In Petroleum Development Oman (PDO) land environment, areal reservoir monitoring is not just 4-D seismic, as is the worldwide default, but can best be achieved by a combination of various geophysical techniques integrated with well based surveillance methods.

In view of the industry-wide achievements with (geophysical) reservoir monitoring techniques to date, their application should be a consideration for any field development

Figure 1. PDO details experience and plans in enhanced recovery monitoring in Oman. (All graphics courtesy of Shell)
plan. For PDO, enhanced oil recovery (EOR) projects are the prime candidates for the application of geophysical reservoir monitoring techniques because of the expected large acoustic effect, the large value through improved reservoir management and concept optimization, and the large EOR capex investment (surveillance is then only small incremental cost). With EOR techniques featuring prominently in PDO’s mid- and long-term strategy, the use of (geophysical) reservoir monitoring techniques will increase significantly.

An opportunity review in PDO identified several main blockers for time-lapse (4-D) seismic: limited changes of acoustic properties at seismic scale caused by low yearly production rates, poor sweep, or by the stiff carbonate matrix, the dense surface infrastructure, the small areal scale of an injection pattern, the lack of suitable baseline surveys, difficult reservoirs, and the complex geology in the overburden.

For PDO, the combination of different geophysical monitoring methods does work and can predict changes in the reservoir conditions and fluid flow away from the wells. The critical success factor for those geophysical reservoir monitoring projects is the full integration of the results of the geophysical surveillance data and the well-based monitoring data into the dynamic reservoir model.

Involvement at the beginning of a field development program (FDP) by geophysicists is essential for the success of such projects, as tailor-made solutions require adequate attention for project management, scoping, justification, technical design, tendering and contracting from both the geophysics department as well as the asset team.

Geophysical monitoring methods
Geophysical monitoring methods cover a wide range and their application will depend on the physical parameter(s) that change during the production process. Below is a brief summary of methods and their main applicability:
Active seismic methods. There are very powerful in monitoring pressure and saturation changes in porous reservoirs. Due to thermal effects because the change of acoustic impedance, the physical property measured by seismic methods tends to be well above the noise level.
• Time-lapse (4-D) seismic, the best-known geophysical reservoir monitoring tool, is a well established technique in mainly offshore clastic provinces around the globe. Sources and receivers are positioned at surface. A baseline survey is compared to a monitor survey to measure changes induced by production and injection. For reasons described earlier, time-lapse seismic has limited scope in PDO’s acreage.
• Vertical seismic profiling (VSP) methods. Receivers are positioned in a well and the sources are at surface. Although they allow in principle to get a more detailed picture of the reservoir, they are not as well established due to limitations in processing, and imaging.
• The Virtual Source technique (Shell patent, Bakulin and Calvert, “The Virtual Source Method: theory and case study,” Geophysics 71, 2006), which ideally uses geophones deployed in horizontal observation wells, is very promising, as it addresses the processing and imaging issues.
• Cross-well-tomography. Sources and receiver are placed in different wells and produce a high resolution image of a 2-D profile between the two wells.

Passive seismic listening (Microseismic). Receivers, placed permanently in a well, record seismic activity, mainly caused by (re)activation of faults, fractures and barriers within the reservoir.

Surface deformation methods. These methods are well established and aim to detect deformation at surface expressed as a change of slope, a horizontal movement, a subsidence, or uplift. Measurements are carried out by deploying accurate leveling, tiltmeters, precise GPS stations, or by satellite remote sensing (InSar). InSar is cheap and easy, and combined with GPS stations used for calibration, is now standard deformation monitoring method in PDO.

Electromagnetic induction methods. These technologies can be deployed with active source or in a passive mode and measure the conductivity between source and receiver. Although well established, their application in reservoir monitoring is a new concept and tests to date are limited. Requirements in electrical properties of the subsurface and its restrictions to 2-D profiles limit the applicability of the technology.

Gravity measurements. Gravity measurements for reservoir monitoring purposes are currently hardly applied. Deployment of tools could be at surface or in wells. Given their expected depth of investigation of 30 to 160 ft (10 to 50 m), they are closing the spectrum to the methods that measure in and around the well bore.

Despite the number of geophysical reservoir monitoring techniques and their possible combinations, measurements from the wells are needed for calibration. Production metering, temperature observations, repeat saturation logging, gradio and pressure, and stress/strain measurements are some of the many options.

The integrated interpretation of geophysical and well reservoir monitoring data with other reservoir and production data is a complicated task. Generation of and integration with dynamic reservoir models and geomechanical models is required to arrive at a quantitative interpretation of the monitoring data.

PDO experience
Over the last years the geophysics department supported several geophysical monitoring activities for the assets (Figure 1). The success rate of the recent projects clearly reflects the steep (and still ongoing) learning curve for these technologies and is encouraging the application of these technologies more widely.

Projects 1-3. Failures of the early attempts had a variety of underlying reasons ranging from equipment failure, geophysical issues all the way to optimal alignment of the geophysical activities with the complex operations for the development of the reservoirs. The learnings from these projects were fully incorporated in the recent microseismic projects (which have been recording flawlessly since late 2005) and also in the planning of the upcoming full-field implementation of microseismic (scheduled for 2008 and onwards) for one of PDO’s assets.

Optimizing the use of and extracting the relevant information from microseismic data is critical. Integration with other data (geophysical monitoring and well based monitoring techniques, production data, geomechanical models) is required to develop a comprehensive understanding of the reservoir, which will allow effective reservoir management.

