Value of Information for a 4D-Seismic Acquisition Project

Abstract

Abstract The decision of any data acquisition should always involve a cost - benefit analysis. While cost identification is usually easy, the value gained with new data is specialy difficult when there are mostly indirect benefits. This is the case of 4D seismic projects where the main contribution is to reservoir production management. Recently, the interest on 4D seismic and the number of such projects have increased significantly. Despite of the high cost, there is a lack of robust and systematic process to evaluate their value. Bp TT faced this challenge when analyzing a Life of Complex Seismic (LoCS) project for multiple gas reservoirs in the Greater Cassia Complex, located in the Columbus Basin, Trinidad and Tobago. This Complex comprises many producing and future development fields including Amherstia, Immortelle Parang, Kapok, Cannonball and Cassia.A significant fraction of present and future potential reserves of several Tcf is under shallow gas, which has detrimental impact on seismic image quality. Such condition required consideration of expensive 4C OBC (Ocean Bottom Cable) seismic acquisition options. Additional benefits from 4D seismic for monitoring dynamic reservoir performance could be foreseen with a permanent installation. A robust and systematic value of information study was developed as part of the LoCS project in order to support the decision about the 4D acquisition options. This work describes the Decision Tree process used in the LoCS value of information study as well as discusses the main results used by bpTT to make a based decision about the acquisition program. Introduction In 2004 BpTT faced the challenge of valuing a number of seismic survey options over the Greater Cassia Complex in the Columbus basin, Trinidad. The complex contains several Tcf's of gas reserves in five currently producing fields; a number of new field developments and future exploration potential. Development of these reserves is complicated by the presence of 27 stacked reservoirs with reserves trapped in over 100 separate segments. With an aggressive well program planned for the next 10 years to fully manage and develop the potential in the Complex included in the BU Long Term Plan (LTP), the team was charged to identify and value the style of survey required to:Improve the static seismic image over the southern half of the Complex affected by shallow gas imaging problems.Value the dynamic benefits that 4D seismic could offer for reservoir management and future well placement. This work provides a brief overview for the Life of Complex Seismic (LoCS) project and a detailed description of a value assessment study performed as part of that project. The main focus of this work is to present the process and the decision tree methodology used in the value assessment study as well as to present the main results from the study. After a short overview of previous value assessment studies, the process used here is described. All steps of the process are then described in detail followed by the main value assessment results. The integration of these results with other elements necessary for management decision about the seismic acquisition options is presented. Finally, the main conclusions from the value study are presented. Life of Complex Seismic Project (LoCS). A seismic strategy for the ‘Complex’, described as the ‘Life of Complex Seismic’ (LoCS) strategy would be based on one large survey focused in Southern Greater Cassia, using either temporary or permanent seabed cables or traditional surface tow technology. The main drivers of acquiring this survey were to improve reservoir management and underpin the future drilling campaign. However, there still remained significant uncertainty that 4D could bring value to gas reservoirs and the high up-front costs of a permanent seabed array could not be justified solely on a static prize.