. Another main outcome of this paper was the generation of probability-density functions, and P10, P50, and P90 values for condensate recovery on the basis of the uncertainty in input parameters. The condensate-recovery P50 for rate-based outcome of a 5-B/D per fracture was found to be less than 10%.