Rob Staples

Time-lapse seismic surveys in the North Sea and their business impact

In a time-lapse 3-D seismic (TL3D) project, a “monitor” seismic survey is compared to a “base” survey. Ideally, the base survey is a 3-D data set acquired before production from the field started. Normally, the monitor survey is acquired after several years of production. The observed differences between the base and the monitor survey can be interpreted in terms of changes in fluid saturation and pressure. When embarking on a seismic monitoring project, it is important to keep the following questions in mind: In this article, we will answer these questions and illustrate our solutions with two examples. ### What field information might technically be obtained? Fluid drainage information from TL3D may be used directly to locate fluid fronts in each major flow unit. It may also be used to derive lateral variations in permeability of the reservoir and sealing capacity of faults. These variations can cause areas of bypassed oil and blocks that are not being drained. In general, it will help to understand the observed production from the field. It is a very powerful method to map production-related changes in reservoir fluid saturation and pressure away from the wells. TL3D very often reveals some flow behavior that had not been predicted, even by history-matched flow models. This information can be critical to future reservoir development. ### What impact will it have on reservoir management? Time-lapse seismic may be used to optimize the number and location of infill wells to access untapped reserves or to accelerate production. Ideally, this will minimize infill costs and maximize recovery. It may also indicate the need to shut off thief zones to avoid premature water or gas breakthrough. It will …

Monitoring pressure depletion and improving geomechanical models of the Shearwater Field using 4D seismic

Dedicated 4D seismic data have been used to investigate horizontal and vertical compartmentalization in the high pressure, high temperature (HPHT) Shearwater Field in the U.K. Central North Sea. Geomechanical modeling and conversion of model results to synthetic seismic time shifts allow quantitative comparison of models with real 4D seismic time shifts. Drilling in and around heavily depleted reservoirs is very challenging, and matching the geomechanical model to all data sets (including 4D) provides the best possible constraints on the likely stress fields an infill well will encounter in the overburden and the reservoir.

A New Technique for Pressure - Saturation Separation from Time-Lapse Seismic - Schiehallion Case Study

A technique is shown for estimating pressure and three phase saturation changes from 4D seismic. The technique is a further development of a recently published method which does not require defined rock and fluid physics relationships. A multi-attribute approach is taken, in which 4D seismic attributes are linked (by relationships with a physically reasonable form) to production data, and an optimal combination of seismic attributes are then selected to perform a bayesian inversion separating pressure and saturations. The methodology is applied to the Schiehallion field in the West of Shetlands with encouraging results. It has highlighted saturation changes, previously hidden by the effect of pore pressure on the rock frame, which can be validated against production and tracer data. Additionally, it has shown areas where gas is coming out of solution that have been validated against information acquired in a recently drilled well.

Determination of a seismic and engineering consistent petro-elastic model for time-lapse seismic studies: Application to the Schiehallion Field

Summary A relationship between pressure and saturation changes in the reservoir and the corresponding time-lapse seismic signatures is determined from measurements of the produced fluids and downhole pressures at a number of wells. The results from this process also provide an insight into the governing parameters for the rock and fluid physics model which are then compared directly to conventional petro-elastic predictions. The methodology is applied to the Schiehallion field where it is found that the 4D responses to water replacing oil, and gas breakout, follow the predictions of Gassmann fairly accurately. However, the sensitivity of the seismic to increases in pressure is observed to be roughly half as strong as that estimated from standard core plug measurements.

Integration of 4D seismic data and the dynamic reservoir model reveal new targets in Gannet C

Two sets of 4D seismic data gave major new insights into the structure and dynamic behavior of the Gannet C oil and gas reservoir in the U.K. Central North Sea. The 4D data revealed major extensions of reservoir units previously presumed to be absent or thin over much of the reservoir. Furthermore, in a subsea field with significant uncertainty of production allocation, 4D also proved an invaluable history-matching parameter for the dynamic model. Together, the dynamic model and the 4D data gave rise to the identification of one recompletion opportunity and two infill well opportunities, to produce oil volumes in existing and newly identified reservoir sands. These now have been realized, roughly trebling oil production from the field.