GEOFRAC-Fracture Monitoring System
Downhole seismic tools identify, locate and map fractures using passive and active seismic monitoring technologies. The passive microseismic monitoring records the energy released from fractures induced or reactivated through stress changes during reservoir treatment or exploitation. The active seismic survey records the energy from a man-made active source travelling through the rock volume of interest. Changes in amplitude and propagation velocity, as well as reflections from geological interfaces are used to interpret the structure of the surveyed volume. E.g. VSP, cross-well surveys, seismic surveys to map the geological structure in the surveyed rock volume.
The GEOFRAC system consists of a GEOREC-S acoustic downhole tool, a surface acquisition rack with a special software designed to determine the orientation and extension of fractures induced by hydraulic stimulation or existing fractures worked on by fluid injection. The GEOREC-S tool is fitted with a triaxial accelerometer, pressure and temperature gauges. The tool is attached to a standard seven conductor logging cable and lowered either down the well to be fractured or down the injection well. Digital signals are high-speed transmitted from the tool to the surface acquisition system. During falloffs following hydraulic fracturing, the GEOREC-S tool records, via the accelerometers, the seismic activity associated with the closure of the fracture. Fracture mapping is performed by locating the different micro seismic events, their location being based on the interpretation of the P/S-wave succession, assuming the velocity model is known. Thereby these data are interpreted in term of azimuth and expression of the fracture taking into account the layer properties.
The GEOFRAC system is very useful in the following applications.
* Enhanced Geothermal Systems.
The injection of pressurized water at the Enhanced Geothermal Systems (EGS) can potentially induce seismicity felt on the surface. The GEOFRAC allows to image the induced fracture network for heat transfer and establish alarm systems for the potential induction or triggering of higher magnitude events.
* Carbon Sequestration
Carbon dioxide is permanently stored through its injection in depleted geological reservoirs. The changes in pore pressure induce stress changes with the potential of fracturing outer formations or inducing felt seismic events. The GEOFRAC allows to monitor the integrity of the caprock and containment of injected CO2. and establish alarm systems for the potential induction or triggering of higher magnitude events.
* Underground Gas Storage
As for Carbon sequestration, depleted underground reservoirs are used for the storage of natural gas. A recent offshore project in Spain resulted in the triggering of a series of felt tremors and a M 4.2 earthquake. The GEOFRAC allows to monitor reservoir integrity and containment of stored gas and establish alarm systems for the potential induction or triggering of higher magnitude events.
* Waste Water Disposal and Earthquakes
The majority of induced seismic events have been associated with waste water injection. A close correlation has been observed between water injection wells and the location of MS events and earthquakes of magnitude 2.0 or greater. (United States Department of Energy).
* Underground Coal Gasification
Unmineable coal seams are converted into gas and extracted for the production of energy. This process induces volume changes that can potentially result in subsidence. The GEOFRAC allows to image fracture development and rock displacements in the treated volume for early detection of subsidence.
Temperature : 180°C (356°F)
Pressure : 1,500bar (21,500psi)
Maximum number of levels :48
External diameter : 50 mm (2”)
Total tool length : 1,450mm (57”)
Compatible casing diameter :3" to 13"
Maximum cable length : 6000 m
Distance between levels : 5 m to 50 m
Cable equalization : Fully automatic
Digitalization : 24bits
Dynamic range : 120 dB
Sampling rates : 0.25ms 0.5ms 1.2ms
Input noise : 0.1 microV rms
Gain settings : 24, 36, 48dB
* Assess the development of the hydraulic fracture during a minifrac test.
* Determines the main direction of fluid injection paths in anisotropic réservoirs.