Successful development of oil and gas shale reservoirs usually requires hydraulic fracture stimulation to create induced fractures that connect the producing well bore with naturally occurring, hydrocarbon-filled fracture networks. The use of microseismic monitoring during hydraulic stimulation has proven to be a key tool for improving the efficiency of the fracturing operations by providing reliable information on reservoir parameters important to successful unconventional resource development. An operator would ideally monitor most of the wells drilled and completed early in the development cycle of a shale play because the ensuing cost savings and recovery improvements can then be applied to a larger number of development wells. However, the logistical difficulties and cost associated with repeated deployment of either downhole or surface-based geophone arrays has proven to be a limitation on the widespread and frequent use of these techniques, and so the entire range of benefits presented by microseismic monitoring has not been fully realized to date.

This map shows the approximate location of eight buried arrays installed to provide Haynesville hydraulic stimulation monitoring services. More than 180 sq miles (466 sq km) of the Haynesville is currently being monitored.

Monitoring on a larger scale

In the past, the monitoring of multiple wells in a large development area has been limited due to the costs and the technical limitations. These challenges have now been overcome by recent installations of specially designed, permanent near-surface geophone arrays in the Haynesville, Marcellus, and Bakken shale plays. These arrays have proven both commercially and technically effective at monitoring large areas from 15 sq miles (39 sq km) up to more than 150 sq miles (390 sq km), providing high-resolution stimulation monitoring results at a per-well unit cost far below that achieved by other methods. The low unit cost is allowing operators to monitor each well drilled within the array footprint, thereby accelerating the efficiency gains in cost and hydrocarbon recovery on subsequent wells drilled in the area.

The primary use for these buried arrays has been for hydraulic stimulation monitoring. However, the capability to economically monitor a large number of wells stimulated in a development area also provides the possibility of using several advanced microseismic monitoring applications not available through other methodologies. These new uses include monitoring production-related microseismicity to directly map individual well drainage areas and the building of hydraulic stimulation models with enough “history matching” to be truly predictive tools. In addition, it allows for the determination of the reservoir “interconnectivity” between individual wells to improve how reservoir simulation model predicts field performance and estimates recoverable reserves. According to Peter Duncan, CEO of MicroSeismic Inc. “This technology is changing the way our clients design their frac programs. They are able to better understand the reservoir dynamics and the stress regime, allowing them to make more strategic decisions based on the additional information gained in this process.”

A different methodology

Microseismic monitoring has historically been achieved using downhole or surface-located receiver arrays. Downhole or reservoir-depth geophone arrays of 10-20 receivers arrays, located in an offset well bore less than 1,500 ft (456 m) from the volume being monitored, take advantage of their close proximity to the microseismic signal to record the event waveform on individual receivers at signal to noise (S/N) ratios high enough (S/N > 4) to allow accurate use of first-break and particle motion processing for event location. Surface-based geophone arrays of 10,000-20,000 receivers overcome the large attenuation of microseismic signal caused by the overburden and near surface through the use of proprietary technology to “beam-form” or stack the output from the entire array of geophones, much like the techniques used in modern 3-D reflection seismic processing. The beam-forming technique allows identification and location of microseismic events that are below the background noise levels (S/N < 1) on individual geophone traces. One such beam forming technique, Passive Seismic Emission Tomography, has been successfully extended to provide high-resolution microseismic monitoring using two to six near-surface permanently installed 3-C geophones per square mile to monitor areas of 100 sq miles (260 sq km) and more at very low per-well costs. Figure 1 shows a typical 25-sq mile (65 sq km) Haynesville installation.

This figure shows buried array results from a Haynesville hydraulic stimulation monitoring job. Events are color-coded by stage and sized by event magnitude.

To date, nine permanent systems, known as “buried arrays,” have been installed in the Haynesville and Marcellus gas shales (all ~ 25 sq miles of coverage) and one in the Bakken oil shale (> 150 sq miles). Figure 2 shows the approximate location of the eight Haynesville buried arrays.

Once installed, commercially available acquisition equipment is used for continuous recording during the stimulation treatments of any well within the array footprint. Multiple wells can be stimulated and monitored at the same time. Real-time acquisition and processing is also available with specialized radio transmission equipment.

