When drilling a well for exploration and recovery of oil or gas, it is known to drill a deviated well, which is a well whose borehole intentionally departs from vertical by a significant extent over at least part of its depth. When a single drilling rig is offshore, a cluster of deviated wells drilled from that rig allows a wider area and a bigger volume to be tapped from the single drilling rig at one time and without expensive and time-consuming relocation of the rig than by utilising only undeviated wells. Deviated wells also allow obstructions to be bypassed during drilling, by suitable control of the deviation of the borehole as it is drilled. However, to obtain the full potential benefits of well deviation requires precise knowledge of the instantaneous location and heading of the bottom-hole assembly (including the drilling bit and steering mechanism such as adjustable stabilisers). Depth of the bottom-hole assembly (or axial length of the borehole) can be determined from the surface, for example by counting the number of standard-length tubulars coupled into the drillstring, or by less empirical procedures. However, determination of the location and heading of the bottom-hole assembly generally requires some form of downhole measurement of heading. Integration of heading with respect to axial length of the borehole will give the borehole location relative to the drilling rig.
In this context, the word "heading" is being used to denote the direction in which the bottom-hole assembly is pointing (i.e., has its longitudinal axis aligned), both in a horizontal and vertical sense. Over any length of the borehole which can be considered as straight for the purposes of directional analysis, the borehole axis in a deviated well will have a certain inclination with respect to true vertical. A vertical plane including this nominally straight length of borehole will have a certain angle (measured in a horizontal plane) with respect to a vertical plane including a standard direction; this standard direction is hereafter taken to be true Magnetic North, and the said angle is the magnetic azimuth of the length of borehole under consideration (hereafter simply referred to as "azimuth"). The combination of inclination and azimuth at any point down the borehole is the heading of the borehole at that point; borehole heading can vary with depth as might be the case, for example, when drilling around an obstacle.
Instrumentation packages are known, which can be incorporated in bottom-hole assemblies to measure gravity and magnetism in a number of orthogonal directions related to the heading of the bottom-hole assembly. Mathematical manipulations of undistorted measurements of gravitational and magnetic vectors can produce results which are representative of the true heading at the point at which the readings were taken. However, the measurements of magnetic vectors are susceptible to distortion, not least because of the masses of ferrous materials incorporated in the drillstring. Distortion of one or more magnetic vector measurements can give rise to unacceptable errors in the determination of heading, and undesirable consequences. Distortion of magnetic vectors in the region of the instrumentation arising from inherent magnetism of conventional drillstring components can be mitigated by locating the instrumentation in a special section of drillstring which is fabricated of non-magnetic alloy. However, such special non-magnetic drillstring sections are relatively expensive. Moreover, the length of non-magnetic section required to bring magnetic distortion down to an acceptable level increases significantly with increased mass of magnetic bottom-hole assembly and drillstring components, with consequent high cost in wells which use such heavier equipment, e.g. wells which are longer and/or deeper. Hence such forms of passive error correction may be economically unacceptable. Active error correction by the mathematical manipulation of vector readings which are assumed to be error-free or to have errors which are small may give unreliable results if the assumption is unwarranted.
It is therefore an object of the invention to provide an improved method of surveying a borehole.