This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 201609, “Cellulose Nanocrystal Switchable Gel for Improving CO2 Sweep Efficiency in Enhanced Oil Recovery and Gas Storage,” by Ali Telmadarreie, University of Calgary and Cnergreen; Christopher Johnsen, University of Calgary; and Steven Bryant, University of Calgary and Cnergreen, prepared for the 2020 SPE Annual Technical Conference and Exhibition, originally scheduled to be held in Denver, 5–7 October. The paper has not been peer reviewed.

The entanglement of biopolymers is a well-known phenomenon that, when controlled, can result in a smart fluid with strong gelation properties. The authors write that, when a suitable salt is incorporated into the cellulose nanocrystal (CNC), the fluids undergo gelation upon contact with bulk-phase carbon dioxide (CO2) but remain a flowing liquid otherwise. In this study, this composition-selective trigger was applied to improve sweep efficiency in CO2 enhanced oil recovery (EOR) and sequestration.

Introduction

Hydrogels are hydrophilic structures that swell when hydrated and have various applications in industry. Hydrogels are of interest in EOR because of their ability to respond to stimuli such as pH, temperature, light, and ionic strength. CNCs are nanoparticles derived from cellulose, one of the more sustainable natural resources available.

CNC hydrogels could have specific applications as a solution to media het-erogeneity and poor gas-sweep efficiency. The hydrogels can be tuned to set over time, allowing the intentional placement of gels into already-swept areas of a reservoir. CNC hydrogels are unique in that they can be formed when contacted with CO2 and broken by the application of nitrogen (N2) gas. The pH of the solution will be increased as the nitrogen partitions across the gel, reversing the CO2 reaction. This gives the gel-forming solution the added benefit of being transmittable throughout a reservoir.

Material and Procedure

Spray-dried CNCs with an average length of 100–200 nm and a width of 15 nm were used. Imidazole was used as the salt mixed with water and CNC suspension to create a pH-triggered gel system. CO2 gas and N2 gas were used as received. Mineral oil with a viscosity of approximately 20 cp was used at the oil phase. Solution preparation, and the process for gel strength in bulk testing, are provided in the complete paper.

All tests were performed at a pressure of 400 psi and an ambient temperature of 21°C. Two sets of flow experiments were performed. The first included flow in a single sandpack saturated with water to investigate the in-situ gelation and reversibility of the gel. The second set used a dual-sandpack system. The shorter sandpack with higher permeability was saturated with water to create a path of less resistance compared with the longer sandpack with lower permeability saturated with viscous oil. Further details of these experiments are provided in the complete paper.

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