Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2001-07-13
2003-10-14
Barlow, John (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
C033S304000
Reexamination Certificate
active
06633816
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to making downhole measurements during the drilling of a borehole to recover natural deposits of oil or gas and, more particularly, to using continuous downhole measurements to directionally drill the borehole.
DESCRIPTION OF THE RELATED ART
Oil and gas are commonly recovered from natural mineral deposits in subsurface geologic formations in the earth's crust. Drilling rigs at the surface are used to bore long, slender boreholes into the earth's crust to the location of the subsurface oil or gas deposits to establish fluid communication with the surface through the drilled borehole. The downhole drilling equipment used to drill boreholes may be directionally steered to known or suspected oil or gas deposits using directional drilling techniques, and the direction and orientation of downhole survey instruments are monitored at discrete survey stations along the borehole.
Surveying of boreholes is commonly performed using downhole survey instruments. These instruments typically contain sets of three orthogonal accelerometers and magnetometers which are coupled within a bottom hole assembly (BHA), which is in turn coupled in the drillstring from 20 to 200 feet above the drill bit. These survey instruments are used to measure the direction and magnitude of the local gravitational and magnetic field vectors in order to determine the azimuth and the inclination of the borehole at each survey station within the borehole. Generally, discrete borehole surveys are performed at survey stations along the borehole when drilling is stopped or interrupted to add additional joints or stands of drillpipe to the drillstring at the surface.
The trajectory of drilled boreholes within any segment of interest is generally determined by the mode of drilling and by the configuration of the drilling equipment that is used to drill the borehole at that segment of interest. In directional drilling using a bent sub and a mud motor, there are two modes of drilling that produce distinctive borehole trajectories. The first mode involves rotation of the entire drillstring, including the BHA that contains the survey instruments, and the drill bit that is coupled to the bottom (leading end) of the drillstring. This mode of drilling, with the bent sub in its aligned or straight configuration and the mud motor inactive, is known as “rotating.” The second mode of drilling has the bent sub in its deployed or angular configuration with the drill bit being rotated by the active mud motor instead of rotating the drillstring. The mud motor is powered by pressurized drilling mud pumped down the hollow interior of the non-rotating drillstring. This mode of drilling is known as “sliding.” Rotating produces a generally linear trajectory of the drilled borehole, although there are typically borehole deviations from true linear trajectory due to the effects of gravity, geologic heterogeneities, misalignment between the BHA and the borehole, stiffness of the drillstring and transition effects that occur due to switching from one drilling mode to the other. Drilling while sliding produces a curved trajectory of the drilled borehole generally conforming to an arc. Again, there are typically borehole deviations from true arc configuration of a borehole segment drilled by sliding due to the same factors that cause borehole deviations from linear trajectory with drilling by rotating.
Many factors may combine to unpredictably influence the trajectory of a drilled borehole. It is important to accurately determine the borehole trajectory in order to determine the position of the borehole at any given point of interest and to guide the borehole to its geological objective. “Position,” as that term is used herein in reference to boreholes, indicates the total vertical depth, longitude and latitude of a point of interest. Surveying of a borehole using existing methods involves the intermittent measurement of the earth's magnetic and gravitational fields to determine the azimuth and inclination of the borehole at the BHA under static conditions; that is, while the BHA is stationary. These “static” surveys are generally performed at discrete survey “stations” along the borehole when drilling operations are suspended to make up additional joints or stands of drillpipe into the drillstring. Consequently, the along hole depth or borehole distance between discrete survey stations is generally from 40 to 90 feet or more corresponding to the length of joints or stands of drillpipe added at the surface.
Reliable measurements of the earth's magnetic and gravitational fields are available at the survey stations, and can be used to obtain reliable estimates of the azimuth and inclination of the borehole at the survey stations. Although the azimuth and inclination at a survey station of interest can be determined using measurements of the earth's magnetic and gravitational fields, the depth cannot be measured, and must be determined by other means. The vertical depth and position of a survey station is determined by mathematically combining the segments of the borehole between discrete survey stations starting with the surface location of the drilling rig and progressing downward to the geologic objective of the borehole. The problem is that undetected borehole variations occurring between discrete survey stations cause substantial errors in calculating the vertical depth and position of a survey station of interest. Undetected borehole variations accumulate as mathematical combination of borehole segments is used to calculate and track borehole vertical depth and position.
It would be beneficial, therefore, to detect borehole variations and to accurately model the trajectory of the borehole between discrete survey stations in order to determine and track the spatial position of the borehole relative to the geologic objective of the borehole. Existing borehole survey techniques use various methods, including the tangential method, balanced tangential method, equal angle method, cylindrical radius of curvature method and the minimum radius of curvature method, to model the trajectory of the borehole segments between survey stations. Generally, these existing methods model the trajectory of the borehole segments between discrete survey stations based on the assumptions that either the azimuth and inclination remains constant from one survey station to the next or, more often, that the azimuth and inclination of the borehole smoothly transition from the values measured at one survey station to those measured at the next survey station. For example, the minimum radius of curvature method models the borehole segment between survey stations based on the assumption that the trajectory of the borehole segment conforms to an imaginary true arc whose length corresponds to the through-borehole distance from the nearest uphole survey station to the nearest downhole survey station. The length of the arc is assumed to be equal to the length of the drillpipe added to the drillstring during drilling of the borehole segment of interest. These imaginary arcs, one for each defined borehole segment, are stringed and mathematically combined together in sequence, from the known surface location of the drilling rig to the bottom of the borehole, in order to estimate the vertical depth and position of the borehole at any given point of interest.
The inability of these existing borehole survey methods to detect and account for borehole deviations that occur between survey stations is problematic. Existing methods of surveying boreholes, including the minimum radius of curvature method, introduce significant error that lead to inaccurate determinations of borehole vertical depth and position resulting in substantial losses of otherwise recoverable reserves due to inaccurate steering of directional boreholes. Borehole surveys using existing survey methods to model the trajectory of borehole segments between survey stations introduce substantial vertical depth and position error, and the extent
Phillips Wayne
Shirasaka Ichiro
Tejada Mauricio
Barlow John
Jeffery Brigitte L.
Le T.
Ryberg John
Salazar J. L. Jennie
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