As the oil and gas industry continues the trend towards instrumented oilfields and smart well completions, more and more operators require real-time information on geomechanical processes taking place within the reservoir, in the areas remote from the actual wellbores. Passive monitoring of acoustic emissions (microseismicity) associated with stress changes within and around the reservoir can also be used to image the reservoir’s dynamic state, providing operators with real-time feedback of the reservoir’s response to EOR activities, including waterflooding or fluid injection.

The Middle East was one of the first geographical areas to adopt ESG’s ResMap™ (real-time microseismic reservoir monitoring) system for production enhancement. In this particular case, a passive seismic monitoring solution was developed to optimize waterflood treatments for a mature oilfield in Oman.

Challenge

The client was looking for ways in which they could optimize their waterflooding operations. They were interested in high resolution delineation of underlying reservoir structures, and obtaining useful information to assist in reservoir development and management decisions.
When conducting EOR treatments, the client wanted to:

• Ensure they were maintaining reservoir pressure
• Avoid ‘wetting out’ situations, (loss of control of waterflood conformance)
• Mitigate fault and fracture reactivation
• Monitor caprock integrity

The client was particularly interested in identifying any fault structures that might have had an impact on planned water injections. Attempts to use a conventional 3D survey were thwarted however, due to the reflectivity of a high velocity, shallow hard carbonate formation. ESG’s real-time microseismic reservoir monitoring system became the best option to define pre-existing fault structures while also providing response data to water injections.

ESG Solution

ESG was contracted to provide the client with a microseismic system for the purposes of monitoring waterflood activity. Primary objectives included monitoring for microseismicity to track deformation and stress changes associated with fluid injection/extraction within the reservoir and identifying and assessing the existence of pre-existing faults and fractures on the advancing waterfront.

The first step in the process was to create a feasibility study of the area. An initial velocity model, based on VSP data and a dipole sonic log recorded in a nearby well, was used to calculate location uncertainty and array detection limits. The finalized velocity model (Figure 1) was incorporated into ESG location algorithms which were used to calculate hypocentral event locations for the acquired microseismic dataset. 

Based on the feasibility study, a custom monitoring system was developed. The ResMap™ system was comprised of a network of seven shallow wells with triaxial geophones; two deep wells with triaxial geophones; and one observation well with a downhole sensor array containing 6 dual-triaxial geophones levels, spaced 50 meters apart. The proposed seismic monitoring array offered an optimal trade-off between the ability to record relatively large magnitude events particularly on the near surface sensors, while maintaining good sensitivity for recording smaller events on the deeper sensors. 

Acquisition was performed by the Paladin™ seismic recorders, which were located at the wellheads of each sensor location. Digitization of the continuous raw seismic signal occurred at the Paladins and this data was transmitted via wireless connection back to the central event acquisition computers located at a central station. All components at each site were powered using solar panel arrays. Figure 3 provides an example of one of the wellhead acquisition stations. 

ESG’s installation team went out to the site and installed, calibrated and tested the system, using perforation shots to orient the sensors and begin monitoring water injections.

Outcome

Upon the system’s successful installation, water injection tests were carried out at fracturing pressure. The associated microseismic events revealed the existence of a fault plane within the reservoir, while conducting a detailed analysis of the microseismic waveforms, geophysicists were able to use advanced Moment Tensor Inversion processing techniques to identify the components of failure and as a result they were able to define the orientation of the fault structure. Continued monitoring of this fault provided understanding on how the degree of activation and orientation of the structure changed over time and in response to continued injection treatments. Fault behavior was incorporated into the geomechanical reservoir model and became an important component to designing effective EOR strategies.

ESG’s ResMap™ system was also used to determine optimal pressure rates for injections. The client was able to correlate the frequency of microseismic events with a range of pressures to determine at exactly what rate they began to induce fracturing. In this particular case the client wanted to avoid fracturing the formation, so they decreased their injection rates until there was no detectable microseismicity, ensuring optimal pressure rates.

Finally, the client was able to use ESG’s ResMap™ system to identify the flood front location of the water injection. The pressure transients caused by a waterflood treatment results in stress redistributions that can be detected by the sensor arrays. In this instance, the client was able to use the microseismic event locations (in conjunction with an injection pressure increase) to identify the waterflood front and infer how the water was moving through the reservoir.

ESG’s ResMap™ solution can be custom designed to monitor a variety of EOR treatments, providing enhanced understanding of the dynamic subsurface processes occurring within the reservoir.