This invention relates in general to directional drilling of wells, and in particular to a method and apparatus for more efficient and accurate determination of an angular offset between a directional sensor reference point and the high side of a bent sub.
In one method of directional drilling of wells, the drill string includes a rotary drill bit on the lower end connected to a downhole drill motor. The drill motor operates in response to drilling fluid being pumped down the drill string, causing the drill bit to rotate relative to the drill string. Also, the drill string can be rotated along with the drill bit for conventional drilling.
The drill motor assembly includes a bent sub, which is a tubular housing that positions the bit at a slight angular offset, usually about three degrees, from the longitudinal axis of the drill string. As a result, if the drill string is held stationary while the drill motor rotates the drill bit, the bent sub will cause the well to be drilled in the direction of the inclination of the bent sub.
Accordingly, the operator can steer the drill bit in a desired direction if the operator knows the direction of the tool axis (the axial direction of the sensor frame) and the particular orientation of the bent sub about drillstring axis, namely via three parameters, azimuth, inclination and bent sub tool face. Note, for purposes of this discussion, sensor tool face is the same as directional sensor tool face which can be either gravity tool face (sometimes simply called xe2x80x9ctool facexe2x80x9d) or magnetic tool face. Sensor tool face is a measure of the orientational state of the sensor about the tool axis. The bent sub tool face is a measure of the orientational state of the bent sub about the tool axis.
Downhole directional instruments (or sensors) including magnetometers and accelerometers (inclinometers) provide the azimuth, the inclination and the directional sensor tool face. Typically, three magnetometers and three accelerometers are used to measure the three components of the gravity vector and the Earth magnetic field vector in the sensor frame, respectively. For convenience, the magnetometer and accelerometer axes are aligned. Usually, the angle between the sensor x-axis (x and y are perpendicular to the tool axis and to each other) and a geometrical direction is defined as the sensor tool face. For example, in a horizontal well when the x-axis of the directional sensor frame is vertical, the sensor tool face is defined as an angle of zero. In directional wells, the gravity tool face is defined as the angle between the x-axis and the line intersect between the x-y plane and the vertical plane parallel to the tool axis.
In vertical wells, the gravity tool face is not well defined as the x-axis is approximately in the horizontal plane. In this case, one uses the magnetic tool face to indicate the tool orientation about the tool axis. The magnetic tool face is usually defined as the relative angle between the x-axis and the line intersect between the x-y plane and the vertical plane parallel to the magnetic north vector. Under any situation (vertical, horizontal or deviated well) a tool face measurement is available, indicating the tool""s orientation about the tool axis.
The directional information,, inclination, azimuth, and tool face, sensed by the downhole instruments will be transmitted to the surface, preferably by using a measurement while drilling (xe2x80x9cMWDxe2x80x9d) telemetry system. The system receives the digital signal from the downhole instruments and operates a poppet valve to provide pulses in the drilling fluid inside the drill pipe. These pulses are detected at the surface and decoded into a readout that informs the operator of the tool""s direction.
When the operator is drilling straight, the drill string is rotated along with the drill bit. This makes the borehole slightly larger in diameter than the drill string. When the operator desires to make a turn, he will cease rotating the drill string, and will position it in a desired orientation (e.g. he/she will position the high side of the bent sub in the direction he/she wants to turn). The drilling fluid pumped through the drill string causes the drill bit to rotate. This causes the drill bit to move in the desired direction.
As mentioned, in order to accomplish the directional drilling task, the operator needs to know the orientation of the bent sub. The relationship between the directional sensor sonde and the bent sub is fixed for each bottom hole assembly. From the directional sensor measurement, the directional sensor tool face is known. If the angular difference between the directional sensor reference point and the bent sub is measured on the surface, then the operator can use this measurement and the directional sensor tool face reading to determine the orientation of the bent sub, namely, the bent sub tool face. Such angular difference is sometimes called tool face offset.
In the prior art, the angular difference is determined by the use of a scribe line on the exterior of the instrument housing. The scribe line indicates the direction of the x-axis of the directional sensor frame
At the well site, the downhole assembly is assembled and suspended vertically in the derrick. Next an operator draws a visible indicator or vertical chalk line from the high side of the bent sub, up to the directional sensor housing. The operator then measures the angular difference between the scribe line and the chalk line. The measurement may be made with a protractor. Now, the operator must decide whether to add or subtract this angular difference to the directional sensor tool face for purposes of determining the orientation of the bent sub. Obviously, the decision as to whether to add or subtract the angular difference is critical. Operators are trained to follow a procedure to correctly determine whether the angular difference should be input into the surface computer as a positive or negative number to be added to the tool face reading to obtain the bent sub orientation
The disadvantage of this system is that it is subject to operator error. The operator must measure the angle properly and then properly subtract or add the angular difference. However if a mistake is made in the decision as to whether the angular difference is to be added or subtracted (e.g. the proper correction is xe2x80x9cplusxe2x80x9d sixty degrees and one mistakenly subtracts sixty degrees) serious adverse consequences can result. For example, such a mistake can result in the operator drilling in the wrong direction for a period of time and at a significant expense.
One object of the invention is to eliminate any error which may be made in measuring and applying the angular difference for purposes of determining the orientation of the bent sub. Another object is to implement equipment for achieving same.
In this invention, it is not necessary to form the prior art scribe line on the instrument housing to indicate the internal reference point of the directional sensor. In fact, the magnetometer and inclinometer can be inserted into the instrument housing with a random orientation. However, a locater must be placed on the instrument housing marking the approximate mid-point of the magnetometer between its upper and lower ends. This may be a circumferential locater groove formed in the instrument housing.
At the well site, the tools are assembled and suspended vertically. A technician draws a chalk line from the high side of the bent sub vertically upward onto the directional instrument housing where the chalk line intersects the circumferential locater groove. The operator connects the magnetometer to the surface instrument panel by using a probe and an electrical lead. The magnetic field Bbefore is measured and recorded. The Bbefore is unlikely to be the unperturbed Earth magnetic field due to the amount of steel in the drilling rig and other magnetic fields present on the rig floor. However, measuring the true Earth magnetic field is not relevant at this point.
The operator then takes a magnetic source and places it adjacent the circumferential groove and in alignment with or in a prescribed fixed relationship with the chalk line. The magnetic field Bafter is then measured. The difference between Bafter and Bbefore is Bsource, the magnetic field generated by the source alone. Since the direction of Bsource is that of the chalk line or is in a known relationship with the chalk line, the direction of Bsource in the magnetometer sensor frame relative to directional sensor tool face (usually the x-axis direction) is or gives the angular tool face offset. This information is recorded in the directional sensor and/or in the surface computer. This angular offset is used later for determining the orientation of the high side of the bent sub for directional drilling.
If the magnetometer has a very large dynamic range, the operator can use a very strong magnetic source. Then Bafter is approximately equal to Bsource. The Bbefore measurement is not necessary.
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