What's new in exploration

Sometimes, when things get a bit slack, the mind wanders. That happened a few weeks ago when I came across the theory of Krauklis waves. First proposed by Pavel Krauklis in 1962, the wave is a “slow dispersive mode that propagates in a fluid layer bounded by elastic media” (http://adsabs.harvard.edu/abs/2011AGUFM.T51I..07K). It, theoretically, is a special wave mode bound to and propagating along fluid-filled fractures, and it may also influence seismic body waves from seismic surveys, according to the Swiss Federal Institute of Technology Zurich.

The rock physics site (www.rockphysics.ethz.ch/research/krauklis) has a visualization of Krauklis wave propagation. The wave is “distinct by its large amplitudes, high dispersion and confinement to the fractures filled with fluid” (Goloshubin, G., et al, EAGE, Saint Petersburg, 2012). “They can propagate back and forth along a fracture and emit a periodic seismic signal. Seismic data may contain this characteristic frequency and eventually reveal fracture-related petrophysical parameters of the reservoir” notes ETHZ.

According to the Harvard study referenced above, results indicate that the “Krauklis wave has dominate amplitudes compared to all other waves and can store most of the energy of seismic waves in fractured media.” This could make it an impediment to seismic surveys, but it could also be a useful tool as well.

While discussion of the existence of Krauklis waves and their value is still in initial stages, some thought has been given to their utilization, especially in fracture evaluation and, potentially, in cross-well tomography. According to scientists familiar with the theory at Lawrence Berkeley National Laboratories, Krauklis waves could provide valuable information about fracture length, width and height and could, therefore, be helpful in hydraulic fracture evaluation. The only hitch is that, so far as I could determine, no one is actively trying to record Krauklis waves yet. It’s a “stay tuned” topic.

That brings up another mind-wandering episode in which I rang a friend to ask if he knew anything about Krauklis waves. We didn’t get far with Krauklis, but he set me onto another promising technology. That technology, being undertaken by Paulsson, Inc., with funding by the Research Partnership to Secure Energy for America (RPSEA) under a new JIP, aims to develop and test a high-resolution, fiber-optic based 200-level, 3C borehole seismic array for deep and horizontal wells.

Previously, in 2011, RPSEA funded Paulsson to develop a fiber-optic based small-diameter, drillpipe deployed borehole vector seismic imaging system to provide high-resolution reservoir images not previously available. Based on the success in developing and demonstrating the technology in the first phase, RPSEA and Paulsson, Inc. have partnered to develop a comprehensive plan for a second phase of the project to manufacture and test an ultra-long borehole 3C seismic array for deep offshore Gulf of Mexico wells and unconventional horizontal onshore shale-oil wells. The new tool for Phase 2 will consist of a slim-hole, 200-level 3C borehole seismic array with a pod spacing of 50 ft, making the proposed array 10,000 ft long. The system has been designed to be deployed in wells to 30,000 ft at pressures up to 30,000 psi and temperatures in excess of 482°F.

Rather than using standard geophones, the system utilizes fiber-optic seismic sensors (FOSS) that provide: the ability to record very high frequencies; large-aperture seismic antenna; accurate P and S wave velocities; high-resolution salt, faults and lithological reflections; and, recording of very small microseismic events (M–4.0). Benefits of the system will include:

• Largest borehole seismic array. The system can deploy up to 1,000 3C seismic sensor pods to 30,000 ft and at 30,000 psi into both vertical and horizontal wells. Current technology can only deploy 40–100 3C pods into near vertical wells to about 20,000 ft at 17,000 psi.
• High-temperature rated sensors. FOSS have been laboratory qualified to operate at temperatures over 572°F. Current technology is capable of operating to about 302°F.
• Outstanding vector fidelity. Better than -80 dB cross axis isolation. Current technology offers about -50 dB.
• Extremely large bandwidth. 0.01–6,000 Hz. Standard geophones operate between about 8–200 Hz.
• High-sensitivity seismic sensors. FOSS are about 50 times more sensitive than industry-standard coil geophones.
• Extreme sensitivity at all frequencies. Field tests have shown that the system can record microseismic events at magnitudes smaller than M-3.5 at frequencies up and over 2,000 Hz.
• Multi-sensor systems. Simultaneously with the fiber-optic seismic 3C vector sensors, the system uses the lead-in fiber to acquire Distributed Acoustic Sensor (DAS) and Distributed Temperature Sensor (DTS) data from the surface to the end of the array. The DAS and DTS technologies provide detailed velocity and temperature information along the entire array and the lead-in cable.
• Intrinsically safe. The system does not require electric power for either the sensors or the hydraulic clamping system.

The JIP will hold a preliminary meeting on May 20 in Houston. The meeting will consist of three topics: 1) presentation of the Phase I results; 2) open discussion of where this technology should go, gaps, and suggestions for improvements; and, 3) interest and structure of a JIP. Those interested in participating can register to attend by contacting Bill Head, RPSEA, or Bjorn Paulsson, Paulsson, Inc.