Reservoir Simulation Fundamentals-IPIMS modules
hello guys ,I thought of sharing this nice basics of reservoir simulation,If you are interested please reply otherwise I will stop.
Episode 1
What Goes Into Reservoir Simulation?
The basic tool for conducting a reservoir simulation study is a simulator. The development of this tool requires a good understanding of the physical processes occurring in reservoirs and a high level of sophistication and maturity in advanced mathematics and computer programming. Although simulation engineers generally do not get involved in actual software development, the effective use of reservoir simulators requires that they at least appreciate what goes into this development.
Like any tool, a reservoir simulator has its strength and limitations, and it is well to keep in mind the various assumptions that factor into its development. This is not to suggest that all simulation engineers must be expert programmers; rather, they must be intelligent users. Therefore, knowledge and understanding of the simulation process are necessary precursors to a reservoir simulation study.
At first, a simulation study might look like a once-and-for-all exercise. In truth, however, it is an evolutionary process, throughout which we continually refine our conceptual understanding of the system. While we cannot overemphasize the importance of accurate reservoir description in a good reservoir simulation study, we must at the same time acknowledge that the data needed for an accurate description is seldom available; invariably, studies start out with less than complete data. However, by carefully analyzing and interpreting the voluminous information generated during the study, we should be able to refine and extend the input data base. Such refinement should lead to a better understanding of the system and, ultimately, to a better reservoir description. Of course, this requires some agility and creativity; there is no such thing as a casual simulation engineer.
It is, therefore, apparent that there are three basic interwoven components that go into a simulation study. These are:
· The tool: reservoir simulator
· The intelligent user: simulation engineer
· The pertinent information: reservoir description
Figure 1 depicts the necessary interactions among the simulation engineer, the simulator and the reservoir description.
Figure 1
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The engineer is clearly the prime mover in conducting the simulation study, and must be in control of other study components.
This control involves:
· being cognizant of the simulator’s limitations and the assumptions that go into its development
· being able to adequately describe the reservoir
· being fully conversant with the analytical techniques involved in interpreting the results.
Based on the initial results, it is not uncommon for the simulation engineer to revisit the appropriateness of the reservoir description through concept refinement.
Why Do We Need Reservoir Simulation?
The information we obtain from a newly discovered field is scanty at best. It is also disjointed to a certain extent, because bits and pieces of information are emanating from different parts of the field. Our first task is to integrate these pieces of information as accurately as possible in order to construct a global picture of the system. A reservoir simulation study is the most effective means of achieving this end. As field development progresses, more information becomes available, enabling us to continually refine the reservoir description.
Once we establish a good level of confidence in our reservoir description, we can use the simulator to perform a variety of numerical exercises, with the goal of optimizing field development and operation strategies. We are often confronted with questions such as
· what is the most efficient well spacing?
· what are the optimum production strategies?
· where are the external boundaries located?
· what are the intrinsic reservoir properties?
· what is the predominant recovery mechanism?
· when and how should we employ infill drilling?
· when and which improved recovery technique should we implement?
These are but a few of the critical questions we may need to answer. A reservoir simulation study is the only practical laboratory in which we can design and conduct tests to adequately address these questions. From this perspective, reservoir simulation is a powerful screening tool.
What are the Simulation Approaches?
The complexity of the problem at hand, the amount of data available, and the study’s objectives invariably dictate the choice of reservoir simulation approach, granted that we have already taken into account the appropriate computational environment (both in terms of hardware and software).
Broadly classified, there are two simulation approaches we can take: analytical and numerical.
· The analytical approach, as is the case in classical well test analysis, involves a great deal of assumptions—in essence, it renders an exact solution to an approximate problem.
· The numerical approach, on the other hand, attempts to solve the more realistic problem with less stringent assumptions—in other words, it provides an approximate solution to an exact problem.
From here on, we use the term simulation rather loosely to refer only to the numerical approach.
The domain of interest can form another level of categorization for simulation approach and model selection. For instance, a study may focus on a single well and its interaction with the reservoir within its drainage area (i.e., single-well simulation in radial-cylindrical coordinate system). The other extreme case may be the study of an entire field (field-scale simulation in rectangular coordinate system) in which the overall performance analysis of the field is called for. In between these two extremes comes the case where only a section of the reservoir is targeted (window-study).
