Classifications

• • E—FIXED CONSTRUCTIONS
  • E21—EARTH OR ROCK DRILLING; MINING
  • E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
  • E21B47/00—Survey of boreholes or wells
  • E21B47/02—Determining slope or direction
  • E21B47/022—Determining slope or direction of the borehole, e.g. using geomagnetism

Definitions

• the present invention relates to a method of
• a method of qualifying a survey of a borehole formed in an earth formation comprising:
• the earth field parameter can, for example, be the earth gravity or the earth magnetic field strength
• the borehole position parameter can, for example, be the borehole inclination or the borehole azimuth.
• the ratio of the difference between the measured earth field parameter and a known magnitude of said earth field parameter at said position, and the theoretical measurement uncertainty of the position parameter forms a preliminary check on the quality of the survey. If the measured earth field parameter is within the measurement tolerance of this parameter, i.e. if the ratio does not exceed the magnitude 1, then the survey is at least of acceptable quality. If the ratio exceeds magnitude 1, the survey is considered to be of poor quality. Thus the ratio forms a preliminary measure for the quality of the survey, and the product of this ratio and the theoretical measurement uncertainty of the position parameter (as determined in step d) forms the best guess of the survey quality.
• Fig. 1 shows schematically a solid state magnetic survey tool
• Fig. 2 shows a diagram of the difference between the measured and known gravity field strength in an example borehole, against the along borehole depth;
• Fig. 3 shows a diagram of the difference between the measured and known magnetic field strength in the example borehole, against the along borehole depth
• Fig. 4 shows a diagram of the difference between the measured and known dip-angle in the example borehole, against the along borehole depth.
• a solid state magnetic survey tool 1 which is suitable for use in the method according to the invention.
• the tool includes a plurality of sensors in the form of a triad of
• accelerometers 3 and a triad of magnetometers 5 whereby for ease of reference the individual accelerometers and magnetometers are not indicated, only their respective mutual orthogonal directions of measurement X, Y and Z have been indicated.
• the triad of accelerometers measure acceleration components and the triad of magnetometers 5 measure magnetic field components in these directions.
• the tool 1 has a longitudinal axis 7 which coincides with the longitudinal axis of a borehole (not shown) in which the tool 1 has been lowered.
• the high side direction of the tool 1 in the borehole is indicated as H.
• the tool 1 is incorporated in a drill string (not shown) which is used to deepen the borehole.
• the tool 1 is operated so as to measure the components in X, Y and Z directions of the earth gravity field G and the earth magnetic field B. From the measured components of G and B, the magnitudes of the magnetic field dip-angle D, the borehole inclination I and the borehole azimuth A are determined in a manner well-known in the art.
• the theoretical uncertainties of G, B, D, I and A are determined on the basis of calibration data representing the class of sensors to which the sensors of the tool 1 pertains (i.e.
• dA th,s theoretical uncertainty of borehole azimuth A due to the sensor uncertainty
• dA th,g theoretical uncertainty of borehole azimuth A due to the geomagnetic uncertainty
• a preliminary assessment of the quality of the survey is achieved by comparing the differences between the corrected measured values and the known values of the earth field parameters G, B and D with the measurement uncertainties of G, B and D referred to above. For a survey to be of acceptable quality, said difference should not exceed the measurement uncertainty.
• Figs. 2, 3 and 4 example results of a borehole survey are shown.
• Fig. 2 shows a diagram of the difference ⁇ G m between the corrected measured value and the known value of G, against the along borehole depth.
• Fig. 3 shows a diagram of the difference ⁇ B m between the corrected measured value and the known value of B, against the along borehole depth.
• Fig. 4 shows a diagram of the difference ⁇ D m between the corrected measured value and the known value of D, against the along borehole depth.
• ⁇ G m difference between the corrected measured value and the known value of G
• ⁇ B m difference between the corrected measured value and the known value of B
• ⁇ D m difference between the corrected measured value and the known value of D
• the above indicated ratio of the gravity field strength ⁇ G m / dG th,s represents the level of all sources of uncertainties contributing to an inclination uncertainty. If, for example, at a survey station m the drill string the ratio equals 0.85 then it is assumed that all sensor uncertainties in the drillstring are at a level of 0.85 times dI th,s . Therefore the measured inclination uncertainty for all survey stations in the drillstring is:
• ⁇ I m abs[( ⁇ G m / dG th,s )dI th,s ]
• ⁇ I m measured inclination uncertainty due to sensor uncertainty.
• the measured azimuth uncertainty is determined in a similar way, however two sources of uncertainty (sensor and geomagnetic) may have contributed to the azimuth uncertainty. For each source two ratios i.e. magnetic field strength and dip-angle are derived, resulting in four measured azimuth uncertainties:
• ⁇ A s,B abs[( ⁇ B m / dB th,s )dA th,s ]
• ⁇ A s,D abs[( ⁇ D m / dD th's )dA th,s ]
• ⁇ A g,B abs[( ⁇ B m / dB th,g )dA th,g ]
• ⁇ A g,D abs[( ⁇ D m / dD th,g )dA th,g ]
• the measured azimuth uncertainty ⁇ A m is taken to be the maximum of the these values i.e.:
• ⁇ A m max[ ⁇ A s,B ; ⁇ A s,D ; ⁇ A g,B ; ⁇ A g,D ] .
• LPU i LPU i-1 + (AHD i - AHD i-1 ) ( ⁇ A i m sin I i m + ⁇ A i-1 m sin I i-1 m ) / 2;
• UPU i UPU i-1 + (AHD i - AHD i-1 ) ( ⁇ l i m + AI i-1 m ) / 2.
• AHD i along hole depth at location i
• ⁇ A i m measured azimuth uncertainty at location i
• ⁇ I i m measured inclination uncertainty at location i
• UPU 1 upward position uncertainty at location i.
• the lateral position uncertainties and the upward position uncertainties thus determined are then compared with the theoretical lateral and upward position uncertainties (derived from the theoretical inclination and azimuth uncertainties) to provide an indicator of the quality of the borehole survey.

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• Geology (AREA)
• Physics & Mathematics (AREA)
• Life Sciences & Earth Sciences (AREA)
• Engineering & Computer Science (AREA)
• Mining & Mineral Resources (AREA)
• Environmental & Geological Engineering (AREA)
• Geophysics (AREA)
• Fluid Mechanics (AREA)
• Geochemistry & Mineralogy (AREA)
• General Life Sciences & Earth Sciences (AREA)
• Geophysics And Detection Of Objects (AREA)
• Measuring Magnetic Variables (AREA)
• Investigation Of Foundation Soil And Reinforcement Of Foundation Soil By Compacting Or Drainage (AREA)
• Earth Drilling (AREA)
• Paper (AREA)

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