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.
Produced-gas-injection pilots in the Eagle Ford formation have demonstrated the greatest success in increasing oil recovery. However, many inconsistencies between laboratory investigations and field trials need reconciliation. Further research is necessary to bridge the gap and improve the scaling from laboratory to field.
The study on which the complete paper is based elicits the learnings and challenges from the application of different IOR/EOR technologies to unconventionals at various scales (micro to macro to field scale). In addition, ideas for future research to improve the understanding of the complex mechanisms of EOR in unconventional oil reservoirs are presented. These include optimizing gas-injection schemes (huff ’n’ puff, continuous injection) on the basis of key parameters such as permeability and investigating fracture placement for improving drainage area and interwell communication.
Current Status of EOR in Unconventional Reservoirs
EOR methods in unconventional reservoirs are quite different from those in conventional reservoirs because of the complex, poor-quality properties of the plays. Most of the studies have been limited to experimental work, mathematical approaches, and numeric simulation. A few pilots have been conducted in North Dakota and in Montana in the Bakken formation. A wide range of conclusions has been presented by researchers on the applicability and understanding of EOR to unconventional oil reservoirs. The ultratight matrix and high conductivity of the natural fractures might be the two most important factors that impair success of conventional EOR methods.
EOR Field Trials
Most of the field trials or pilots have been performed in the Bakken and Eagle Ford formations. Field trials for EOR have been limited in the Permian Basin Wolfcamp formation. The field trials in the Bakken are distributed between Canada and US.
Challenges
- Most of the investigations for enhanced oil recovery in unconventional reservoirs have been limited to experimental studies and numerical simulation.
- Wide gaps exist between the microscale (laboratory scale) and macroscale (field trials) studies for different EOR methods in unconventional reservoirs.
- CO2 EOR ranked as the most suitable method over natural gas in the laboratory studies, but field trials showed more success with natural gas or enriched natural gas.
- Experimental studies concluded that CO2 diffusion was the mechanism for improved recovery, whereas the pilot tests did not show a clear indication on the field scale.
- The earlier studies concluded that injecting miscible gas by huff ’n’ puff might be more beneficial than continuous injection for the following reasons:
- Ultratight permeability might prevent a continuous flooding process.
- Diffusion was assumed to be a more-dominant mechanism.
- Conformance problems reported in pilot tests have not been well understood.
- Whether a high intensity of natural fractures played a role in the conformance problems was not clear, although this was not observed in the Canadian Bakken.
- Conducting studies to integrate all tools of laboratory, simulation, and pilot tests is essential to advance the understanding of the applicability of each IOR method to unconventional reservoirs.
- Identification of sweet spots requires taking into account various rock and fluid properties.
- Improved understanding of the petrophysical, geological, and geochemical conditions of the reservoir rock helps in designing the best development strategy of these shale resources.
- Horizontal wells with multistage fractures are used for unconventional development. The effective number of wells and fracture stages is not well-understood.
- The current industry practice is to complete the wells with equally spaced fractures. This may result in short transverse or axial fractures that are problematic during hydraulic fracturing because of stress shadowing.
- Horizontal wells are currently placed in an equally spaced fashion, which leads to variations in total production per well. Developing a standard by which to place fractures and wells has always been challenging.
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.