Resaturated pay: A new infill target type identified through the application and continuous improvement of 4D seismic at the Forties Field

Abstract

Forties is a giant North Sea oil field with an estimated 4.2 billion to 5 billion barrels of oil initially in place. The field has been producing since 1975 and by year-end 2015 had produced more than 2.7 billion barrels of cumulative oil production. The peak production rate exceeded more than 500,000 barrels of oil per day (BOPD) in 1979, and since then more than 3.8 billion barrels of water have been injected into the field. The majority of the field is now water swept due to the combined effect of water injection and extensive depletion supported by a large underlying aquifer. The ability to identify the few remaining unswept portions of the reservoir has elevated 4D seismic into a key technology to derisk all new wells drilled at Forties. The types of 4D targets at Forties are ever-evolving as continuous improvements in seismic acquisition and processing technologies deliver higher quality time-lapse seismic data. With every subtle improvement in 4D repeatability, additional and sometimes unexpected information about the reservoir has come to light. Historically the primary and most robust 4D effect was an increase in impedance (i.e., a “hardening” effect), which could be mapped confidently to identify areas where water was replacing oil. As 4D repeatability improved, it became evident that a decrease in impedance (i.e., a “softening” effect) was observed in many areas in the reservoir. Further analysis revealed that this type of 4D response was highlighting areas where oil had resaturated previously swept reservoir, forming a new type of infill target in the Forties portfolio called resaturation targets. Since drilling the first 4D resaturation target in 2011, 22 of these targets have been drilled with an 86% success rate, delivering 15 million barrels of new oil production from the Forties Field.

Four-Dimensional History Matching Using ES-MDA and Flow-Based Distance-to-Front Measurement

“…From an industry standpoint, 4D seismic data have been primarily used as a qualitative constraint on the reservoir model [7][8][9][10][11][12] as the understanding of the reservoir evolves and revisions are made in a multidisciplinary framework, encompassing the domains of geology, seismic, and reservoir engineering [13]. However, while it is possible to identify the changes occurring on the subsurface, like saturation and pressure variations (e.g., [14,15]), using 4D data in a qualitative way, we have no information on the magnitude of the change or with what certainty.…”

Quantitative integration of 3D and 4D seismic impedance into reservoir simulation model updating in the Norne Field

“…Four‐dimensional seismic response can be integrated either qualitatively (such as interpreting likely causes of 4D anomalies due to changes in saturation and pore pressure) or quantitatively (by adding seismic derived attributes inside in the objective function of a history‐matching process). Most 4D seismic interpretation remains qualitative in the literature (Johnston ) and it was recently discussed in Byerley, Singer and Rose () and Maleki et al . (2018a).…”

“…Byerley et al . () used time‐lapse seismic data to implement an optimized well‐operating strategy for the Surmont heavy oil field, and Maleki et al . (2018a) identified the production effects in the Norne Field utilizing the joint interpretation of 4D seismic inversion with the flow simulation model results.…”

“…Fourdimensional seismic response can be integrated either qualitatively (such as interpreting likely causes of 4D anomalies due to changes in saturation and pore pressure) or quantitatively (by adding seismic derived attributes inside in the objective function of a history-matching process). Most 4D seismic interpretation remains qualitative in the literature (Johnston 2013) and it was recently discussed in Byerley, Singer and Rose (2016) and Maleki et al (2018a). Byerley et al (2016) used time-lapse seismic data to implement an optimized welloperating strategy for the Surmont heavy oil field, and Maleki et al (2018a) identified the production effects in the Norne Field utilizing the joint interpretation of 4D seismic inversion with the flow simulation model results.…”

Integration of 3D and 4D seismic impedance into the simulation model to improve reservoir characterization

“…4D seismic response can be integrated either qualitatively (such as interpreting likely causes of 4D anomalies due to changes in saturation and pore pressure) or quantitatively (by adding seismic derived attributes inside in the objective function of a history matching process). Most 4D seismic interpretation remains qualitative in the literature (Johnston, 2013) and it was recently discussed in Byerley et al (2016) by utilizing time lapse seismic data (impedance domain) to identify areas where water was replacing oil for the Forties Field. However, the multidisciplinary nature of reservoir modeling demands more quantitative approach to integrate 4D seismic data.…”