Experimental Program Investigates Miscible CO₂ WAG Injection in Carbonate Reservoirs

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 188471, “Experimental Data Acquisition and Modeling for the Study of Miscible CO₂ WAG in Carbonate Reservoirs Under Oil-Wet Conditions,” by T.  Joubert, S. Duchenne, M. Bourgeois, SPE, R. de Loubens, SPE, and M.  Petitfrere, Total, prepared for the 2017 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 13–16 November. The paper has not been peer reviewed.

Prediction of miscible water-alternating-gas (WAG) injection performance relies on proper calibration of thermodynamical and petrophysical models. Swelling, miscibility, and stripping phenomena must be captured in the equation of state (EOS), and the oscillations of gas and water saturations require using history-dependent relative permeabilities. This paper provides a robust methodology for miscible CO₂ WAG experimental-data acquisition and history matching.

Introduction

Miscible CO₂ injection in oil reservoirs leads to low residual oil saturations in the swept areas. However, macroscopic sweeping can be poor because of the high mobility of CO₂. One way of improving macroscopic sweep is to inject CO₂ in the presence of mobile water to reduce its mobility (in tertiary or WAG modes).

The prediction of WAG efficiency and the sizing of surface installations rely partly on the ability of the three-phase relative-permeability model to calculate proper mobility for each phase in any part of the reservoir. Numerous three-phase relative-permeability models have been proposed in the literature. Among these models, one proposed by Larsen and Skauge (1998) is considered by the authors as a starting point for simulation work. The experimental program discussed in this paper was launched in order to clarify several points, including the existence and the amplitude of residual oil saturation in miscible flooding and the validity of existing three-phase models.

Experimental-Program Definition for WAG Study

The experimental program is composed of several coreflooding experiments performed at reservoir conditions of 260 bar and 94°C. Materials and methods are discussed in detail in the complete paper.

Flow Regions and Saturation Paths. WAG mechanisms involve alternating two fluids of different density and mobility properties at the injection well. These differences lead to gas migration toward the top of the reservoir. The consequence is a large variability of flow sequences in space as illustrated in Fig. 1, which shows a cross section between two vertical wells from a WAG phenomenological simulation. Several saturations paths were extracted to identify six flow regions that can be grouped into three categories:

Two-phase flow with monotonous saturations variations (Flow Regions 1 and 2)

Two-phase flow with oscillating saturations (Flow Regions 3 and 4)

Three-phase flow with oscillating saturations (Flow Regions 5 and 6)

Depending on reservoir properties and the development plan, the proportions between these flow regions can vary significantly and should be assessed for a given reservoir before the experimental-program definition. A reliable three-phase relative-permeability model should be valid over each flow region, or at least over the most significant one for a given case.