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Integrated surveillance is critical for understanding reservoir dynamics and improving field management. A key component of the surveillance is areal monitoring of subsurface changes by use of time-lapse geophysical surveys such as 4D seismic. The complete paper reviews the advances in these technologies with recent examples from the Gulf of Mexico (GOM) and deepwater Brazil.
4D seismic has played a pivotal role in monitoring offshore fields for some time. However, until recently, its application in the GOM had been limited, largely because the effects of the Loop Current and infrastructures make it difficult to repeat the feathering of streamers used in conventional 4D-seismic acquisition. The key for 4D success is to repeat everything in baseline and monitor surveys to make sure the time-lapse difference observed in the data reflects real subsurface changes rather than differences in data-acquisition conditions or processing work flows. To solve the challenge of streamer-positioning repeatability, an operator used ocean-bottom nodes (OBNs) to achieve highly repeated surveys at the Mars Field, and excellent 4D results were obtained. Since then, 4D seismic has been deployed successfully at the portfolio scale in the GOM and that operator’s 4D surveillance strategy has been changed from streamer seismic to ocean-bottom seismic (OBS), which includes OBNs and ocean-bottom cables.
In addition to source and receiver positions, other conditions in data acquisition can play important roles in 4D repeatability—for example, the variation of water properties during a seismic survey and between different surveys including water depth (tides) and seismic velocity in water. Such variations have been a well-known issue that limits the repeatability of 4D seismic, in particular for OBS data where multiple migration is often used. Seismic travel-time analysis is traditionally used to estimate errors and make corrections, but the process is often complicated by coupled factors including tides, water velocities, positioning errors, and the clock drift of OBNs, which collectively make it difficult to achieve precise water static corrections. To address this challenge, a seafloor device called the Pressure Inverted Echo Sounder (PIES) was developed for direct measurement and continuous monitoring of water-column properties during marine seismic surveys. A PIES is deployed at the seafloor and monitors two-way water time by transmitting an acoustic signal and measuring the time it takes to be reflected back from the surface. Such information has been used to make travel-time corrections of seismic data for enhanced data repeatability in the operator’s 4D processing projects.
The 4D-seismic technologies and strategies developed in the GOM were soon deployed in the operator’s deepwater fields globally. The high-quality data allowed monitoring of subsurface changes and influencing of fundamental decisions in reservoir management at an early stage of field life. The information was used to update reservoir models and improve history matching and had an immediate business impact on reservoir management (e.g., optimal adjustment of well rates).
4D-data-quality improvement and clear 4D snapshots of reservoirs can greatly reduce subsurface uncertainties, simplify data interpretation and integration, and provide confidence to make expensive decisions such as drilling infill wells in deep water.
High-quality 4D data not only offer more-detailed reservoir monitoring but also allow observation of subsurface changes in a relatively short period of time. This ability opens up new opportunities for frequent seismic monitoring to understand the dynamic behavior of the field better and minimize delays in important operational decisions. To realize this in practice, the cost implication of frequent monitoring needs to be addressed.
The scalability of OBN acquisition enables flexible survey configurations and target-oriented seismic monitoring. On the basis of this unique feature, the authors proposed instantaneous 4D seismic (i4D), a concept designed to monitor specific targets by use of a small patch of shots and receiver nodes with reduced vessel time and cost. A typical target would be water-injection wells in which rapid time-lapse effects could occur. This technology was first deployed in 2012, when a campaign of i4D surveys was conducted covering all active injectors in the operator’s deepwater GOM portfolio. Efficient survey planning and execution resulted in highly repeated data. Subsurface changes induced by production and injection activities in the target areas were captured clearly, including the first high-quality subsalt 4D signal, which was obtained with only 120 nodes.
