Technology
PSET®
Passive Seismic Emission Tomography, PSET®, utilizes 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 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. However, MicroSeismic, Inc.'s proprietary PSET® technology utilizes a dense FracStar® array of surface geophones to "beam steer" or sum 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.
PSET® technology allows provision of the following services:
- Hydraulically induced fracture mapping
- Delineation of reservoir-scale faults or reservoir discontinuities
- Identification of areas of reservoir compaction
- Injected fluid front monitoring
- Continuous reservoir monitoring
Hydraulically induced fracture mapping
Wellbore-deployed geophones have been used extensively by the industry to monitor discreet microseismic events for a number of years. Broad application is limited due to the lack of available wellbores in many new or expanding tight gas basins. Surface based PSET® monitoring provides much the same information as a downhole survey without the wellbore entry requirements. Wellbore-based systems have a detection threshold of approximately 1500' for discrete events, thus introducing a directional bias or uneven monitoring volume if the observation well is not ideally located. FracStar® arrays are designed for each specific job, with the ability to build out the array to monitor a reservoir volume of any size. This provides full coverage of the induced frac geometry and also allows mapping of associated geological features such as re-activated faults. PSET® processing of the passive data allows for full definition of the 3-D geometry of the induced fractures and relating the fracture growth to time dependent pumping characteristics.
Delineation of reservoir-scale faults or reservoir discontinuities
Conventional reflection seismic mapping of fault systems has several limitations. Resolution is restricted to the minimum wavelength of the active source, and only a static view of the fault system is gained. Most reservoirs contain faults that can not be resolved by conventional seismic surveys, faults that influence reservoir performance in unexpected ways by either inhibiting flow or providing flow pathways. Production or injection can create pressure differentials that re-activate small, pre-existing faults or fractures, also creating unanticipated performance from the reservoir.
MicroSeismic, Inc.'s PSET® technology can be utilized to image areas of heterogeneous behavior at the reservoir level. Faults or discontinuities that form flow barriers create unique microseismic signatures as pressure and fluid fronts are prevented from progressing in expected patterns. Faults acting as flow conduits exhibit significant microseismic activity as the moving fluids cause fault creep and failure. Active (or re-activated) faults produce microseismic energy from the small-scale rock failures associated with fault creep.
Identification of areas of reservoir compaction
The depletion of fluids from a reservoir causes pressure drops within the fluid matrix. If there is insufficient influx or injection of replacement fluids, overburden weight will be transferred to the rock matrix in the reservoir. For most classes of reservoir rock that have been subjected to tectonic forces, the rock matrix is already at critical stress levels. The additional stresses caused by fluid depletion will precipitate matrix failure, creating localized reservoir compaction. This failure produces a rich microseismic energy source, with field testing showing tens to hundreds of events a day in fields where compaction is occurring. The literature indicates that zones of higher compaction correlate to zones of higher depletion.
MicroSeismic, Inc.'s PSET® arrays can locate these areas of compaction, thus providing invaluable information regarding drainage areas for individual wells, fault blocks, or entire fields without resorting to expensive, risky and limited success wellbore solutions.
Injected fluid front monitoring
Primary, secondary and tertiary recovery of hydrocarbon reserves frequently involves the movement of one or more non-hydrocarbon fluids through the reservoir, movement that creates pressure transients at the flood front that can induce microseismic activity. Monitoring this microseismicity with PSET® technology gives a direct indication of the flood front location.
Water injection, gas re-injection and aquifer influx usually increases reservoir pressures depleted by oil and gas production. Steam and CO2 injection de-viscosifies and increases the formation volume factor (swells) of various qualities of crude oil. All of these drive and depletion mechanisms create microseismic activity because they change the existing stress fields as the flood front sweeps through the zones of lower pressures. MicroSeismic, Inc.'s PSET® surveys can map the areas of microseismic activity, which in the case of active reservoir drive mechanisms would correlate to fluid front location. PSET® surveys provide an inexpensive, low risk, fast and high quality method for determining the zones of active flood front advance, both for a one-time picture and on an ongoing basis for long term monitoring.
Continuous reservoir monitoring
All the above listed services are valuable as stand alone surveys. However, the full value of PSET® for producing fields is realized 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. PSET® suffers from none of the drawbacks inherent in conventional reflection seismic 4-D 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. PSET® 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.
MicroSeismic, Inc.'s PSET® technology is set to revolutionize long term monitoring of in-situ reservoir performance. PSET® surveys will allow subsurface teams to significantly deepen their understanding of the dynamic forces at play within the reservoir. PSET® output will provide an invaluable means of history matching reservoir simulation models, reinforcing and building on other tools utilized by the teams. It should be emphasized that each reservoir is unique, and that PSET® results will require close collaboration between MicroSeismic, Inc. and the operating company to ensure that maximum value is obtained from the data by interpreting the results within the context of each unique producing environment.
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