Flood Management: Solving Conformance or Sweep Efficiency Problems: Part 1
In Part 1 of a series of articles, the author highlights candidate selection and problem clarification. This section describes how the success rate of solving conformance problems is improved by reducing assumptions and improving your problem understanding prior to executing a solution.
Since the advent of secondary recovery mechanisms, waterflooding or gas displacement, the oil industry has suffered from a variety of problems that yield inefficient displacement of hydrocarbon fluids during flooding. The industry has also suffered from a very low success rate at correcting these poor sweep efficiency problems. The first three articles in this series are designed to enhance our ability to identify, characterize, solve, and evaluate performance of solutions applied to conformance or sweep efficiency problems. This process structure will allow us to discuss key elements that should generate a higher success rate in solving conformance problems. Part 1 of this series focuses on candidate selection and problem clarification or problem understanding which are the first two elements of Fig 1.
The process wheel shown in Fig. 1 is used to identify the steps most production engineering companies take as they work through their conformance or sweep efficiency problems. This provides a basis by which we can discuss the various elements of solving these problems. The companies I have worked for chose to call this “conformance engineering.” Let’s define conformance engineering as: The process of utilizing reservoir and wellbore information to understand the flood performance and then use that understanding to adjust aspects of the flood which results in improved oil recovery.
That is the working definition this series of articles will use for conformance engineering. I think we can all agree that this includes a large variety of sweep efficiency problems. These range from deep reservoir displacement fluid control issues to near-wellbore control issues and every combination of these elements. This includes issues for controlling displacing fluids at the injector or the producer.
We can also use this to understand and characterize human-induced floods vs. natural processes. Strong water aquifer drives have the same displacement mechanisms as waterflooding. They have significant differences to elements like the pressure source, which is broad for aquifer drive and a very specific point source of pressure for pattern floods. In addition, a strong aquifer drive utilizes gravity segregation to enhance the efficiency of the drive where a pattern waterflood can suffer from gravity underrun. This is one of the reasons a peripheral waterflood can be a very effective secondary recovery process. We also consider fields where natural gas cap expansion drives oil displacement. In some fields, the gas cap is supported by gas reinjection. Prudhoe Bay is a prime example where there is no market for natural gas, so gas reinjection into the gas cap and gas cap expansion becomes a strong recovery mechanism for this field.
For the sake of this JPT article series we will consider any flood process whether natural or induced to be part of flood management basics.
In the candidate selection step, and depending on your displacing fluid, a common statement is we have a “water or gas problem.” That statement is incorrect. What we have is a symptom of the problem and that is excess water or gas. We do not know what our problem is. In fact, we don’t even know if that fluid indicates a problem. We need to change our perspective and recognize that during any flood or displacement process, a time will come when large quantities of displacing fluid will reach our production wells. Our key question is whether this high production volume of displacing fluid precedes the expected recovery efficiency for this stage in the flood process. Some industry members like to call this “good water” vs. “bad water.” However, since we want to consider all flood or displacement processes, I suggest we use the terms “efficient displacement fluid” and “inefficient displacement fluid.” Our candidate selection or identification process should focus on finding wells, or patterns where the displacement process has a lower displacement efficiency than expected. If this occurs early in the life of a flood or well, it is often very easy to identify. In situations where we are dealing with a multidecade flood, this is a more difficult task. Rapid changes in the displacing fluid production volume is a common symptom, but the specific nature of the problem is still unknown. Once the candidates are identified, our attention should then focus on other symptoms.
One thing that helps in this effort is to organize the information and knowledge into a simple structure utilizing some basic understandings about the flood. This may seem very simplistic at first but staying focused on the basics can help to avoid getting lost in the massive forest of information. Throughout the entire flood management and conformance control process it helps to maintain focus on two very basic questions:
1. How is fluid moving through the reservoir and why?
2. How does the wellbore, both past and present, interact with the reservoir?
These basic questions should be a fundamental focus for every reservoir and production engineer. These questions are stated very simply, but they contain all key principles of what we need to understand. To answer those questions effectively takes multidisciplined training, large quantities of data, massive effort, teamwork, and time.
This article focuses on the basic information elements since a comprehensive understanding of these items would take multiple books across multiple disciplines. The following sections are designed to help us compartmentalize the information we have available. Each section of information discussed below takes years of training and experience to become proficient, and many subsections of each general section have experts within them. There is still value in providing this structure so that we can discuss each section independently. This simplistic view is my preference on how to break down the information you will need to answer the two basic questions above.
Reservoir and Wellbore Characterization. The process of taking all available discovery, primary drilling, geophysical, geological, and completion information to generate a comprehensive description of both the reservoir and each individual wellbore. Note however these descriptions are not static. They continue to be enhanced or improved as additional data is provided.
Reservoir and Wellbore Monitoring. The process of utilizing all physical data that is captured over the life of the field to generate a time-sensitive understanding of the changes taking place in the reservoir and in individual wellbores.
Reservoir and Wellbore Testing. The process of identifying important physical information that is needed to gain a better understanding of physical conditions or changes that have taken place in the reservoir and/or the wellbore and executing a test to evaluate those key physical parameters. Routine or regular testing of these critical parameters are often incorporated into the monitoring program.
