Time-Lapse Seismic for Optimal Reservoir Management - A Global Perspective
Authors: Johnston David H. 
Published: May 2016

Abstract
Time-lapse or 4D seismic data has proven value in reservoir management, increasing reserves and recovery by locating bypassed and undrained hydrocarbons and optimizing infill well locations and flood patterns. 4D seismic can also decrease operating costs by reducing uncertainty in the reservoir models, optimizing completions, and minimizing the number of dry holes. ExxonMobil's 4D experience demonstrates this added value to reservoir management over a wide range of geographical areas, geological settings, and production scenarios. This paper describes three 4D case studies; from the Norwegian North Sea, the Gulf of Mexico, and West Africa. In each case, the interpretation of 4D seismic data adds value to the field in ways that are not always anticipated beforehand. Most 4D seismic surveys contain at least one surprise. The value of that information can be significant.

Introduction
The concept behind time-lapse or 4D seismic is deceptively simple. We use repeated 3D seismic data, shot over a producing field, to help us monitor and predict the interwell changes in reservoir properties that result from the field's depletion. The fourth dimension is calendar time. Changes in reservoir properties, such as in fluid saturation and pressure, result in changes in elastic properties that, under favorable circumstances, can be detected by the seismic data. For thermal recovery processes, 4D seismic is used to detect and monitor temperature changes, and in compacting reservoirs it is used to detect porosity and stress changes. Time-lapse seismic is field-scale reservoir surveillance.
However, the devil is in the details. Issues related to the application of 4D seismic technology include (1) detectability - the magnitude of the seismic response to production changes, (2) repeatability - the similarity between two seismic surveys, and (3) interpretability - how the data is integrated with geologic and engineering data. Considering these issues, each field is unique. For example, different depletion mechanisms result in different fluid-saturation distributions, different pressure variations, and, in turn, different 4D seismic responses. The 4D seismic response also depends on reservoir rock and fluid properties which are unique to each field.
Repeatability of the 4D seismic image is dictated by a number of factors which include survey geometry, overburden complexity, and background noise. We have found that understanding the relationship between seismic-survey geometry repeatability and image repeatability is essential to determine the level of acquisition effort required to reduce 4D interpretation uncertainty to an acceptable level. For example, lessons learned regarding 4D acquisition in the North Sea may not translate to other basins in the world.
Finally, 4D interpretation strategies depend on the objectives of the survey. Qualitative map- and volume-based approaches satisfy many reservoir management needs. However, for fields with complex depletion strategies, more quantitative interpretation methods may be required to help separate saturation from pressure changes. In addition, geologic, simulation, and geomechanical models that are conditioned using 4D interpretations are more predictive. That increased predictability extends the usefulness of the seismic data and adds value throughout the field's life cycle through better reservoir management decisions.