Figure 2
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Figure 2 schematically represents these three simulation approaches.
What are the Basic Steps of a Simulation Study?
In general, a reservoir simulation study involves five steps:
· Setting objectives
· Selecting the model and approach
· Gathering, collecting and preparing the input data
· Planning the computer runs, in terms of history matching and/or performance prediction
· Analyzing, interpreting and reporting the results
A critical step is selecting the model and the approach. We must decide at the outset how many dimensions will be adequate. Such decisions depend on the flow dynamics involved and the amount of detail required. There may be cases when a two-dimensional representation is sufficient to describe a thin reservoir, whereas a three-dimensional model is unavoidable in the case of a thick reservoir. The expected flow structure dictates the choice of the model’s coordinate system. For example, in most single well studies, we may use radial-cylindrical flow geometry. However, if a well is vertically fractured, we should assign an elliptical-cylindrical flow geometry. We also have to decide how best to represent physical phenomena. For instance, if compositional variation is important, we may have to employ a compositional simulator rather than the more commonly used "black oil" model. Similarly, if we intend to study a coalbed methane reservoir, we must use a specialized model that accounts for the desorption process.
One of the most labor-intensive aspects of reservoir simulation study is data gathering, collection and preparation. Oftentimes, this requires collaboration among technical personnel with varying levels of expertise. For instance, geological and geophysical data are extremely crucial and need to be processed in the form that is useful for reservoir description. Data will be often be sparse or incomplete. In such situations, statistics or other tools can prove quite helpful. Because of the large volume of data being processed at this stage, and the likelihood of internal inconsistencies in the data, the engineer must have strong organizational skills and sound judgment.
In simulation studies, time (both the engineer’s and the computer’s) is of the essence. A typical simulation study requires a large number of runs, which must be carefully orchestrated to yield the desired information. As we make each run, we must carefully analyze the results, and appropriately label them to avoid confusion and costly duplications. In addition, we must avoid runs that yield no new information. It is pertinent, therefore, to take into account the inferences from the previous runs in planning the next suite of runs.
Perhaps the most important step in a simulation study is analyzing and interpreting the results. It is at this stage that our creative and discerning abilities are put to the test. As tempting as it may be to do so, we should not view every number that comes out of the computer as the absolute answer. Instead, we must always be asking questions such as, "what if? what then? why? so what?" In this way, we bring into play our experience, common sense, and perhaps sometimes extraneous knowledge.
A simulation study’s ultimate objective is to forecast reservoir performance. If we have selected the correct model, adequately prepared our data, conducted the appropriate computer runs, and made good, informed analyses, we should be confident of our ability to predict performance. Any mistakes we make in the previous steps will have a cumulative impact on performance prediction.
We must communicate study results in an appropriate manner to other technical personnel and to management. This should be in the form of a comprehensive technical report with sufficient details for others to assess the study’s quality.
How are Reservoir Simulators Used?
A reservoir simulator can be an effective tool for screening, analysis and design. The thought process that goes into appropriately using a simulator for these purposes, however, is quite involved. Figure 3 illustrates the interactions inherent in this thought process.
Figure 3
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The cornerstones of a reservoir simulator are the mathematical model, laboratory investigation (laboratory data), field observations and the computer code. In using reservoir simulators, these cornerstones generate signals, which propagate and interact with each other such that a continuous feedback takes place for the mutual benefit and enhancement of all the parts. For example, laboratory investigation, field observations and the computer code can highlight the need for improvement in the mathematical formulation. Similarly, a computer code originated from a robust formulation, together with pertinent field observations, may shed light on the validity of the experimental approach taken in the laboratory. This dynamic interaction illustrates the self-enhancing nature of reservoir simulators.
What Does a Simulation Study Require?
A simulation study is a challenging and demanding task, loaded with opportunities to learn more about the reservoir. To reap the full benefits of this powerful tool, it is imperative to recognize the proper roles of the engineer and of the reservoir simulator. In a successful study, neither of these can afford to dominate the other. The engineer should not demand from the simulator what it is not meant to do, but neither should he or she become overly dependent on the simulator. In a nutshell, the success of a simulation study hinges on a combination of a good engineer and the right simulator.
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