Sea Technology

APR 2017

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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12 st / April 2017 www.sea-technology.com GPS positioning access. The repeat- ability is quantified by measuring the background 4D noise as the normal- ized RMS average amplitude change in the seismic image (NRMS), mea- sured away from the reservoir where no production-related changes are expected. NRMS statistics for PRM monitor surveys are typically around 5 percent. In repeated deepwater OBN sur- veys in water depths around 2,000 m, where nodes have been recovered after each survey and redeployed for the next, node positions are typically repeated to within 5 to 10 m; the real- time ROV navigation accuracy in that water depth. The NRMS repeatability statistic for deepwater OBN is typi- cally measured at around 6 percent, ri- valling that achieved by PRM. Marine streamer data, susceptible to nonre- peatable current conditions, generally have a much higher 4D noise compo- nent. With the low 4D background noise of deepwater OBN, it has been dem- onstrated that seismic changes at the reservoir, like those due to water injec- tion, can be measured with a reduced scale node patch: 150 to 300 nodes rather than the 1,500 or so nodes used for full field imaging. This reduced scale in receiver effort and the corre- sponding reduction in source point ef- fort can significantly reduce the cost of reservoir monitoring with nodes. Permanent Reservoir Monitoring Permanent reservoir monitoring installations are rare in general and very rare in deepwater. PRM systems have been shown to be of great value in optimizing field development and enhanced oil recovery, but they are costly and complex engineering and facilities projects requiring custom- designed components, trenching tens to hundreds of kilometers of cable among seafloor infrastructure, wet- mate connectors, umbilicals, and data recording and storage equipment in- stalled on platforms or FPSOs. Capital expense and risks only go up with increasing water depth. Many parts of the asset operating company outside the geoscience group are in- volved. Getting everyone to accept the PRM value proposition may itself be a hurdle to getting a system in place, especially if the return on investment prediction is not well understood. Motivation Encouraged by the high quality and high value of 4D signals obtained from the sparse ocean bottom node seismic method, even with a reduced scale node and source patch and even in complex subsalt settings, we are moti- vated to find a way to enable more fre- quent and more cost-effective monitor surveys with nodes. We are especially motivated to find a deepwater solution where we expect the commercial ben- efits over expensive PRM installations to be large. Nodes today record continuously and have a deployment life of about six months before they must be re- covered, downloaded and recharged. Some nodes today have limited acous- tic communication, but none have suf- ficient bandwidth to transmit the high bit count of seismic data. To eliminate the slow and costly deployment and re- trieval of the node array by ROV each time a monitor survey is desired, we need a node with a much longer op- erational lifetime, so that many moni- tor surveys can be acquired before bat- tery exhaustion. We also need a node with sufficient subsea communication bandwidth to rapidly download data after each monitor survey, without re- covering the node. A battery-powered node will never be permanent, but the technology exists to make a node that is sufficiently long-lived to permit many monitor surveys with a single initial deployment and a single final recovery—a semipermanent reservoir monitoring solution. New Technology We have developed a new node called ZLoF that is similar in size and weight to the Z3000 deepwater pro- duction node but has sufficient energy storage to enable a five-year lifetime on the seafloor and a 300-day active recording life, enough for 12 surveys of 25 days each before recovery from the seafloor. This node will record only when needed, be awakened by remote command, perform a system health check, report status, synchronize its internal clock, and initiate data record- ing. After the active source patch is completed, data are downloaded from each node on the seafloor without recovering the node. Nodes are then commanded back into "sleep" mode. The new node has about the same mass and form factor as current deepwater nodes that have successfully been used

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