The three major phases explain the seven steps needed for measuring micro-gravity at the seafloor, and interpreting such data for reservoir monitoring.
A feasibility study is done to assess if and how a gravimetric monitoring program can aid reservoir management
PLANNING: Feasibility Study
The choice of locations and numbers of stations on the seafloor are important for optImized data acquisition.
PLANNING: Station Layout
On each station concrete benchmarks are permanently deployed on the seafloor.
PLANNING: Station Deployment
To carry out the services vessels arrangements, time schedules, and equipment has to be settled.
EXECUTION: Survey Planning
Relative measurements are done by the ROVDOG instrument package on the seafloor.
EXECUTION: Data Acquisition
The main purpose of the data processing is to correct for various effects, quality check the data and finally achieve high accuracy.
EXECUTION: Data Processing
After data processing of a repeat survey, the timelapse data is compared.
INTERPRETATION: Time-lapse Interpretation
The first step is to conduct a feasibility study to assess if and how a gravimetric monitoring program can monitor pressure and saturation changes in a producing reservoir and how the information can be valued in the reservoir management. A field in its development phase should have an elaborate evaluation of the uncertainty in all factors that impact the overall recovery and predicted production rates. A field already in production should likewise be evaluated for the contribution gravity and subsidence monitoring can offer to aid in production rate predictions and remaining reserve estimates.
The choice of locations and number of stations on the seafloor are important for optimized data acquisition. Based on a variety of geophysical, geotechnical and economic criteria, the selection of station locations can be made. Reference stations outside the area of the field are important as they define the level of no change.
On each station benchmarks are permanently deployed on the seafloor to serve as stable references for placing the recording instrument. The benchmarks assure that the exact locations are repeated from one year to another and allow seafloor subsidence measurements to be made with sub-cm accuracy.
A developing field should have an elaborate evaluation of the uncertainty in all impacting factors.
To carry out the services, a suitable survey vessel is used. Vessels for the subsea inspection and survey market carrying a standard work-class ROV can be utilized. Quad offers either a full package including vessel or the measurement work onboard a client-provided vessel.
Repeated relative measurements are done by the ROVDOG instrument package. The ROVDOG contains multiple pressure cases with a gravity and pressure sensor within each. The ROVDOG is connected to the ROV via cables, and the instrument frame is physically handled by a purpose-built manipulator arm. When safely placed on a seafloor benchmark, the ROVDOG is levelled and recording starts (lasting typically 15-20 minutes). Data are continuously sent through the umbilical to the vessel, and recorded in real time on computers by Quad’s operators.
The main purpose of the data processing is to correct for various sensor and environmental effects, to Quality Control the data and finally achieve true relative gravity values with high precision. Some of the editing is done manually. Key sensor readings and statistics are gathered. Stations which are repeated twice or more during a survey provide a good statistical basis for determining outliers and anomalies. Time-lapse comparisons are done as part of the QC, and some of the processing parameters are fine-tuned. Additional measurements can be added towards the end of the survey, to strengthen weak points in the network.
Measurements are done by the ROVDOG instrument package
Based on agreements between multiple visits at benchmarks during a survey, intra-survey repeatability can be assessed. After a repeat survey has been acquired and processed, time-lapse changes can be calculated. The amount of seafloor subsidence (or uplift) and associated uncertainty is mapped. This can be compared against models, based on reservoir geometry and pressure change distribution estimated from a reservoir model. Updated compressibility values may be retrieved from the subsidence data. Areas of mis-match may indicate non-expected pressure change or anomalous rock stiffness. Iterative flow model updates are possible to better match the subsidence data.
The gravity changes are corrected for subsidence, to make the remaining Δg dependent on reservoir mass changes only. The gravity changes then have information on the amount of water influx, hydrocarbon production (such as gas depletion) or waste injection. Also for gravity, changes can be compared to those predicted (forward modelled) from a reservoir flow model, mis-matches can be identified, and history matching of the model can be done. This is best done jointly with all available constraints from production/pressure data.
Subsidence and gravity data can also be inverted for reservoir changes without having a flow model to compare with. Such inversions may be constrained in various ways, and typically by geometry sontraints.
The total anomaly may be used to assess the total compaction or total water influx into a reservoir, using Gauss’ theorem.
Changes can be compared to those predicted from a reservoir flow model
Example of a history match of gravity data and a flow simulation model (from Vevatne et al. EAGE 2012)