Advanced 4-D tools are helping operators reap measurable rewards.

Time-lapse 3-D, or 4-D, seismic technology was first introduced in the early 1980s but has gained widespread interest and use only in the past 5 years. The literature increasingly includes successful case studies with hints, if not quantification, of the economic benefit realized in managing the reservoir. For some companies, 4-D seismic programs are now an essential part of their global reservoir management strategy. To others, 4-D seismic tools are still perceived as a niche technology, limited to North Sea clastic oil reservoirs and a smattering of other special interest applications such as monitoring steamfloods and gas storage facilities.

So it is time to step back and evaluate how 4-D programs have been successful to date and what it will take for the industry to realize the full potential of 4-D seismic tools to add value to producing assets.

What do we mean by 4-D success?

The concept of success is more complex than one may think, both to define and to measure. We can define success in a variety of ways. Technical success implies that the survey was acquired within an agreed acquisition specification, processed to preserve the 4-D effect and interpreted to indicate reservoir changes. This doesn't mean it has been a business success, where the information is used to impact reservoir management, such as to steer an injection well or locate bypassed pay. Even if a 4-D program impacts field operations, it is an economic success only when that impact generates more monetary benefit than it cost to acquire the 4-D result. Measuring and, especially, predicting economic success is a notoriously difficult but essential part of time-lapse seismic reservoir management.

Published 4-D successes to date have been based primarily on a qualitative approach, meaning that time-lapse difference images are analyzed and interpreted to provide a visual indication of the relative movement of fluids in the reservoir. For example, Figure 1 is included in the Gannet-C study published by Shell. The interpretation of "changed" and "not changed" areas clearly indicates the expected draining of the reservoir and also suggests undrained compartments. In addition, the positioning of the original oil/water contact was altered and extended in recognition of the changed areas observed away from the base of the pinnacle-shaped reservoir trap. These observations give valuable insight into the structure and production of the reservoir but are essentially qualitative in nature.

Quantitative reservoir monitoring

Qualitative 4-D interpretation has been successful, but usually requires a large reservoir change to generate useful results. As the use of 4-D techniques spreads into areas outside the North Sea, to shorter elapsed time intervals and into carbonates as well as clastics, the demands on the seismic data will increase. Even now, reservoirs that produce both saturation and pressure changes pose a significant 4-D challenge since the effects are difficult to identify separately. These challenges are driving the development of quantitative 4-D methods to determine absolute reservoir saturation change, differentiate between saturation and pressure change, and extract the maximum potential value from 4-D seismic technology.

The key to the technical success of 4-D seismic applications lies in the repeatability and resolution of the seismic data. Improvements in seismic acquisition are enabling detection of smaller differences in seismic signal with confidence, effectively increasing the number of fields where the technique is applicable and decreasing the time interval between repeat surveys. For some operators, the concept of repeat surveys on a yearly or shorter timescale is becoming a reality.

A service that integrates technologies to solve specific reservoir problems has improved seismic resolution through digital single-sensor measurements and calibrated marine sources. Highly accurate acoustic positioning and steerable streamers result in the ability to know and control the position of the cable throughout its length. In this way, the cable can be placed in a specified position for improved 4-D repeatability and 3-D coverage beneath surface obstacles such as offshore platforms.

Trials using new streamer steering systems show that significant improvements in repeatability can be achieved and that performance measurements (such as residual noise measured in normalized root mean square) are significantly better than those recorded with conventional seismic systems. A North Sea operator identified a 130% improvement in the repeatability normalized root mean square for the survey.
Calibration with wireline and vertical seismic profiling data enables more accurate signal amplitude and phase measurements, velocity models and multiple predictions. It produces improved well ties, imaging, long-offset amplitude variation with offset and acoustic inversion, all of which result in higher resolution and fidelity of the final reservoir attributes.

Quantitative 4-D fluid saturation mapping techniques allow calibration of 4-D seismic responses coupled with flow simulator models. As part of a major drive to improve recovery from Statoil's Gullfaks field in the Norwegian sector of the North Sea, time-lapse seismic data have been used to guide quantitative mapping of oil saturation changes. Maps were generated (Figure 3) showing the probability of an area being drained, partially drained or undrained according to various oil saturation changes. The maps reduce the uncertainty in further development because they are quantitative and provide more powerful input to simulation models.

