Modern Chemical Enhanced Oil Recovery

Theory and Practice

James J.Sheng

• Publisher: Gulf Professional Publishing
• Year: 2010
• ISBN:
  9780080961637
• Topic: Technology & Engineering
• Subtopic: Fossil Fuels
• Index: Technology & Engineering

Crude oil development and production in U.S. oil reservoirs can include up to three distinct phases: primary, secondary, and tertiary (or enhanced) recovery. During primary recovery, the natural pressure of the reservoir or gravity drive oil into the wellbore, combined with artificial lift techniques (such as pumps) which bring the oil to the surface. But only about 10 percent of a reservoir's original oil in place is typically produced during primary recovery. Secondary recovery techniques to the field's productive life generally by injecting water or gas to displace oil and drive it to a production wellbore, resulting in the recovery of 20 to 40 percent of the original oil in place.

In the past two decades, major oil companies and research organizations have conducted extensive theoretical and laboratory EOR (enhanced oil recovery) researches, to include validating pilot and field trials relevant to much needed domestic commercial application, while western countries had terminated such endeavours almost completely due to low oil prices. In recent years, oil demand has soared and now these operations have become more desirable. This book is about the recent developments in the area as well as the technology for enhancing oil recovery. The book provides important case studies related to over one hundred EOR pilot and field applications in a variety of oil fields. These case studies focus on practical problems, underlying theoretical and modelling methods, operational parameters (e.g., injected chemical concentration, slug sizes, flooding schemes and well spacing), solutions and sensitivity studies, and performance optimization strategies. The book strikes an ideal balance between theory and practice, and would be invaluable to academicians and oil company practitioners alike.

• Updated chemical EOR fundamentals providing clear picture of fundamental concepts
• Practical cases with problems and solutions providing practical analogues and experiences
• Actual data regarding ranges of operation parameters providing initial design parameters
• Step-by-step calculation examples providing practical engineers with convenient procedures

Chapter 1 Introduction

1.1 Enhanced Oil Recovery’s Potential

Today fossil fuels supply more than 85% of the world’s energy. Currently, we are producing roughly 87 million barrels per day—32 billion barrels per year in the world. That means every year the industry has to find twice the remaining volume of oil in the North Sea just to meet the target to replace the depleted reserves. Of the 32 billion barrels produced each year, almost 22 billion come out of sandstone reservoirs. The reserves and production ratios in sandstone fields have around 20 years of production time left. The proven and probable reserves in carbonate fields have around 80 years of production time left (Montaron, 2008). With global energy demand and consumption forecast to grow rapidly during the next 20 years, a more realistic solution to meet this need lies in sustaining production from existing fields for several reasons:

• • The industry cannot guarantee new discoveries.
• • New discoveries are most likely to lie in offshore, deep offshore, or difficult-to-produce areas.
• • Producing unconventional resources would be more expensive than producing from existing brown fields by enhanced oil recovery (EOR) methods.

Figure 1.1 shows the US oil volume distribution in 1993 (Green and Willhite, 1998). The total oil discovered up to 1993 was 536 billion barrels, with the total produced being 162 billion barrels (30% of the total discovered) and the reserves being 23 billion barrels (4% of the total discovered). This is the number that could be produced economically using conventional methods. The remaining oil in the reservoirs was 351 billion barrels, or 66% of the total discovered. If EOR can recover half of the remaining (i.e., 176 billion barrels), then we could double the currently projected recoverable reserves. Similarly, we could have additional reserves of 2 trillion barrels worldwide.

FIGURE 1.1 US oil volume distribution in 1993.

1.2 Definitions of EOR and IOR

Depending on the producing life of a reservoir, oil recovery can be defined in three phases: primary, secondary, and tertiary. Primary recovery is recovery by natural drive energy initially available in the reservoir. It does not require injection of any external fluids or heat as a driving energy. The natural energy sources include rock and fluid expansion, solution gas, water influx, gas cap, and gravity drainage. Secondary recovery is recovery by injection of external fluids, such as water and/or gas, mainly for the purpose of pressure maintenance and volumetric sweep efficiency. Tertiary recovery refers to the recovery after secondary recovery. It is characterized by injection of special fluids such as chemicals, miscible gases, and/or the injection of thermal energy.

