Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Well logging
Patent
1996-07-15
1999-03-02
Moskowitz, Nelson
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Well logging
367 35, 181104, 175 45, G01V 140, E21B 47026
Patent
active
058779965
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a transducer arrangement for use as receivers in acoustic borehole measurements, in particular for the purpose of geological steering, stratigraphic imaging and control during drilling of deviated and horizontal wells, whereby transducer sensitivity is focused generally in radial directions normal to the drillstring axis in close proximity to the drill bit.
Various methods are known for use before and during drilling to provide information of importance to decisions being taken and control being effected during the drilling operations. Decisions on oil and gas reservoir target locations and well paths are primarily based on 2 and 3-D surface seismic surveys combined with well logs from nearby wells. During drilling it is very useful to know the drill bit position relative to the seismic section concerned. However, an exact vertical depth match between the geological prognosis from seismic surveys and drill bit position is not easily available. Formation velocity profiles are normally provided after drilling from wireline operated sonic logs and borehole geophysical surveys, i.e. Vertical Seismic Profiling (VSP).
For all wireline and drillpipe conveyed services, drilling must be stopped and the drill string pulled out of the hole prior to the survey. Borehole seismics of this kind offer useful post-drilling information about the subsurface structural features and the well trajectory relative to these. However, the well may already be in the wrong place in relation to geological zones of interest.
Wellsite petrophysicists, geologist and directional drillers today use formation evaluation logs based on Measurement While Drilling (MWD) and Logging While Drilling (LWD) in addition to directional surveys to determine well location on the geological map predicted from seismic data and from nearby wells. MWD and LWD logs can confirm whether the target zone is met at the predicted depth or not, and thereby give the driller a real time guidance on the depths and direction to head for. Natural gamma ray and resistivity devices constitute supplementary means for obtaining useful information in this connection. The major problem with current petrophysical devices is the very shallow depth of penetration, mainly less than 1-2 meters into the surrounding formations.
To overcome problems related to mal-positioning of the well path due to uncertainties in survey data and seismic maps, MWD borehole sonar methods have been proposed. The main objective of such methods and devices is to allow high precision detection and interpretation of rock strata and fluid contacts sideways and ahead of the drill bit.
Reference is made to the system described in international patent application, publication number WO93/07514. This international patent publication relates to a system and method for performing seismic prospecting and monitoring during drilling of a well. Both acoustic and electromagnetic energy is employed. The energy may be imparted by the drilling operation itself, or may be generated by another downhole source. Acoustic sensors employed may have their direction of sensitivity in a radial direction from the axis of the drill string or drill string tool.
The present invention is particularity directed to be applied during drilling of highly deviated and horizontal wells. As in the known system just referred to, this invention comprises a seismic source and a receiver or sensor array incorporated into the drill string closely behind the drill bit. A stratigraphic image is obtained showing reflections from dipping and horizontal layers above and below the well path. A continuous lateral survey coverage is provided, from which the variations in formation velocities, formation dip angle and the well path depth relative to the near-wellbore geological layers can be extracted. This type of remote information combined with conventional MWD directional survey data enables true real-time steering of the well path in the process of landing the well efficiently into the reservoir, providing stratigraphic cont
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Krokstad Asbj.o slashed.rn
Mjaaland Svein
Nakken Erik Iversen
Den Norske Stats Oljeselskap A.S
Moskowitz Nelson
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