Business performance

The potential benefits of 4-D methods are well documented. These benefits range from monitoring reservoir fluid movements to identifying bypassed pay and fluid flow barriers, and can be used in at least two distinct ways. First, for near-term reservoir intervention, 4-D results impact the planning and location of new injection and production wells, generally resulting in immediate increases in reservoir off-take and cash flow, and avoiding the cost of putting a well in a poor location. Second, for long-term reservoir management, integration of 4-D results with the dynamic reservoir model allows optimization of the reservoir development plan, resulting in increased recovery with potentially fewer wells and increased net present value of the asset.

To illustrate this, Statoil has used 4-D seismic results to map bypassed pay and increase reserves in the Gullfaks field. It has used this information to select 34 additional well locations and identify 600 million bbl of additional reserves - a significant business impact.

To have the most business impact on the management of the reservoir and deliver the most economic value, 4-D seismic results also need to be available soon after acquisition, enabling the operator to use the results immediately to match production data to seismic differences and drill infill wells.
How does 4-D methodology impact the economic performance of the asset, and how can it be measured?
Oil companies realize that getting more from their existing assets is good business as long as it's cost effective. Not only can 4-D seismic technology help improve the net present value of the field by increasing ultimate recovery, but well intervention and infill drilling can result in short-term increases in cash flow and reductions in production costs per barrel. As new fields are discovered and developed in deepwater basins around the world where drilling costs are especially high, 4-D seismic programs become much more important to reduce reservoir intervention costs. Planning and budgeting for 4-D applications in these circumstances are becoming key parts of the initial field development plan.

Economic impact

Even though it is difficult to measure (and predict) economic impact quantitatively and to relate the cost of 4-D seismic programs to improvements in the value of the asset, it is clear that the 4-D objective must be to generate economic success. Predicting economic value is an essential part of the 4-D planning process.

Decision-tree analysis is a well-established methodology for predicting the value of information as well as incorporating prior perceptions of project risk and uncertainty. This approach can be used to quantify the value of 4-D seismic data in drilling a new infill well.

In Figure 4, the value of the 4-D information has been calculated for a field in the Norwegian sector of the North Sea. In this instance, the 4-D seismic program cost US $4.6 million. In the decision tree, Bayes' Theorem is used to modify the chance of drilling success without 4-D tools (Cs) and to estimate the value of drilling success with 4-D technology. Note that the 4-D value exceeds the 4-D cost over a wide range of 4-D reliability, suggesting that 4-D programs will almost always add economic value. It can also be seen that when the operator's knowledge is low (Cs=0.5) and 4-D seismic reliability is high, maximum value is created. Obviously, if the operator had "perfect information" (Cs=1.0) with respect to the reservoir, there would be no requirement for new information; however, this is rarely the case.

Finding where you are on the graph is simple. Past drilling performance can be used as a measure of the operator's knowledge, and geophysical performance measurements based on resolution and repeatability can indicate 4-D reliability. Advanced 4-D seismic technologies are more reliable and result in operators achieving better value from the investment in time-lapse information and their asset. Using these measures as a guide, it is easy to estimate the potential value addition attributable to time-lapse surveys.

Some independent industry analysts estimate 4-D seismic may contribute up to 11.5 million b/d by 2020, at an average finding and development cost of less than $2/bbl.

Past, present and future

Operators have been slower to adopt 4-D seismic technology than the seismic industry would like, possibly because the concept was ahead of its time when introduced. The industry was acquiring 4-D seismic data with less reliable conventional 3-D seismic tools, and the results often were disappointing in quality and slow to materialize. This paradigm has changed with the advent of new 4-D tools and cluster computing technology.

Today, new adopters of time-lapse technology do not face the same challenges as the early pioneers. Economic value readily can be justified based upon realistic expectations of technical success and business impact.

The new generation of repeatable 4-D seismic technology, including faster turnaround using onboard processing and Linux computing coupled with quantitative 4-D interpretation techniques will, if properly managed, significantly improve the technical success of 4-D seismic, and with this, the industry's ability to positively impact field operations and economics.

For some companies, 4-D seismic reservoir management is already considered best practice and state of the art for their mature fields. In the future, 4-D seismic information will be used in the early phases of new field developments when the operator's knowledge of reservoir performance is relatively low, in deepwater developments where well intervention and drilling costs are high and in carbonate reservoirs where acoustic differences tend to be small. The key to realizing this value lies in the reliability of new 4-D technology.

Reference

1. "Seismic Applications Throughout the Life of the Reservoir," Oilfield Review, Summer 2002.

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