Lessons Learned From 4D Projects

Abstract

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.

Value of Information in the Oil and Gas Industry: Past, Present, and Future

“…Another common practice is to assess the information value after the fact, for example, in the form of historical look-backs to document the impact of the information (Aylor 1999;Waggoner 2000). What they typically mean by this is a qualitative assessment along the lines of "this information will increase our confidence in our decision."…”