Advanced Technology
Leonard LeBlanc!-- --
Houston
Seismic-while-drilling limited by bit types, risers, well deviation
Seismic-while-drilling (SWD) operations on four wells off Vietnam conducted by BP and Statoil have identified a number of limitations for the equipment and outlined areas for further research and equipment development:
- Only tri-cone and bi-center bits generate the P-waves needed for surface geophone pickup. PDC bits are not effective, and drilling engineers suggest the development of noisy PDC bits.
- Well deviations exceeding 65 degrees generate drillstring-wellbore or drillstring-casing interaction noise, resulting in pilot signal loss. This rules out horizontal wells until downhole sensors and alternative noise sources can be developed.
- Riser noise interferes with the pilot signal, limiting offshore operations to 350-meter depths. Researchers believe riser noise can be overcome with additional equipment and water depth extended substantially.
- The well operations team must be deep integrated with geology and geophysics personnel to provide an effective product, and geology/geophysics teams must be able to provide confidence levels for data.
- Different surface arrays may produce vastly different results and data and additional research is recommended.
- When SWD is employed, careful drilling is not required when searching for coring points and casing seats, and the setting of casing seats was found to be optimal.
SWD involves the use of the drillbit as a seismic source, an array of geophones on the surface at some distance from the well, and an accelerometer on the kelly or topdrive to monitor the pilot signal from the drillbit. The geophone signals are compared with the pilot signal. SWD equipment from both Western Atlas and Schlumberger was used in the four wells.
Reference: Jackson, M., Einchcomb, C. Aluvihare, G., Klungsoyr, T., "Seismic While Drilling Operational Experiences in Vietnam," Downhole Talk, October, 1996.
More methane, less clathrates
Over millions of years, low temperatures and high pressures on the ocean floor created the right conditions to trap rising gas (primarily methane) by forming frozen gas-water clathrates (or hydrates). Frozen clathrate pads formed over large areas of oceanic seabed throughout the world. Early investigations of these clathrate pads suggested they were a far greater source of energy than all the natural gas accumulations drilled to date. However interesting, the clathrate accumulations generally resided at great ocean depths and there was no economic method to recover them.
Recent research may help in the eventual recovery of these frozen resources. Scientists working on a Deepsea Drilling Project hole in the western North Atlantic Ocean found that the absence of bottom seismic reflectors beneath clathrate pads did not indicate unsaturated clathrates, as formerly supposed, but free methane and water. These results, if projected globally, show that gas clathrate accumulations may be over-estimated by a factor as high as three.
Interestingly, the same drill core/bottom reflector findings show a gradual transition vertically to clathrates from free methane flow, and that free methane accumulations under the clathrate pads are far larger than previously supposed (ref. 1).
Here is an intriguing and speculative production scenario for the future: Wells to tap these methane accumulations could be completed with only an insulated production string cemented to the frozen clathrate cap. As the free gas flows, pressure beneath the clathrate cap should drop and the temperature should rise slightly. The loss of pressure and generation of slight heat should begin breaking up the frozen clathrate formation nearest the methane flow or preferably along the free gas-clathrate interface, gradually releasing additional supplies of methane in the process.
Produced water could be a problem, but ideal placement of the completion string and control of pressure release and warmup may be effective in eliminating coning and at the same time release methane molecules from the clathrate lattice before conversion to water takes place (ref 2). Pressure-release fracturing of the hydrate structure will probably stop at some distance from the production tubing, necessitating a new completion in the gas hydrate cap. Conceiveably, this scenario could create a continuing supply of free methane, leaving wells to produce for decades instead of years. Will the process actually work? Maybe not in this fashion, but its a sure bet the hydrate accumulations can be tapped for energy in the future.
References: (1) Holbrook et al, "Methane Hydrate and Free Gas on the Blake Ridge from Vertical Seismic Profiling," Science, Vol. 273, 27 September, 1996. (2) "Ice resists melting in warm conditions," Science News, Vol. 150, October 19, 1996.
Multiple gradiometers provide additional view under salt
Bell Geospace is industry testing a 3D gravity gradiometry device that apparently produces an improved image of the lithology beneath salt structures. The device was previously used by the US Navy for oceanographic purposes. The device uses three separate gravimeters, set up to measure gravity in different directions. The device is allowed to free fall through the water column, and the contrasting changes in gravity among the three are measured and computed. This provides a 3D picture of gravity changes in direction and an important view of the subsalt features and contents.
Synthetic gas-to-liquid conversion cited as solution to remote gas
A process to convert natural gas to synthetic crude oil and transportation fuels developed by Syntroleum Corporation is now in the commercial realm. Texaco has signed an agreement to impliment cost-saving improvements to the process. Texaco has not indicated application sites to implement the process, but it is expected to begin with stranded gas discoveries. The process involves the use of several catalysts to take the gas through a sequence of conversion processes. The process effluent is free of metals, particulates, aromatics, and sulfur, according to the developers.
Copyright 1996 Offshore. All Rights Reserved.