The Ekofisk Field is a large oil reservoir located in the Norwegian sector of the North Sea. About 8.2 meters of seafloor subsidence has occurred above this thick, high porosity chalk reservoir since production started in 1971. The seafloor subsidence is a result of reservoir compaction in the chalk, driven primarily by pressure depletion early in the field life and water weakening at later stages.
Reservoir compaction monitoring via radioactive marker bullets has established a clear association between compaction and changes in water saturation and reservoir pressure. These data have also confirmed the mechanical behavior of the chalk observed during laboratory tests. The compaction of high porosity chalk is a result of both net effective stress increases linked to pore pressure depletion and water weakening induced by water injection. Field compaction data from a compaction monitoring well located in the crestal part of the field also suggest a vertical compaction to subsidence ratio of 1.20 to 1.25.
Owing to compaction-related issues, many wells have been redrilled over the life time of the field. The new wells have yielded new data, such as porosity, water saturation, and layer pressure, spanning the entire field production period. These data, however, have been affected by compaction and, consequently, have introduced biases when used as hard data to build a geological description of the reservoir. A porosity loss of as much as 10 p.u., for example, can be predicted for high porosity, crestal chalk which has undergone a total reservoir compaction approaching 40 feet over a 30-year production life.
More than 200 wells have been drilled since 1971 at Ekofisk, with only 42 of them being considered representative of the initial geological state of the field. In order to utilize the data from all 200-plus wells, a software program was developed to handle the complex decompaction and forward compaction logic needed to correct the well logs for compaction effects. Using the decompaction and forward compaction capabilities of the developed software, data from all 200-plus wells were used to evaluate the total reservoir compaction volume and shape. This was, in-turn, compared to 4D seismic-based compaction results as well as compaction results from geomechanical simulations.
The developed software and evaluations have made it possible to now build a geological model of the Ekofisk reservoir using data from the 200-plus wells drilled on the field, each representing the initial state of the reservoir prior to production in 1971. Ultimately, this is expected to improve the flow simulations of the reservoir leading to better estimates of production and reserves.