Monitoring microseismic events provides valuable information about the oil and gas production process. It can help operators maximize production and ultimate recovery and, in a growing number of applications, warns of potential environmental and safety issues. The value of monitoring these microseismic events can be as much about observing how the events change over time as it is about knowing they are happening in the first place, making permanent monitoring the ideal method. However, the expense, planning and design work required for permanent microseismic monitoring systems make it a major commitment.

A new continuous seismic data acquisition service provides a simple, cost-effective method of previewing the microseismic environment to help operators determine if they should install permanent microseismic monitoring systems and the best way to go about it. The CONDAQ continuous seismic data acquisition service from Baker Hughes seamlessly acquires both high-quality vertical seismic profile (VSP) and microseismic data during VSP surveys without an additional wireline logging run, eliminating the costs that this would require.

The service obtains VSP and microseismic data using exclusive infield software to deliver two datasets for VSP and microseismic analysis with no need for extra rig time or operational planning. High-speed wireline telemetry allows data to be recorded continuously without interruption. An automatic process extracts VSP data from the continuous stream. The VSP data, which are equivalent to the data acquired from a series of conventional individual shot records, are immediately available at the well site without compromising data quality or complicating processing.

The value of microseismic monitoring
Operators of U.S. shale plays are familiar with the value that short-term microseismic monitoring offers in hydraulic fracturing operations. Monitoring during the stimulation process is carried out using an array of temporarily deployed receivers. The measurements identify fracture properties such as azimuth, length and height and also indicate stimulated reservoir volume and complexity. These data can be used to monitor the performance of the ongoing stimulation operation as well as optimize future well spacing and design.

In addition to hydraulic fracturing applications, microseismic monitoring can be used to track induced microseismicity that occurs over time, providing valuable information about the nature of what’s occurring in the rock formation. A better understanding of reservoir production can be obtained or the progress of an injection plan observed. It also can be used to monitor well integrity by identifying and analyzing compaction and potential well stability problems. The ability to see substantial microseismic activity in or near a region where it is not expected can raise an alarm and lead to an investigation of what is taking place, the possible consequences and how they might be mitigated. Other applications include monitoring injection and drawdown of underground gas storage to understand cyclic pressure and stress changes on local formations and the possible ramifications for the nearby environment.

Daunting commitment
Because induced microseismic events usually evolve over time, understanding the way they change can be as important to oilfield operations as the events themselves. Consequently, maximizing the benefit of induced microseismic data requires a permanent—or semi-permanent—downhole monitoring system. Yet because of the significant expense, planning and design work required to install a permanent system (and the difficulties involving well access), many operators are hesitant or unwilling to make the commitment without being certain that meaningful microseismic activity is occurring. This means that it’s advisable to conduct a trial and feasibility study to understand if there are potential benefits from using a permanent sensor system. This typically requires a dedicated wireline logging run with associated costs from service charges and rig time plus operational inconvenience.

The new continuous seismic data acquisition service eliminates the need for a dedicated logging run by providing a seamless, cost-effective alternative that allows operators to confidently assess whether using permanent microseismic monitoring is the right way to go. The keys to the service are its advanced high-speed-telemetry seismic receiver systems that allow continuous recording and exclusive infield software.

Doubling up on VSPs
As part of this new method, established VSP techniques are used for geophysical appraisal of the region around a well. The resulting high-resolution 2-D and 3-D images yield important structural and stratigraphic information that assists in understanding the reservoir, including identification of faults and salt flanks. Amplitude-variation-with-offset calibration and anisotropy measurement also can be obtained. Integrated with surface seismic and well-logging data, VSP data can be used to define formation rock properties and pore pressure indicators and identify other reservoir details with high resolution. The results give operators a clearer understanding of the best ways to capitalize on the reservoir.

During the traditional approach to VSP acquisition, seismic data are recorded in bursts while a source is putting seismic energy into the ground. In the interval between these, the receivers are often stationary and acquiring no data, an unproductive use of rig time. By continuously acquiring data through this new service, this time is spent more efficiently recording microseismic data in addition to the VSP data being recorded. The continuously acquired data corresponding to the VSP shots are automatically extracted infield to provide records equivalent to a conventional VSP. These can be displayed, quality-controlled and processed to generate a normal VSP dataset.

A second microseismic dataset consisting of the continuous recordings also is available to be analyzed and used to help determine the viability of permanent monitoring in a well. A number of microseismic indicators can be pulled to characterize the nature of the microseismic environment. For example, the distribution and characteristics of the noise levels in and around the well can be established. Any spontaneously occurring recordable microseismic events also will be captured, providing information about their nature and frequency of occurrence. Combining this with other information gained from the VSP dataset from the same well is highly beneficial. Used with VSP-derived attenuation and velocity data, sensitivity maps can be generated. These show what events can be detected and from how far away. These sensitivity maps can be produced for different permanent monitoring configurations, allowing
for optimization of planned designs.

Recently, the continuous seismic data acquisition service saved 24 hours of offshore rig time in an application to determine the viability of and best techniques for employing a permanent microseismic monitoring system to help ensure well integrity. While shooting a VSP in a well in the same area where proposals for a permanent monitoring system had been submitted, the service obtained both a VSP and a microseismic dataset. Information from the microseismic dataset enabled the level of microseismicity to be assessed and the measurements to be gathered for a variety of parameters useful in refining the plans for a permanent monitoring system. The ability to simultaneously gather VSP and microseismic data eliminated the need for dedicated microseismic monitoring rig time, with savings estimated at $600,000.

Ongoing applications of the new service later this year are expected to further confirm its value in assessing the viability and optimizing the design and placement of permanent microseismic monitoring systems.