4DGeoViz Blog

Basics of Enhanced Oil Recovery: My Trusty Reference

I recently completed a project in water flooding. The application of Enhanced Oil Recovery (EOR) is likely to become the norm in many fields in the 21st century. Some interesting questions about the mechanics of the methodology came up during the project and I was reminded about the value of a standard reference from my days as an undergrad at the University of Wyoming. Mechanics of Secondary Oil Recovery, by Charles Robert Smith in the Department of Petroleum Engineering at the University of Wyoming was first published in 1966. My copy is a reprint from 1975. Despite the date, this book covers the basics in a simple condensed style for water flooding, gas injection and an interesting chapter on the thermodynamic impacts of adding heat to the reservoir. It is important for new practitioners in this important field to get up to speed on these concepts and become knowledgeable practitioners.

Secondary oil recovery uses two or more wells to where energy is added to the reservoir system typically via injection of fluids to one well and pushing oil-bearing fluids towards producing wells. The book provides a good review of permeability concepts and Darcy’s law of fluid flow through a porous medium. There is a good basic discussion of relative permeabilities in the gas-oil-water system and the role of hysteresis in planning systems.

Water flooding is discussed in terms of various standard well-spot patterns. The five-spot pattern is a typical pattern where a field has been developed on a regular grid and an injection well is located in the center of a square area and production occurs at wells on the corners. Assuming reasonable conditions of homogeneity and isotropy in the reservoir sediments and approximately constant reservoir thickness in the vicinity of the spot pattern, an engineering analysis of the equipotential field and flow to a corner well can be developed for a quarter of the spot pattern using lines of symmetry. Additional more complex spot patterns are discussed. Today’s reservoir simulation technology allows for generalized analysis and design of customized spot patterns that can be site specific. Mechanics of Secondary Oil Recovery does not delve into more specialized cases, but provides the foundation for generalized analysis, design and planning to meet present-day requirements.

Carbon dioxide injection also is discussed in detail. Today’s needs for carbon sequestration dovetail nicely into applications for CO2 injection that provide both a receptor for CO2 in concert with enhanced oil recovery. In an historical note, the book notes that the gas injection concept dates back as early as 1864 with an actual application in 1911 using simple air injection. It is important to design the injection application properly to ensure a consistent pressure distribution that does not lead to fingering and breakthrough of the gas prematurely at the production well. Modern petrophysical analysis and simulation technology allows for sufficient analysis and planning to make this method more amenable to practical use. In addition, geochemical conditions may limit the applicability of CO2 in some subsurface environments, so it is important to have a comprehensive understanding of the reservoir chemistry prior to selecting a method.

Finally the subject of increasing the thermal content of the reservoir is discussed. The book introduces the concept with a discussion of in situ combustion in the reservoir. This method is perhaps a little outdated given today’s environmental sensibilities. Modern techniques include adding steam or other hot fluid that can be generated at the surface via solar, natural gas combustion or other more environmentally friendly method. Once injected, these fluids serve both to reduce the viscosity of oil-in-place and increase pressure via the temperature rise. Many production environments have plenty of solar energy or excess gas available to heat injection fluids. Therefore, it is an accessible technology.

The application of EOR is likely to become the norm for a significant portion of oil and gas production going forward. Having a good basic reference like Mechanics of Secondary Oil Recovery will allow the knowledgeable practitioner to implement EOR successfully.