Project 4. The success of the integrated use of production/well data with surface deformation and microseismic data in a compacting gas reservoir has been shown by Shell Technology in Bourne, Oates, Maron, “Monitoring Deformation of a Carbonate Field in Oman: Evidence for Fult Re-activation from Microseismic, InSAR and GPS data,” Geophysical Reservoir Monitoring forum, Bahrain, March 2005. An early 4-D (time-lapse surface seismic) attempt in the same field was unrealistically optimistic and reflects the limited understanding of the time-lapse seismic in the industry in the mid-90s (poor repeatability, datasets were not optimized for reservoir surveillance, i.e., they had very different acquisition geometries).

Project 5. The steam injection pilot in a fractured carbonate reservoir was aerially monitored using borehole seismic, microseismic and surface uplift. The microseismic network suffered from high noise levels. Although the limited amount of events that could be picked are rather scattered, a relationship to steam extent becomes apparent, when they are combined with surface uplift, temperature and production data. The microseismic events surround the region of reservoir dilation (as interpreted from surface uplift data) and are also consistent with the temperature observations. They cluster around fault planes at the southern extent of the steam front. Temperature changes can cause changes in effective stress leading to a destabilization and ultimately slippage of faults and fractures, which generates microseismic events. The combination of microseismic and surface deformation is considered to be critical to monitor the steam injection in this fractured carbonate field to optimize reservoir management.

Investigations indicated that the noise levels in the microseismic network can be reduced substantially. A plan for full field monitoring through combined microseismic and surface deformation is made for the full field steam project expected to commence in 2009 (Project 8 Figure 1).

Project 6. Microseismic networks were installed in two waterflood pilot patterns. As expected, the water injection did not generate microseismic events. A gradual increase of injection rates is currently being tested. Time-lapse seismic effects are expected to be very small in such a waterflood. Areal mapping has therefore been tested with time-lapse cross-well induction logging (EMI). This cross-well EMI, a new technique for reservoir surveillance, shows almost perfect repeatability. In one injection pilot, the zero difference repeat reflects the fact that the water bypassed the matrix through 1 ft (0.3 m) — which is well below the resolution of this technique — highly permeable streak. In another pilot, a 30-ft (10-m) thick water piston can be seen on the time-lapse EMI. Preliminary results are positive, although various processing instabilities still have to be resolved.

Despite all difficulties and challenges, several data sets have managed so far to deliver a proof of concept, particularly the newer microseismic projects, the surface deformation monitoring, and the cross-well EMI for waterflood monitoring.

Upcoming opportunities
The promising results of geophysical reservoir monitoring efforts outside PDO as well as the recent successful proof of concepts in PDO (see Figure 1), in combination with their potentially significant impact and their small incremental cost, suggest that these technologies should be considered for all FDPs for planned EOR activities.

Geophysical reservoir monitoring is currently being designed for four fields (Projects 7 through 10 in Figure 1), where steam injection is expected to start between 2007 and 2010.
The two fractured carbonates fields will be producing using steam-assisted gas-oil-gravity drainage (saGOGD). This production process depends on steam in the fractures (thus heating the matrix and creating gravity drive for oil from matrix into fractures). Full field implementation of time-lapse seismic is depending on the outcome of an ongoing stepwise de-risking driven by the uncertainty in the acoustic effects for saGOGD in carbonates. Microseismic and surface deformation are proven techniques in this environment and detailed design is ongoing.

Several heavy oil sandstone fields will be producing using steam-pattern flood. Initial pilots are currently planned in two fields. Time-lapse seismic is expected to work well in these fields. However, the steam flood patterns are rather small, making resolution of the geophysical methods a key issue. A microseismic network is considered, but depth of the reservoir and the nature of the reservoir are concerns that need to be addressed. Surface deformation monitoring will not be considered for the pilot projects due to the depth of the reservoir and their limited areal extent (it will be looked at for the full field monitoring).

Reservoir monitoring process
The process for execution of a geophysical reservoir monitoring project is linked to the development project process. Early involvement (at concept selection stage before the drafting of a FDP) of geophysicists is essential for the success of reservoir monitoring, as tailor-made solutions require adequate attention for project management, scoping, technical design, tendering, and contracting from both the geophysics team and the asset team.

Based on recent experiences, a five-step approach evolved for geophysical reservoir monitoring projects.

Opportunity screening and selection of relevant technologies. This phase can only seriously start once the development concept (e.g., waterflood with five spot or a crestal steam GOGD) is selected. A feasibility study to evaluate the potential of each monitoring technology is followed by a selection and ranking of relevant technologies. Input from the assets is essential. The “value of information” is assessed before final approval from management is obtained to include the selected techniques for consideration in the FDP.

Detailed design. Selected/high-graded technologies will be taken through a thorough feasibility study, and, if supported, a detailed design and preparation of an implementation plan will be prepared.

Implementation. This includes contracting, procurement, project management, and installation of relevant hardware. Good project management and planning is critical to success of a geophysical reservoir monitoring project. This is because of the multiparty, long-term and non-conventional nature of a project such as microseismic; it requires coordination between many different disciplines such as well engineering, operations, health, safety and environmental, communications/telecoms, computing, processing, and drilling.

Data acquisition and processing. Any baseline acquisition should be completed before first oil or first steam.

Detailed integrated interpretation. Although impressive results have been achieved to date both in PDO and Shell, the integrated interpretation of monitoring data is subject to ongoing development.

What next?
The encouraging successes of geophysical reservoir monitoring activities in PDO, Shell and the industry are the basis for PDO’s substantial project portfolio for years to come. However, a focused effort is required to standardize workflows for a systematic and fully integrated analyses of monitoring data to ensure that the acquired data will be used to its full extend and ultimately supports reservoir management decisions.

Geophysical reservoir monitoring will continue to be an integral part of PDO’s EOR projects and is expected to be an enabler for proactive reservoir management.