Maximizing recovery efficiency

For most unconventional resource plays, well bores are drilled perpendicular to regional maximum horizontal stress to generate propagation of induced fractures normal to the wellbore azimuth. This is generally regarded as critical to maximizing recovery efficiency. Buried arrays are currently being employed in the Haynesville to provide detailed mapping of these stresses on a local level and have already shown that the orientation of the stresses can vary considerably over relatively small distances. This is critical knowledge for development drilling as the optimum wellbore azimuth can now be tailored to specific portions of the field.

Figure 3 shows typical results from a Haynesville well monitored using a buried array. Induced fracture growth is predominantly in the direction of maximum horizontal stress. Stage spacing appears to be optimized in this well, with little overlap or gaps between stages.

Microseismic monitoring using a buried array provides high-resolution microseismic mapping that is being relied upon to guide subsequent well locations, wellbore azimuths, well spacing, stage spacing, and fluid/proppant rate optimization. Operators are gathering detailed geological and engineering data early in the field development cycle and over a much broader area than possible with conventional monitoring techniques, thus accelerating the maximization of hydrocarbon recovery and the optimization of drilling and completion costs.

The large two-dimensional aperture of the buried array and the significant reduction in background noise due to geophone burial allows for the direct observation of a number of microseismic events discretely across most of the array. Analysis of these events provides the source mechanism for the microseismic events, allowing determination of the event magnitude, the type of fracture plane movement (such as strike-slip), the orientation of the fracture plane (dip and azimuth), and the direction of fracture movement (rake).

This knowledge, combined with the understanding of the orientation of the overall fracture propagation during stimulation, provides critical information about the interaction between induced fractures that generally propagate in the direction of maximum horizontal stresses and the reactivation of natural fractures which are usually oriented in some direction other than current maximum horizontal stress. The buried array aperture also allows for differentiation between the shear and dilation components of the fracture movement. Mapping the dilation or opening of the fracture walls may help provide an understanding of where the proppant has been placed.

More than just frac monitoring

Active use of a buried array opens up exciting new applications for microseismic monitoring.

Research work is currently underway in a US basin to determine if production from a well completed in a gas shale produces enough reservoir stress change to reactivate natural and induced fractures. A buried array is being used to monitor the microseismicity generated during the first two to three months of gas production from a series of new wells. Production-related microseismic activity is at its highest during this period because the large difference in wellbore flowing pressure and the contributing fracture network pressure will generate the highest stress changes experienced during the well’s production life. Production-related microseismicity should be generally limited to fracture networks actually producing gas, so it provides a direct measure of the drainage area for of any well within the array footprint. Knowledge of individual well drainage areas would allow for true optimization of development economics by providing the ability to eliminate unneeded wells and completions activity while maximizing overall hydrocarbon recovery.

Monitoring a significant number of wells with a buried array early in the development cycle will also generate improvements in the accuracy of both hydraulic stimulation and reservation simulation models far beyond that which is available using conventional monitoring techniques. Extensive microseismic mapping of the interaction between individual frac stages and the local rock fabric within the buried array footprint, combined with core, well and production logs, 3-D seismic structural and attribute volumes, and local or regional geomechanical knowledge, will allow for statistically valid “history matching” of the models and ultimately the creation of a model that allows completions engineers to accurately predict the behavior of fracture creation and growth in that area. Multiwell monitoring using buried arrays will also allow reservoir engineers to better model the long-term production behavior of a gas shale reservoir by directly mapping the interconnectivity between offset wells and the interaction of producing wells with the natural fracture and fault systems. Incorporating the variability and connectivity mapped during full field microseismic monitoring into the input for a gas shale reservoir simulation model will provide more robust estimates of the true volume of reservoir contacted by the development wells and the anticipated production rates and expected recovery of hydrocarbons.

Long-term benefits

This figure shows a typical near-surface, permanently installed geophone array used for hydraulic fracture monitoring and long-term reservoir monitoring in the Haynesville gas shale. (Images courtesy of Microseismic Inc.)

Microseismic monitoring has evolved over the last decade to become an integral part of unconventional resource development. Permanently installed buried arrays are beginning to play a vital role in the future of unconventional resources development as they are installed over an increasing number of fields worldwide. The information provided by these specialized installations will significantly further the science of unconventional resource development and will lead to very satisfactory investments through the overall improved recovery and completion efficiency made possible.