The Promise of New Reservoir-Monitoring Technologies

Norm Warpinski, Chief Technology Officer, Pinnacle Technologies

Reservoir monitoring is a fast-developing area of production technology that has the objectives of measuring, controlling, and predicting the performance of the reservoir. From the measurement perspective, reservoir monitoring may include temperature, pressure, flow rate, composition measurements, periodic logging, 4D seismic, microseismic, de-formation measurements, and a host of other possibilities. Most of the measurements made in reservoir monitoring today are wellbore-specific, targeting the conditions in a production or injection wellbore. On the other hand, technologies such as 4D seismic, microseismic, and surface deformation have the potential to image a volume of the reservoir away from wellbores, providing an understanding of how the reservoir is performing as a complete system. It is these imaging technologies that hold great promise for the future improvements in the understanding of overall reservoir performance. It is my opinion that as surface-deformation monitoring is better understood by the industry, there will be a trend toward its use over 4D seismic on many onshore fields because of its low cost, fast turnaround, and wide areal interrogation of the reservoir.

The first recorded instance of surface-deformation monitoring that I am aware of was associated with the disposal of low-level radioactive waste in cement fractures at Oak Ridge in the 1960s. The leveling-type measurements were relatively crude, but the fractures were shallow and wide, and the results were reasonably good. Today, surface-deformation measurements are quite sophisticated, using tiltmeters for precision measurements, interferometric synthetic aperture radar (InSAR) for large-scale reconnaissance, and global positioning systems (GPS) for a host of intermediate and combined applications. Currently, these types of measurements are limited to onshore applications, but subsea measurements are possible in some cases.

Tiltmeters measure the horizontal gradient of the vertical displacement with great precision (up to one nanoradian), and an array of tiltmeters properly situated over a reservoir can be used to extract the surface deformation that is taking place because of processes occurring deep underground. These tools are sensitive enough to pick up Earth tides caused by the moon’s pull on the Earth’s crust over time. Results can be processed to calculate reservoir-level volumetric changes. GPS and InSAR, on the other hand, measure the actual displacement of the surface, but with considerably less precision. InSAR has the least resolution of the techniques, but it has great appeal for the large areal extent that can be covered and minimal (often zero) ground equipment. Being able to identify subcentimeter ground deformation over hundreds of square kilometers for a few thousand dollars has great appeal.

Surface-deformation monitoring has value because all of the downhole processes that we employ to extract hydro-carbons create changes in the reservoir that are manifested in surface movement. Thermal processes, in particular, generate large volumetric strains in the reservoir that are easily detected on the surface, but primary production, waterflooding, fracturing, and other enhanced-recovery technologies also result in volumetric changes that can be monitored from the surface. In some applications, a downhole array of tiltmeters can be employed to monitor deformation at the reservoir level, as well as at the surface, to provide an enhanced view of the reservoir changes.

Clearly, if we can monitor the volumetric changes in the reservoir, then we can track fluid fronts, thermal fronts, reservoir compaction, fracturing behavior, effectiveness of completion techniques, and many other stimulation and production operations. In many applications, this type of technology can be used as an alarm system for monitoring possible surface breaches, movement toward aquifers, and other situations. This near-real-time monitoring and alarm ability will become very important in the near future for advancing CO2 sequestration and waste-injection projects by giving the public assurance that the industry can safely dispose of various waste products. In some areas, these technologies are becoming legislatively mandated because some unmonitored operations in the past resulted in surface breaches or out-of-zone injections.

In the case of CO2 sequestration, containment verification for hundreds of years is conceivable. For CO2 sequestration to become a viable option for the disposal of greenhouse gases, low-cost and timely long-term monitoring is a must. Acquisition and processing of tiltmeter and GPS data is a fast process that can be performed in relatively short time, giving near-real-time information on downhole conditions. Conversely, 4D seismic monitoring is performed only periodically and, at best, the acquisition, processing, and interpretation may take weeks or months.

For deformation data to be useful, it must be integrated with other production and injection information such as rates, volumes, and well-operational sequences to understand the significance of the induced reservoir changes. Integration with other imaging techniques is also invaluable for observing the reservoir behavior from different perspectives. Examples such as integrated surface deformation and downhole microseismic data exist in the literature. Combined surface-deformation data and downhole-distributed temperature measurements also offer promise for more accurately understanding reservoir behavior, particularly in injection operations.
Because the future inevitably encompasses a move toward more-marginal reservoirs and more-complicated production technologies, the use of reservoir monitoring strategies will continue to grow and to be an important element in optimizing completion, stimulation, and production activities. Surface-deformation monitoring, a direct indicator of the changes occurring within the reservoir rock, is likely to become an ever-larger piece of monitoring efforts and a key provider of information to guide process control.

Norm Warpinski, SPE, is chief technology officer for Pinnacle Technologies in Houston, in charge of developing tools and analyses for hydraulic-fracture mapping, reservoir monitoring, hydraulic-fracture design and analysis, and other aspects of reservoir development. He previously worked at Sandia National Laboratories from 1977 to 2005 on various projects in oil and gas, geothermal, carbon sequestration, waste repositories, and other geomechanics issues. Warpinski has extensive experience in various types of hydraulic-fracture mapping and modeling and has been involved in large-scale field experiments from both the hardware and software sides. He also has worked on formation evaluation, geomechanics, natural fractures, in-situ stresses, rock behavior, and rock testing. Warpinski earned a BS degree in mechanical engineering from Illinois Institute of Technology and MS and PhD degrees in mechanical engineering from the University of Illinois, Champaign/Urbana.