For sources, one way to reduce cost is to use small marine sources that have the potential to enable low-cost or autonomous vessels to be used in seismic surveys. The applicable base for this technology includes fields with relatively shallow reservoirs and uncomplicated overburdens—in particular, those in which permanent receivers are installed and the cost for 4D monitoring is primarily driven by the source effort. This small-source concept was tested at a site where a 2,450-in.2 source array had been used routinely for production 4D surveys. To test the small-source scheme, a single air-gun cluster of 360 in.3 was fired in the middle of the source array (between firing large sources) and, simultaneously, a separate small-source data set was acquired in both the 2015 survey and the 2016 survey. The 4D result using the small source was compared with that using the large source. As expected, use of the small source increased the noise level in 4D data. Nevertheless, the noise is largely random, with no effect on the interpretation conclusions, and is considered acceptable for a low-cost solution.
For receivers, a series of low-cost technologies for different surveillance objectives was developed. To combine the advantage of OBN and cable-based permanent-reservoir-monitoring systems, semipermanent OBNs were developed in a joint effort with a major node vendor. Instead of being deployed and recovered in each survey, the nodes are deployed and recovered only once with multiple surveys acquired in between, so that the expensive remotely-operated-vehicle operations are reduced and the total cost is expected to be lower than that of traditional OBN surveys. After each survey, the seismic data are retrieved remotely with a high-speed optical communication system, and the nodes are switched to sleeping mode until the next survey. In 2016, the prototype units of this system were tested successfully in multiple deepwater fields in the GOM.
Seismic data can also be recorded from sensors in wellbores as vertical seismic profiles (VSPs), either with geophones or with distributed acoustic sensing (DAS) with a fiber-optic cable. In comparison with the traditional VSP approach, the DAS-VSP approach is particularly appealing for low-cost frequent monitoring surveys for several reasons:
The concept of 4D DAS-VSP for 3D imaging of deepwater fields was proved by the authors in a survey in the GOM. As part of the field trial, the feasibility of 4D DAS-VSP using small sources was tested and encouraging results were obtained for this low-cost concept. Fig. 1 presents a comparison of 3D DAS-VSP images acquired with different combinations of active elements in the source array, each using a shot box of 9×1.7 km. The depth of penetration (greater than 20,000 ft) and areal coverage are very similar across all source sizes: 5,110, 850, 500, and 250 in.3. Repeated data were acquired with 5,110- and 500-in.3 sources to quantify the 4D noise level, which showed that the average normalized root-mean-square (NRMS) value for the 500-in.3 source down to 15,000 ft is approximately 10%, compared with an NRMS value of approximately 8.5% for the 5,110-in.3 full-volume-production survey source. Considering that the test was performed using a relatively small shot box, further noise reduction of small-source 4D DAS-VSP is possible with increased shot coverage.
4D seismic can be used together with an integrated suite of surveillance tools that includes other low-cost and fit-for-purpose technologies. The relatively expensive seismic surveys are not necessarily acquired at a predetermined schedule but preferably on an as-needed or on-demand basis. A new monitoring survey can be triggered by some other surveillance tool that provides continuous monitoring at a relatively low cost.
Satellite-based remote sensing technologies have been used successfully in high-precision monitoring of surface deformation and subsidence over producing oil and gas fields on land, but they are not an option for deepwater fields. Since 2007, new geodesy measurement technologies have been tested and deployed for long-term seafloor-deformation monitoring. Vertical displacement of the seafloor can be monitored by precise pressure measurement by use of pressure-monitoring transponders (PMTs) deployed on the seafloor, and more-detailed monitoring involving horizontal displacement can be realized with both pressure measurement and acoustic ranging by use of a network of autonomous monitoring transponders (AMTs).
In an integrated surveillance program, not only the reservoir but also the seabed and overburden are monitored to ensure efficient and safe operation of offshore fields, especially those with considerable shallow geological processes or geomechanical changes. To further fill the gap between 4D seismic and PMTs and AMTs for comprehensive field surveillance, sparse OBNs were deployed recently for a 200-day passive recording over an area of interest based on information from 4D seismic and PMTs. Another technology tested recently is high-resolution 4D seismic using short streamer cables (with very closely spaced seismic receivers) and a small source. In addition to seafloor and shallow-subsurface surveillance, this technology can also offer a solution for frequent low-cost monitoring of reservoirs that are shallow relative to the seabed and do not have significant imaging challenges.
Advances in 4D Seismic and Geophysical Monitoring of Deepwater Fields
01 March 2018