Mark K. MacLeod

The Alba Field ocean bottom cable seismic survey: Impact on development

Alba Field, in the Central North Sea (UK Block 16/26), consists of Eocene‐age, high‐porosity, unconsolidated turbidite channel sands sealed by low‐permeability shales at an average subsea depth of 1900 m. The main channel is approximately 9 km long and 1.5 km wide and can be up to 100-m thick (Figure 1). The channel contains discrete bodies of intrareservoir shales that can cause significant drilling, completion, and production problems. Immediately overlying the main Alba sand channel, several wells have encountered thin, discontinuous, oil‐saturated Brioc sands.

Incorporating Seismic Attribute Maps in 3D Reservoir Models

We introduce a new geostatistical method to incorporate seismic attribute maps into a 3D reservoir model. The method explicitly honors the difference in vertical resolution between seismic and well log data. The method, called Sequential Gaussian Simulation with Block Kriging (SGSBK), treats the seismic map as a soft estimate of the average reservoir property. Using this method, the average of the cell values in any one vertical column of grid cells is constrained by the value of the seismic map over that column. The result is a model that contains vertical variability driven by well logs and the vertical variogram model and spatial variability driven by the seismic map and the areal variogram model.

Analysis of time-lapse data from the Alba Field 4C/4D seismic survey

The 1998 Alba 3D Ocean Bottom Cable (OBC) survey was designed to accomplish multiple objectives. The primary goal was to image low impedance reservoir sands with converted wave (PS) reflections; one important secondary goal was to image fluid movement by comparing the OBC data with a 1989 streamer survey. Modelling shows that a strong original oil–water contact reflector should be visible throughout much of the field and that water saturation changes should be observable by analysing the time-lapse differences between the 1989 streamer data and 1998 OBC survey. Differences between the 1989 and 1998 seismic field data confirm that fluid changes are clearly visible near several producing and injector wells. However, extracting additional quantitative saturation information from the seismic data has proven difficult, possibly because of: (a) complex interaction between the fluids, sands and shales within the Alba reservoir; (b) moderate to poor repeatability of the seismic response to reservoir fluids. The focus of this paper is the acquisition and analysis of Alba time-lapse data. We show that production- and injection-related effects are predicted by modelling and observed in the data and then we make an attempt to relate these effects quantitatively to oil production and water injection. Despite the challenges in using the Alba time-lapse data quantitatively, the data have been successfully used qualitatively for well planning risk assessment and for guiding reservoir simulation efforts. Lessons from this work will be used in any future time-lapse surveys at Alba.

Salt dome imaging using 3‐C phones in salt

We describe a method to determine the shape of a salt dome using an oriented three-component geophone placed inside the dome. Shots are fired at the surface at a range of offsets and azimuths that depend on the depth and flank of interest. We present two methods of ray trace migrating the traveltime information. The first uses the ray direction of the first arrival and the P-wave salt and sediment velocities to determine a unique point on the salt flank. The second method uses the ray directions, P and S-wave salt velocities, and the traveltime difference between the direct arrival and a shear wave generated at the salt-sediment interface. When migrating synthetic seismic data, both methods show excellent agreement with the starting model. The main benefit of the P-S method is that it is independent of the sediment velocity.