Enhanced oil recovery is oil recovery by injection of gases or chemicals and/or thermal energy into the reservoir. It is not restricted to a particular phase, as defined previously, in the producing life of the reservoir. Another term, improved oil recovery (IOR), is also used in the petroleum industry. The terms EOR and IOR have been used loosely and interchangeably at times. Some feel that the two terms are synonymous; others feel that IOR covers just about anything, including infill drilling and reservoir characterization.

Workable definitions of EOR and IOR are necessary not just for improved communication, but also to recoverable reserves booking, contract negotiations, government incentives, taxation, and regulatory authorities when looking at fiscal issues (Stosur et al., 2003). The following sections summarize the existing definitions used in the petroleum industry and then propose this book’s definitions of EOR and IOR.

1.2.1 Existing Definitions

Apparently, it has been agreed among petroleum professionals that IOR is a general term that implies improving oil recovery by any means; EOR is more specific in concept and can be considered a subset of IOR. According to Taber et al. (1997a), EOR simply means that something other than plain water or brine is injected into the oil reservoir, whereas IOR is a term used more broadly. According to Green and Willhite (1998), the term EOR is used to replace tertiary recovery because the chronological term does not describe some actual operation such as thermal recovery in a viscous oil reservoir. In this case, thermal recovery might be the only way to be able to recover significant oil. EOR results principally from the injection of gases and chemicals and/or the use of thermal energy. IOR includes EOR but also encompasses a broader range of activities—for example, reservoir characterization, improved reservoir management, infill drilling, horizontal well drilling, and sweep efficiency improvement. Selamat et al. (2008) included workover, step-out drilling, and infill drilling into IOR. Jørgenvåg and Sagli (2008) included any activity into IOR programs that may improve oil rate and recovery, whereas EOR refers to reservoir processes that recover oil not produced by secondary processes (Stosur et al., 2003).

High-pressure nitrogen injection is considered an EOR process (Manrique et al., 2007). Some authors (e.g., Moritis, 2000) classify immiscible gas injection as EOR too. In those cases, processes other than pressure maintenance are involved, and the processes result in more oil recovered.

Thomas (2008) defined EOR as a process to reduce oil saturation below the residual oil saturation (S_or). Recovery of oils retained due to capillary forces (after waterflooding in light oil reservoirs) and oils that are immobile or nearly immobile due to high viscosity (heavy oils and tar sands) can be achieved only by lowering the oil saturation below S_or. Such a case needs a definition of residual oil saturation different from the conventional one.

1.2.2 Proposed Definitions

The terms EOR and IOR should refer to reservoir processes. Any practices that are independent of the recovery process itself should not be grouped into either EOR or IOR. Such practices include reservoir characterization, reservoir simulation, use of hardware and equipment (pumps, down-hole separators, etc.), use of special well types (horizontal wells, multilaterals, smart wells, etc.), improved reservoir management, infill drilling, and so on. Oil here means hydrocarbon, including oil and natural gas.

Improved oil recovery refers to any reservoir process to improve oil recovery. Virtually, this term comprises all but the primary processes (Stosur et al., 2003). The following is an incomplete list:

• • EOR processes
• • Near wellbore conformance control (cement plug/gel treatment for water and gas shutoff)
• • Immiscible gas injection (dry gas, CO₂, nitrogen, alternating or co-injection with water)
• • Water injection, cyclic water injection
• • Well stimulation (acidizing and fracturing)

Enhanced oil recovery refers to any reservoir process to change the existing rock/oil/brine interactions in the reservoir. Here is an incomplete list:

• • Thermal recovery: in situ combustion—forward: dry, wet, ...