Industry Struggles With Enhanced Recovery for Unconventional Reservoirs
Authors: Feder Judy 
Published: September 2019

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper OTC 28973, “Recent Advances in Enhanced Oil Recovery Technologies for Unconventional Oil Reservoirs,” by S. Balasubramanian, SPE, P. Chen, SPE, S. Bose, A. Alzahabi, and G.C. Thakur, SPE, University of Houston, prepared for the 2018 Offshore Technology Conference, Houston, 30 April–3 May 2018. The paper has not been peer reviewed. Copyright 2018 Offshore Technology Conference. Reproduced by permission.
The US Energy Information Administration estimates that most of the growth in US crude oil production is expected to come from tight rock formations within the Permian region in Texas, which is expected to contribute nearly 30% of total US crude oil production in the future. The primary recovery from unconventional oil reservoirs is predicted to be less than 10% and ranges anywhere from 2 to 8% for the various shale plays throughout the United States. Exploiting the vast potential•of unconventional reservoirs and increasing the recovery factors beyond primary depletion by implementing improved- and enhanced-oil-recovery (IOR/EOR) methods is imperative. This paper reviews the IOR/EOR technologies currently being applied to unconventional oil reservoirs.
Introduction
Individual well production has a steep decline before leveling off at a low rate. Infill drilling has been used to achieve short-term increases in production. Drilling and completing new wells with long laterals is not always economically viable. In this context, EOR in unconventional oil reservoirs has gained significant attention and is the motivation of proposed research.
To date, understanding EOR in unconventional plays is in its embryonic stage because of poor understanding of the geological constraints on unconventional reservoir performance. Definitions of key geological parameters that influence primary recovery are understood to some extent. For EOR, the industry is still chasing geological characteristics similar to those that make primary recovery successful (e.g., fractures and brittleness). These parameters may be different for EOR. As the mechanism of EOR becomes better understood, geological parameters that define the sweet spot for a secondary recovery process will need to be established simultaneously. An integrated field laboratory study and practical results will help advance knowledge.
This paper is based on a thorough review of the pertinent published literature on IOR/EOR. Results of EOR application to unconventionals shared by various operators in their investor presentations and press reports were also analyzed. The IOR/EOR studies were classified into laboratory experiments, numerical modeling, and field laboratory trials (pilots). Additionally, the field trials were analyzed on the basis of the representative shale plays.
Most of the studies performed for the application of EOR technologies to unconventional oil reservoirs have been limited to experimental investigations and numerical simulation studies. The research revealed that miscible-gas injection (e.g., produced gases, CO2) is the most promising method among EOR techniques, which include miscible-gas injection, waterflooding, surfactant injection, chemical methods, and polymer injection. Experimental studies showed that CO2 injection had the highest potential for improving recovery in unconventionals, followed by produced-gas injection. The studies also showed that diffusion was the predominant mechanism. Surfactant injection showed the next-best potential to increase oil recovery by altering the wettability of rock in laboratory experiments. The gas-injection pilots showed that sufficient injectivity was achieved mainly because of the injection-induced fractures, and did not reveal any significant effect of diffusion. Conformance control remains a major challenge, especially because of the channeling of the gas through the fractures.