Reservoir and Wellbore Modeling. The process of taking some or all available data on the reservoir and/or the wellbore and incorporating that information into some mathematical, numerical, or inference model that is designed to calculate or describe or potentially predict the physical processes taking place in some portion or all of the reservoir or wellbore.
Integrating Information and the Resulting Assumptions. The sections on organizing reservoir and wellbore information are intentionally brief, because there is no way to do justice to all the information that is contained within each section and subsection. If you have had good technical training in your degree, discipline, or within your company, you have already spent considerable time learning very intricate details on much of this information.
What is not commonly discussed is what to do with the blank spots. In the process of clarifying our problems, we are often faced with missing or limited information, which forces us to make assumptions. These assumptions can be as simple as, “We have a good primary cement job,” or “The casing is only 2 years old, so there is no way we can have a casing hole,” or as encompassing as, “All core, log, seismic, and other reservoir description data indicated there are no natural faults or fractures.” In many cases, these basic simplifying assumptions can become the critical piece to formulating an ineffective solution.
The single greatest factor in determining your success at solving your flood or conformance problems is developing a comprehensive understanding of the problem you are trying to solve. Past studies on failed conformance solutions have shown that 70–80% of the times we fail, we fail because we misunderstand some aspect of the problem. This includes simple items like not checking the casing integrity or confirming no cement channel exists behind pipe. These failures are both examples of misunderstanding the problem which can prevent a mechanical plug from achieving isolation. I have often heard someone tell me that they have used a given product and that the product did not work. My experience indicates that we are the ones who do not work. Products and technology generally fail because they have been applied to the wrong problem or they are applied to an aspect of the problem they were never designed to control. Thus, when we fail to properly characterize and understand all aspects of the problem, the result is a failed application of the technology, and a failed solution.
The primary reason we fail to gain a comprehensive understanding of the problem is that it is often too difficult to evaluate all its important aspects. This difficulty and our human nature to avoid difficulty causes us to take shortcuts. These shortcuts usually result in assumptions, and those often result in simple and quick solutions that have a very high failure rate. What we must realize is that every time we assume, and that assumption is wrong, we have just lowered our potential for success. As we consider the nature of each conformance problem, I suggest that taking a careful look at all the assumptions you are making can help you to avoid many failures. For example, are you assuming good isolation behind pipe? Do you understand the extent of the flow barriers between layers deeper in the reservoir? Have those flow barriers been compromised by induced or natural fractures? Have you considered the historical aspect of all wellbore penetrations? Where are these penetrations and were they properly abandoned for isolation within the producing interval?
Generating a comprehensive understanding with a limited number of assumptions requires diligence, experience, vast quantities of information, and a willingness to spend the time generating a complete history of the problem and how it developed. In most cases this cannot be done by a single individual but requires a team of people to evaluate the many aspects of the potential problem. Elimination of assumptions is not an easy thing to do. The team evaluating these problems requires an experienced leader who will remain diligent and not accept unnecessary assumptions. This leader must know how and when to push for cost-effective answers on the critical aspects of the problem, especially those that relate to what we might consider as a potential solution.
What are two of the key critical aspects of the problem? One is the basic nature of the problem flow path. Is the problem flow path a fluid filled VSC (void space conduit)? Or is the problem flow through permeable rock? The second critical aspect of the problem is understanding where the control exists for the problem. In this case the options are near the wellbore or deep in the reservoir away from the wellbore. To help understand these two critical aspects of conformance problems, a “Conformance Problem Matrix” has been provided with a variety of conformance problem types placed onto the matrix (Fig. 2).
Not every conformance problem is presented in this chart, but if you consider these two major elements of the problem flow, we should be able to place any type of conformance problem on this matrix. Please study and consider this matrix carefully since this will be a key element of Part 2.
In Part 2, to be published in the June JPT, we will provide a more in-depth discussion on the problems and the problem matrix. We will also discuss wellbore intervention solutions and how to overlay these solution types on top of the conformance problem matrix. Additionally, we will introduce the conformance solution continuum and show its relationship to the problem matrix.
Editor's note: David Smith will be presenting "Basics of Conformance Engineering" at the SPE Worshop on Full Life Cycle Mangement of Produced Water, 23–24 May, in Galveston, Texas.
David Smith, SPE, is currently the president and principal advisor for Oilfield Conformance Consulting LLC and an adjunct professor for Missouri University of Science and Technology (MS&T). Prior to his current efforts and for approximately 20 years, Smith was the global conformance engineering advisor for either ConocoPhillips or Occidental Petroleum. Prior to that he was a project manager in conformance water management for Halliburton and held several positions within ARCO that were associated with profile modification and sweep improvement. Smith has been an active SPE member for more than 45 years. He was the technical program chairman for the 2014 SPE EOR/IOR Conference in Tulsa, a past co-chairman of the SPE EOR/IOR TIG (Technical Interest Group), and an SPE Distinguished Lecturer in 2019–2020. Smith holds a bachelor’s degree in geology from Pacific Lutheran University and an MS in petroleum engineering from Stanford University.