A New Method of Semi-Permanent Reservoir Monitoring in Deep Water Using Ocean Bottom Nodes

摘要

Deep water Ocean Bottom Node (OBN) systems have delivered high quality full azimuth 3D and 4Drnseismic data. In this paper, we discuss the development of a new node technology and a new method ofrnemploying nodes for reservoir monitoring, wherein, a high-speed underwater optical communicationsrnsystem is utilized to extract data from nodes that have a long deployment capability, without recoveringrnthe nodes from the seafloor. This highly repeatable semi-permanent 4D monitoring capability can bernimplemented at considerably lower cost than with cable based Permanent Reservoir Monitor (PRM)rnsystems.rnFull azimuth and long offset seismic data acquired with a sparse array of ocean bottom nodes and arndense patch of shots has proven to be a very successful technique for both 3D and 4D imaging belowrncomplex salt overburden in the deep water Gulf of Mexico, where more than 25 OBN surveys have beenrnconducted in the last ten years. Deep water OBN 3D and 4D surveys have also been successfullyrnconducted in offshore West Africa and Brazil. Autonomous deep water OBN systems are proven to bernvery reliable, with 98u0002% data recovery, and nodes are routinely deployed by a remotely operated vehiclern(ROV), amongst field infrastructure and simultaneous operations. Also, for reservoir monitoring, OBNrnsurveys are proven to be highly repeatable, resulting in low 4D noise, NRMS statistics typically less thanrn10%, which is comparable with that achieved using PRM systems.rnWe are motivated to look for ways to extend current proven node technology to semi-permanentrnreservoir monitoring in deep water (＞ 500 m), where the cost and technical, logistic and HSE challengesrnfor PRM installation amongst the infrastructure of a working field are high. Installing a group ofrnautonomous nodes, which is routinely done today, is an attractive alternative to the complex integrationrnof a cabled system with existing facilities. By semi-permanent, we mean the nodes are deployed once, andrnthen multiple surveys are acquired over an extended period of time, before recovery. For this purpose, wernhave designed and built a node which has sufficient energy storage for a five year on-the-seabed lifetimernand a 300 day active recording life, enough for 12 surveys (baseline + 11 monitors) – each having arnduration of about 25 days. An enabling technology that makes the semi-permanent Life of Field (LoF)rnnode possible is a proprietary high speed underwater optical communications technology, whereby, datarndownload from the node is performed in situ on the seafloor. Nodes are deployed in "sleep" mode andrnwhen needed they are remotely switched on, "health" checked for operational readiness, synchronized with a reference time signal, and have their recording systems activated. Upon completion of the active sourcernperiod, each node is re-visited to extract the seismic data via the optical communication system and thenrnswitched off until the next monitor survey, all without handling the node, but by closely approaching thernnode with an ROV or an AUV (Autonomous Underwater Vehicle), to effect the optical link.