A Concise Method for Predicting the Effectiveness of 4D Reservoir Monitoring

Seymour, R.H.; Emery, A.J.

doi:10.3997/2214-4609-pdb.131.gen1997_c033

Cited by 4 publications

References 0 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

4D Cause and Effect: What do reservoir fluid changes look like on seismic?

Khan¹,

Waggoner²,

Hughes³

2000

All Days

View full text Add to dashboard Cite

While most reported 4D effort focuses on 4D processing and interpretation of 4D seismic results, perhaps it is time to look at the problem the other way around. What 4D results are likely to arise from common reservoir depletion and displacement conditions? By better understanding the engineering cause of the geophysical effect, it may be possible for engineers to better contribute to the processing and interpretation of 4D data. This paper looks at this issue through reservoir simulation studies. The reservoir simulator is a powerful tool to predict how fluid saturations and pressures are distributed throughout the volume of the reservoir over time. This information can be used in conjunction with a properly scaled rock physics model to generate seismic attribute maps that reveal the patterns of 4D results that would be expected under various depletion, displacement, and heterogeneity conditions. The preliminary findings of this work are that pressure effects do obscure fluid saturation changes and exacerbate the effects of porosity variations in the reservoir. In addition, it is shown that care must be taken to accurately represent the poor vertical resolution of surface 3D seismic to avoid overly optimistic predictions of likely 4D seismic results. Introduction Time-lapse 3D, or 4D, seismic is an evolving geophysical technique involving the acquisition, processing, and interpretation of repeated 3D seismic data. The technique has developed considerably in the geophysical community since the early 1980's when it was first introduced, to the point where several successful case studies have been published recently1,2, including reporting the economic benefit to the asset. But even with the advances in seismic processing that have produced these 4D successes, there continue to be two aspects of the problem that deserve special attention - will 4D work in a given field, and interpretation of the 4D result. Interestingly, these are also the two aspects of 4D that require input from non-geophysical disciplines, primarily reservoir engineers and petrophysics. The first of these, ‘Will 4D work in a given field?’, has been the focus of several papers2,3,4,5,6,7 and excellent rock physics research and development8,9. But most of this work focuses on developing a rock physics model for a field, and then predicting the change in acoustic response for an assumed ‘average’ change in reservoir properties. For example, a saturation change from 0. to 0.8 will give an 8. 5% change in acoustic impedance, Z, a pressure change from 5586 psi to 4116 psi will give a 4% change in Z, and the combination of a saturation and pressure change will result in a 12% change in Z2. This ‘average reservoir’ approach is useful as an initial screening tool, but it is possible to go much further2,10,11. Specifically, when a reservoir simulation of the reservoir is available, it is possible to synthesize the acoustic response in much more detail than the ‘reservoir average’ approach. In this way, it is possible to predict both the magnitude and location of the acoustic change to give a better estimate of whether ‘4D will work in a given field’. This approach will be described in detail in the sections that follow. It turns out that this same approach is useful in addressing the 2nd aspect needing attention, the interpretation of the 4D result. By synthesizing the acoustic response of common reservoir depletion processes, it may be possible to describe the ‘4D signature’ of the different processes. This may allow a more informed interpretation of 4D results. However, the scope of this paper is limited to describing the approach and demonstrating two simple cases. Studying the Cause/Effect Relationship Knowing the relationship between the fluid changes in the reservoir (Cause), and the acoustic response on the 4D seismic technique (Effect) is critical in both feasibility and interpretation stages of a 4D project. But quantifying this relationship is difficult using field measured results because it is impossible to know what the Cause actually is, and the Effect is obscured by noise.

show abstract

4D Cause and Effect: What do reservoir fluid changes look like on seismic?

