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 4D seismic data. In this paper, we discuss the development of a new node technology and a new method of employing nodes for reservoir monitoring, wherein, a high-speed underwater optical communications system is utilized to extract data from nodes that have a long deployment capability, without recovering the nodes from the seafloor. This highly repeatable semi-permanent 4D monitoring capability can be implemented at considerably lower cost than with cable based Permanent Reservoir Monitor (PRM) systems. Full azimuth and long offset seismic data acquired with a sparse array of ocean bottom nodes and a dense patch of shots has proven to be a very successful technique for both 3D and 4D imaging below complex salt overburden in the deep water Gulf of Mexico, where more than 25 OBN surveys have been conducted in the last ten years. Deep water OBN 3D and 4D surveys have also been successfully conducted in offshore West Africa and Brazil. Autonomous deep water OBN systems are proven to be very reliable, with 98 % data recovery, and nodes are routinely deployed by a remotely operated vehicle (ROV), amongst field infrastructure and simultaneous operations. Also, for reservoir monitoring, OBN surveys are proven to be highly repeatable, resulting in low 4D noise, NRMS statistics typically less than 10%, which is comparable with that achieved using PRM systems. We are motivated to look for ways to extend current proven node technology to semi-permanent reservoir monitoring in deep water (＞ 500 m), where the cost and technical, logistic and HSE challenges for PRM installation amongst the infrastructure of a working field are high. Installing a group of autonomous nodes, which is routinely done today, is an attractive alternative to the complex integration of a cabled system with existing facilities. By semi-permanent, we mean the nodes are deployed once, and then multiple surveys are acquired over an extended period of time, before recovery. For this purpose, we have designed and built a node which has sufficient energy storage for a five year on-the-seabed lifetime and a 300 day active recording life, enough for 12 surveys (baseline + 11 monitors) – each having a duration of about 25 days. An enabling technology that makes the semi-permanent Life of Field (LoF) node possible is a proprietary high speed underwater optical communications technology, whereby, data download from the node is performed in situ on the seafloor. Nodes are deployed in "sleep" mode and when 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 source period, each node is re-visited to extract the seismic data via the optical communication system and then switched off until the next monitor survey, all without handling the node, but by closely approaching the node with an ROV or an AUV (Autonomous Underwater Vehicle), to effect the optical link.