Discovery, appraisal, development, EOR and decommissioning often are described as the principal stages in the life of any E&P project. Successful E&P companies manage the risks and uncertainties associated with environmental, regulatory, technical and, as recent turmoil in the oil and gas markets has shown, economic factors at each stage in the life-of-field development. Seismic data play a key role in defining and managing those risk factors.
Modern ocean-bottom node (OBN) acquisition systems provide the best seismic data quality and clearest reservoir image to inform E&P strategies throughout each stage of field development.
Large-scale OBN surveys
The search for hydrocarbons has extended to remote and difficult-to-access areas; environmentally sensitive areas; areas of dense infrastructure; and geologic basins with complex overburden, structure and stratigraphy. Given these challenges and the limitations of conventional towed-streamer acquisition, autonomous nodal systems successfully balance the competing objectives of subsurface imaging, affordability, acquisition efficiencies, accessibility, repeatability and environmental impact.
The geologic and imaging objectives of a seismic survey are met by all-azimuth long-offset high-fold and dense subsurface sampling: All are requirements of new imaging and noise reduction technologies. OBNs are the affordable choice over ocean-bottom cable (OBC) systems to achieve those objectives. OBC systems that use terminations, connectors, power distribution and data telemetry are subject to technical downtime that drives up acquisition costs. The reliability of the OBN systems and the paucity of technical downtime allows more emphasis to be placed on improving the operational performance of the crew. Additionally, OBN acquisition is comparable in cost to wide-azimuth (WAZ) streamer for exploration where water depth, surface obstructions, shipping lanes, etc. leave gaps in streamer coverage.
Seismic surveys used for exploration and regional analysis must be efficient and cost-effective to acquire. Large-scale OBN exploration surveys are generally characterized by:
- All vessels working at all times;
- Source effort and receiver effort in balance;
- Large receiver area that minimizes repeated shots and efficiently uses blended sources; and
- Minimal impact of startup and ramp-down time.
OBN’s reliability and flexibility enable E&P companies to affordably meet their exploration objectives without compromising data quality and image clarity. Figure 1 shows a typical node-on rope acquisition using blended acquisition.
FIGURE 1. In a typical node-on-a-rope acquisition, nodes are deployed to the seafloor and retrieved using a passive rope. In this case there are two source vessels activating sources independently from each other. The blended seismic records are deblended from the continuously recording nodes after retrieval from the seafloor. (Source: FairfieldNodal)
Superior imaging for field appraisal
As E&P projects mature, the role of seismic data shifts from regional reconnaissance to identification of minimum economic field size. A well-designed seismic survey not only satisfies exploration objectives but also satisfies the requirements of field development and appraisal. The value of the seismic data and the quality of the information it provides directly correlates to reduction of risk and uncertainty in appraisal-stage decision-making, which involves:
- Delineation of economic limits;
- Estimation of reserve volume;
- Well/platform location; and
- Recovery strategy.
Poorly designed surveys, though possibly adequate for exploration purposes, rarely deliver the kind of data required by modern processing algorithms that are essential for field appraisal. Figure 2 is a comparison between collocated lines of a narrow-azimuth OBC survey and a WAZ long-offset OBN survey from the Gulf of Mexico. The images have been processed with similar imaging algorithms to identify the differences attributable to the acquisition only. The OBN image is clearly superior in fault definition, signal-to-noise ratio (S/N) and frequency content.
The true value of an OBN survey must be measured in the context of total cost of ownership of the survey. Technology-driven E&P strategies are highly dependent on quality seismic data. The costs of a mispositioned appraisal well, an unidentified opportunity or even the cost of reacquisition of an inadequate seismic survey dramatically impacts project economics. Total cost of ownership must also factor in safety and environmental risk exposure. Each mobilization of a seismic field crew or unnecessary or unplanned well location adds safety and environmental risk. OBN acquisition provides the lowest risk, highest reliability and highest quality image, resulting in the lowest total cost of ownership compared to seismic data acquired with any other system.
FIGURE 2. In this data comparison between (A) WAZ OBN and (B) narrow-azimuth ocean-bottom system data from the Gulf of Mexico, note the differences in image quality and sharp fault definition in the OBN data. (Source: FairfieldNodal)
Repeatability for development, EOR
The role of seismic in late-stage life of field shifts to reservoir management and surveillance. The technical challenges of reservoir management involve early and accurate characterization of the reservoir in terms of volumetrics, pore fluid identification, lithology/facies prediction and continuity. As field development progresses, the development team must extrapolate and calibrate measurements taken at sparse well locations, such as pay thickness, porosity and hydrocarbon saturation, onto the seismic grid. The essential link and integration between petrophysical data, well performance data and seismic data is generally achieved through various forms of seismic attribute generation and analysis.
Denser WAZ geometries such as those acquired with OBN surveys unlock the power of complex attribute generation, signal processing algorithms and wave equation imaging techniques that feed into reservoir characterization technologies. The attributes of a node survey that directly affect the quality of a seismic image and, therefore, the quality of the reservoir characterization are:
- Better imaging quality through long offsets and WAZ illumination;
- High fold;
- Dense spatial sampling;
- High S/N;
- Excellent vector fidelity;
- Multicomponent receivers; and
- Highly repeatable acquisition.
In the last stage of the life of field, monitoring the changes in reservoir properties over time and the resulting change in seismic response gives critical information to the reservoir management team. The reservoir properties that change over time include pore pressure, pore fluid type and saturation, porosity, and density.
The change in seismic response over time, known as the 4-D response, and the degree to which it can be measured depends on the quality of the initial or baseline survey and the ability to repeat the monitor survey. In the case of OBN acquisition, node positions often are acoustically monitored and, if necessary, adjusted as the node is deployed. Four-dimensional repeatability also benefits from “carpet-type” shooting as positioning constraints can be relaxed in very high-density OBN acquisition geometries. The reliability of node positioning, improved access around surface obstructions and superior image differentiates OBN from any other ocean-bottom system for 4-D seismic.
Throughout the life of field, seismic data play a critical role in regional reconnaissance, field development planning, economic risk reduction, reservoir management and production monitoring. A well-designed OBN survey provides the seismic image clarity that informs critical decision-making at each stage of life of field.