Polymer-Assisted WAG Injection Improves CO2 Flow Properties in Porous Media
Authors: Carpenter Chris 
Published: June 2024

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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 215024, “Polymer-Assisted Water-Alternating-Gas for Improving CO2 Flow Properties in Porous Media,” by Mohsen M. Yegane, SPE, Delft University of Technology and the Dutch Polymer Institute; Thijs van Wieren, SPE, Delft University of Technology; and Ali Fadili, Shell, et al. The paper has not been peer reviewed.

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CO2 flow in porous media is vital for both enhanced oil recovery and underground carbon storage. For improving CO2 mobility control and improved reservoir sweep efficiency, water-alternating-gas (WAG) injection often has been applied. The effectiveness of WAG diminishes, however, because of the presence of microscale reservoir heterogeneity that results in an early breakthrough of gas. In the complete paper, the authors propose polymer-assisted WAG (PA-WAG) as an alternative method to reduce gas mobility and the mobility of the aqueous phase, consequently improving the performance of WAG. In this method, high-molecular-weight water-soluble polymers are added to the water slug.

Introduction
Recently, PA-WAG has received attention as a method to mitigate early gas breakthrough and gravity segregation during WAG injection. However, the flow mechanisms in PA-WAG injection in porous media remain poorly understood. In particular, no experimental study exists to the authors’ knowledge that demonstrates in-situ visualization and discusses how PA-WAG can improve the gravity override and early gas breakthrough of WAG. The objective of this study is to demonstrate experimentally the feasibility of PA-WAG by conducting a series of X-ray computed tomography (CT) -aided coreflood experiments. To this end, coreflood experiments in Bentheimer cores using different injection schemes (CO2 and polymer injection, WAG injection, and PA-WAG injection) were conducted. The aim of CT scanning during the coreflood experiments was to map the phase saturations at different times of injection. Using dual-energy CT scanning, a reduction in gravity override could be visualized, and the CO2 breakthrough was delayed when PA-WAG was used.

Experiments
Table 1 of the complete paper presents the various chemical components that were used in this study. The coreflood experiments were performed using Bentheimer sandstone cores. Bentheimer cores have high permeabilities and a homogeneous mineralogy. The porosity of the core samples was measured using CT scanning.
To introduce the aqueous phases into the core, a dual-cylinder pump was used. The core, core holder, and heating sleeve were placed in a fixed horizontal position on the CT bench because vertical scanning led to undesirable artifacts and yielded no meaningful insights. Fraction-collector sampling was used to collect effluents at the outlet at various time intervals. CO2 was injected into the system by a mass-flow controller sourced from a dedicated CO2 supply. The pump indirectly introduced both the oleic phase during primary drainage stages and the polymer solution used for secondary or tertiary recovery by means of a transfer vessel. The pressure drop across various sections of the core was measured using four pressure transducers. Pressure and temperature measurements were recorded at 10-second intervals. Four coreflood experiments were conducted in this study. As mentioned, all experiments were performed at pressures and temperatures of 20±1 bar and 40±1°C.