Modeling of EOR in Shale Reservoirs Stimulated by Cyclic Gas Injection
Authors: Wilson Adam 
Published: February 2015

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 169069, ’Compositional Modeling of EOR Process in Shale Oil Reservoirs Stimulated by Cyclic Gas Injection,’ by Tao Wan, Xingbang Meng, James J. Sheng, SPE, and Marshall Watson, SPE, Texas Tech University, prepared for the 2014 SPE Improved Oil Recovery Symposium, Tulsa, 12-16 April. The paper has not been peer reviewed.

Cyclic gas injection combined with modern technologies such as horizontal drilling and hydraulic fracturing has achieved promising results in low-permeability formations. This paper focuses on the effects of injected-gas composition and fracture properties on oil recovery, concluding that cyclic gas injection in hydraulically fractured shale oil reservoirs is feasible for improving oil production substantially over primary production.

Introduction

Cyclic gas injection is an effective and quick-responding enhanced-oil-recovery (EOR) method in intensely naturally fractured or hydraulically fractured reservoirs. The short payout is a characteristic that should attract industry’s interest in investing in these types of projects. Recent studies showed that cyclic gas injection could be a viable method to improve the oil recovery in shale oil reservoirs, but those studies were performed on the basis of black-oil models without detailed analysis of the phase behavior of the reservoir-oil/injected-gas system. The reliability of using the blackoil model to simulate the cycling of gas/ condensate reservoirs is questionable because the two-component description may not be able to represent the active compositional phenomena adequately. This paper describes the use of an equation-of-state (EOS) approach to model the phase behavior of the oil and injected-solvent system.

Description of Models

The reservoir-fluid-composition data are from the published data of the fifth SPE comparative solution project, which aimed to illustrate the comparison of results between a four-component black-oil miscible-flood simulator and a fully compositional reservoir-simulation model.

Minimum-Miscibility-Pressure (MMP) Determination

Local displacement efficiency by gas or solvent injection is closely dependent on the MMP. Most gas- or solvent-injection cases operate in a regime in which true miscibility is not achieved; however, high recoveries still could be attained. The analytical method for MMP determination focused on finding the key crossover tie-lines for a dispersion- free displacement when one of the key tie lines becomes a critical-tie line (a tie-line of zero length). This paper shows the use of a cell-to-cell simulation method to determine the MMP of the given solvent composition and reservoir fluids. The pseudoternary diagram is generated from the calculations to study the vaporization or extraction process and interprets MMP as 3,440 psi. The multiple-contact miscibility occurs at approximately 3,400 psi, and a very small increase in recovery is observed at higher pressures. A slimtube simulation model also was developed that has good agreement with the MMP calculated by the tie-line method.