Recent developments in borehole geophysics can reduce the hazards and improve the chances for success when drilling for oil and gas. Seismic Telemetry Logging uses sonic pulses and seismic techniques to scan or "look ahead" of the drill bit to determine the character and the structural conditions of the materials to be drilled. Fracture-prone formations and overpressured gas zones can be detected before they are penetrated, thus permitting mud weights to be altered in sufficient time to prevent lost circulation or dangerous blowouts. Additionally, the degree of compaction, the dipping attitude of the bedding and the position of formations below the borehole are determined so that drilling programs may be optimized and good casing seats established.

The same sonic pulses, which monitor the position of the drill bit, may be used to detect deviations from a true vertical drilling path (crooked holes) and guide directional wells to planned objectives. Precise measurements are made of sonic transit times in materials drilled, thus providing a continuous velocity lag, density log and porosity log as the drilling progresses.

Sonic telemetry is used to transmit data to the surface In less than 30 seconds. The results are, therefore, immediately available on the rig floor where they may be used to alter or optimize the drilling program.

Seismic telemetry logging employs conventional seismic instruments with specifically configured surface sensor spreads and a newly developed down hole energy source (pulser) to provide useful borehole data in a timely manner. The energy source is positioned directly above the drill bit and is activated at will without interrupting normal drilling operations. Geologic structures (faults, reefs, sand bodies, salt diapirs, etc.) around and below the borehole can be delineated. The position and magnitude of overpressured gas zones below the drill bit (2000-4000 feet) may also be determined. The position of the drill bit, both vertically and horizontally, can be established through sonic triangulation with acceptable accuracy at each pulsing interval (30 feet). This provides sufficient deflection data for guiding directional wells to prescribed objectives. Sonic triangulation can be of particular value in offshore environments where multiple wells are often drilled directionally from single platforms, and in cases where horizontal drilling is planned.

Much valuable information is obtained from these techniques. In fact, few exploratory wells are now drilled without them. Great progress has been made recently in improving the science of geophysics, both in concept and equipment. It is expected that this new process of scanning or "looking ahead" of the drill bit with sonic pulses will be an important step in improving drilling operations and finding complex "hidden" structural and stratigraphic oil and gas reservoirs. Additionally, it is expected to provide an extra measure of safety for personnel and to help maintain a clean environment by giving early warnings of overpressured gas zones and other hazardous conditions.

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