Microseismic monitoring provides direct 4D information about stress changes in a reservoir by recording seismic waves generated during very small slip events.
Relatively new to the oil and gas industry, this technology can map the path of fractures from the wellbore into the reservoir as they are created by hydraulic stimulations. When used with recently developed real-time monitoring technology, microseismic information helps engineers optimise production and mitigate risk throughout the life of the field by optimising hydraulic fracturing jobs on the fly.
Previously, microseismic data from a produced or stimulated well was acquired using a tool positioned in a separate well often drilled for that purpose. Today, an innovative technology that reduces noise associated with fluid flow allows this data to be obtained in active treatment or production wells.
This new technology was the subject of a Schlumberger presentation at May's 2008 Offshore Technology Conference in Houston.
Used with the company's permanent seismic sensing system (PS3), an Omega-Lok device couples geophones to the inside of casing as part of the well completion. It also decouples the sensors from the tubing, minimising completion noise passively while maximising formation coupling. A hydraulically activated release mechanism is used to deploy the fit-for-purpose, low-noise-floor sensors. The PS3 tool is connected to the surface by a quarter-inch electric cable passed through the tubing hanger and connected to a surface acquisition unit placed close to the wellhead.
Data from field tests and a recent commercial deployment of the PS3 system showed noise reduced by a factor of 100 compared with other tubing-conveyed monitoring methods.
According to the Schlumberger presentation(*), the system enables long-term monitoring without the cost of drilling offset monitor wells and exploits the full value of microseismic information on a reservoir-wide scale.
This latest announcement comes on the heels of recent news about the performance of the live-well PS3-FW system and the retrievable monitoring-well PS3-MW system during field trials in Abu Dhabi.
Schlumberger believes that PS3 could open up much wider opportunities for borehole seismic instrumentation and reservoir monitoring within the oil industry. With the seismic sensors acoustically de-coupled from the flow noise, and well coupled to the formation, the noise floor is dramatically reduced, allowing detection of much smaller and more distant seismic/microseismic signals, says the company.
The PS3-FW decoupling device, known as the Omega Lok, is held in a compressed state during the running-in operation. Once the tubing string is in place, the release mechanism is activated by elevating the pressure in either the tubing or the annulus or via a hydraulic line.
The Omega-Lok device is then released from the tubing and pushes itself against the inside of the casing, thus becoming extremely well coupled to the formation. The whole process is analogous to setting a packer except this device no longer touches the tubing. Once deployed the sensors are totally decoupled from the production tubing whilst being well coupled to the formation.
The Abu Dhabi trials also included the new PS3-MW version. This retrievable system is based on the PS3-FW technology, but is for use in monitoring wells. Here the Omega-Lok is replaced by tubing eccentralisers that achieve the acoustic coupling of the sensors to the casing.
PSET down the well
Meanwhile, another company that claims to lead the industry in surface-based microseismic monitoring and passive seismic imaging has also been busy on the new technology front.
Houston-based MicroSeismic Inc (MSI) is using its proprietary passive seismic emission tomography (PSET) technology – which utilises wide aperture arrays, either on the surface or in the near-surface – to map the microseismicity associated with hydraulic fracturing and other dynamic reservoir activities.
PSET arrays obviate the need for monitoring wells and provide the capability to map the full extent of frac growth and fault activity with relatively constant resolution over a large survey volume. MSI has performed work in many of the unconventional resource basins of North America. Already well established in hydraulic fracture monitoring, MSI is expanding its services to include PSET based reservoir monitoring to assist the operators of unconventional resource plays better understand and develop tight oil and gas opportunities.
PSET uses a FracStar array of surface detectors to locate very low level acoustic energy emissions associated with hydrocarbon producing activities. There are many potential sources of seismic energy from within the reservoir, all associated with specific dynamic reservoir reactions to producing activities. Mapping of these energy fields can yield valuable information unavailable from other commercially viable technology.
“Accurately locating discreet, reservoir-level microseismic events is not possible using a small number of surface located geophones as with conventional seismological earthquake location techniques due to signal attenuation by the overburden,” notes the company. “However…a dense FracStar array of surface geophones 'beam steers' or sums the output of the entire array to detect and locate microseismic activity. Mapping production or injection related microseismic events provides an understanding of the heterogeneity of the reservoir and the geological/reservoir conditions from which hydrocarbons are being extracted,” it adds.
Overall PSET allows provision of numerous services, including hydraulically-induced fracture mapping, delineation of reservoir-scale faults or reservoir discontinuities, identification of areas of reservoir compaction, injected fluid front monitoring, and continuous reservoir monitoring.
While all of these services are valuable as stand alone surveys, MSI says that the full value of PSET for producing fields is realised through the use of long-term continuous monitoring: “A one-time PSET survey gives the location of dynamic events as a snap-shot in time for the reservoir. Performing long term monitoring gives a truly dynamic view of critical reservoir management issues.”
According to the company, this means that PSET suffers from none of the drawbacks inherent in conventional reflection seismic 4D surveys, such as differing quality of data, changes in acquisition or processing technology, changes in the environment such as water column salinity or temperature (for marine surveys), or prohibitively high recurring acquisition costs. MSI adds that the technology can provide information that allows reduction of other, more invasive, costly and risky data collection methodologies such as fluid saturation and production logging, and borehole and ocean bottom seismic surveys.
However, as International Hydrographic and Seismic Search was going to press, MSI announced that it achieved real-time analysis of surface array based microseismic frac monitoring data sets in the field. This breakthrough in accelerated processing and interpretation will enable operators to make better decisions in tight timeframes with reduced uncertainty during their hydraulic fracture well treatments.
Keys to success of this development were the creation of a field deployable processing system with dramatically increased data processing speed and a major breakthrough in the technique for analysing the massive data volumes acquired during fracing operations.
Peter Duncan, ceo of MSI, commented: “Our customers asked and now we can provide a system that has been proven in the field which provides microseismic data analysis with unprecedented speed and clarity. This is a revolutionary step forward for us. We can now provide to our clients the information they need in the field to modify completion programmes to achieve significant gains in well performance.”
(*)Wilson SA, Le Floch G and Jones R: "Monitoring the Oilfield of the Present Using Live-Well Microseismic Technology," paper OTC 19693, presented at the 2008 Offshore Technology Conference, Houston, Texas, USA, May 5-8, 2008.
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