Khan¹,

Waggoner²,

Hughes³

2000

All Days

View full text Add to dashboard Cite

show abstract

Lessons Learned From 4D Projects

Waggoner¹

2000

SPE Reservoir Evaluation &Amp; Engineering

View full text Add to dashboard Cite

Summary Time-lapse three-dimensional, or four-dimensional (4D), seismic has been under consideration by the industry for reservoir monitoring for more than a decade. It offers the possibility of identifying the interwell distribution of bypassed and untapped oil, of monitoring displacement heterogeneity, and of detecting uneven pressure depletion away from wells. If obtained, these detailed observations could be used to increase ultimate recovery, reduce production costs, and prevent surprises such as unexpectedly early breakthrough. But these benefits are not easily obtained, and are certainly not guaranteed. There are a number of factors that impact whether a 4D project will be successful, and a careful study of these is required to give a realistic expectation of what 4D can do for a specific reservoir. Numerous 4D seismic projects have been active over oil fields world wide, and successes, relative to each project's objectives, have been realized by field operators using a wide variety of data acquisition techniques (land, streamer, and seabed methods), and over a variety of field types, including both clastics and carbonates. This paper draws from this experience to present a generalized 4D project workflow, and reviews results from some of these recent projects as illustrations. In general, sufficient software tools, rock physics data, and experience now exist to conclude that 4D is a low-risk/high-benefit reservoir management tool. The key to a successful project, however, is determining what 4D can do in a specific field, which requires a careful feasibility study, clear reservoir management objectives, and high-quality and experienced seismic processing and interpretation. Introduction Time-lapse three-dimensional (3D), or four-dimensional (4D), seismic has received a great deal of industry attention and activity over the past few years, as evidenced by the number of conferences organized specifically for 4D seismic and by the number of papers presented at more general conferences. In addition, both the Society of Exploration Geophysicists (SEG) and Society of Petroleum Engineers (SPE) designated Distinguished Lecturers in 1998 that presented excellent material regarding 4D techniques1 and the integration of 4D data with other types of data to improve reservoir description.2 These presentations have been well attended all over the world as the industry seeks to learn more about 4D seismic. What's All the Excitement About? The majority of people are probably interested in the ability of 4D to monitor fluid movement within the reservoir, and subsequently to identify bypassed reserves that can be produced through targeted offset drilling. Another commonly stated benefit of 4D is improved characterization of the reservoir to allow more reliable predictions from reservoir simulation studies, especially as it relates to the effectiveness of water or gas injection processes. These are but two of the extremely valuable reservoir management benefits of 4D seismic; others can be found in the numerous papers on the subject. Is the Excitement Justified? As with most things, the answer is both yes and no. As concluded by several authors,1,3-13 4D has potential, and several case histories to date have shown 4D to work to some degree. It is important to recognize that 4D is a simple idea based on physically limited measurements, difficult processing, and a complex earth. In some sense, it is amazing that it ever works, but experience shows that it can work, and it is that experience that forms the basis for the cautious optimism presented in this paper. Planning for Success Western Geophysical and its predecessors have been doing 4D seismic research, development, planning, and commercial projects, since the early 1980's. From that experience has grown a sound understanding of what it takes to do a successful 4D project. This paper is a collection of brief case histories within the framework of the following systematic and generalized 4D work flow: establish a clear reservoir objective; perform a careful feasibility study on the field of interest; do a rapid analysis of existing overlapping datasets; characterize the static reservoir properties; acquire and (re)process new 3D seismic data; analyze time-lapse differences; and characterize the dynamic reservoir properties using 4D results. Most of the cases have been published or presented at recent conferences, to which the reader is referred for more detailed description and analysis. Establish a Clear Reservoir Objective. Two of the general reservoir objectives were mentioned previously, and repeating a longer list would only serve to heighten the expectation that 4D will solve all problems. In fact, there are a large number of problems that surface seismic cannot address because of the physical limitations of the measurement. For example, 4D will never be able to see the movement of a heavy oil/water interface in a 10 ft thick carbonate at 15,000 ft depth under a large gas cloud. Even if it could, that information would only be of use to you if that was the condition in your reservoir. The goal here is to define what needs to be learned about the reservoir so that the 4D project can be properly planned and the results can be measured against whether the needed information was, in fact, provided by the seismic data. To date, most 4D projects have been performed primarily as a geophysical exercise, rather than with a primary reservoir objective, in order to "test" the 4D technique. For these studies, the stated objective is to take two existing 3D datasets, which happen to have some overlap, and see what the difference between them shows. An example of such a rapid analysis is shown in a later section. The most common result is a suggestion of a difference and a recommendation that the datasets be reprocessed, because the acquisition and processing were of different vintages and for different purposes. However, because the objective is not driven by a reservoir need, there are few examples of even a good geophysical result being used to influence a development or production decision. The important point in setting the reservoir objective is that it be set by the reservoir engineer or asset team to gain needed information. Not only is every reservoir different, but the objective for a particular reservoir will change during its development and production lifetime. Once set, the objectives need to be evaluated as part of the feasibility study that follows to avoid unrealistic expectations.

show abstract