Instantaneous 4D Seismic (i4D) – An Innovative Concept to Monitor Offshore Water Injector Wells

Time-lapse, or 4D, seismic is frequently used in the offshore to monitor fluid movement processes such as waterflood or aquifer drive. These processes happen at relatively slow timescales (years). As a result, proper monitoring of these effects requires repeat surveys to be acquired every 3 to 5 years or so. Recent examples in industry show that fast reservoir changes (~months) are also taking place. The observed changes are often associated with water injector wells. For example, Alsos et al (2009) show an example of a water injection well that produced large time-lapse signals in less than 1 year after its startup. Due to cost and other considerations, it is not feasible to repeat field-wide 4D surveys often, nor install permanent monitoring systems in many of those fields. Yet there is a need to monitor such phenomena more frequently, and also with short reaction times. In the offshore, we propose a new method for time-lapse seismic surveys focused on water injector wells. Our socalled “Instantaneous 4D (i4D)” method aims to acquire dedicated high-quality 4D data with short turn-around time at cost levels low enough to make it affordable to repeat such data acquisition programs reasonably frequently. We have implemented the method as a variation on surveys that use ocean-bottom seismometers. We suspect that our method can also be combined with other ways of seismic data acquisition (for example, permanent reservoir monitoring systems; vertical seismic profiling systems in wells). Our method is not intended to replace full-field surveys. Rather, it adds small surveys more frequently in between regular full-field surveys to enable better observation of fast 4D reservoir effects along with slow changes (Figure 1). It can be seen as filling the “gap” between the current practice of infrequent full-field surveys, and the installation of the permanent reservoir monitoring system, which can be expensive.