With MWD (Measurement While Drilling or Measure While Drilling) method, the directional and horizontal wells, and other wells with three-dimensional well placement can be achieved. However, for more than two wells of three-dimensional well placement, such as intersection wells, paralleling wells or 3D sidetrack wells, the ellipse uncertainty of MWD will increase as well as the well gets deeper, cumulative errors of MWD will exceed tolerances. If only MWD is used, it is difficult to provide a sufficiently accurate relative distance for wells. Therefore, you need to measure the relative distance between the drill bit and the drilled well, to eliminate the accumulated error of the MWD method.
In the prior art, a magnetic field range finder method is mostly used for achieving a measurement of relative distance. The magnetic field source comprises a magnetic field of a magnetized casing of the drilled well, a magnetic field generated by current in a solenoid of the drilled well, and a magnetic field of permanent magnets mounted on the drilling bit. Magnetic field intensity is weaker by the distance in cubic attenuation. A sensitive distance of permanent magnet is farther than a solenoid, and sensitive distance of the solenoid is farther than the magnetized casing, so the permanent magnet is the most practical and applicable. A permanent magnet as source approach has been successfully used in many applications, such as heavy oil SAGD wells, heavy oil THAI wells, coal bed methane intersection wells, underground coal gasification “U” shape wells and “V” shape cluster wells, mining emergency rescue drilling, freeze hole drilling in ground freezing construction, offshore platforms multi-wells drilling, horizontal directional drilling for pipeline construction, saline mine connection wells, other obstacles. What is needed is a focus on improved permanent magnet-based ranging, in order to increase measurement distance and improve measurement reliability.
For example, U.S. Pat. No. 5,258,755 uses the permanent magnet with NS pole perpendicular to the axis of the drill bit and a solenoid with its NS polar axis parallel to the axis of the drill bit, the solenoid is energized, but in practice, it is difficult to achieve, the solenoid should be replaced with a permanent magnet. In U.S. Pat. No. 5,258,755 the solenoid is connected with alternating current to generate an alternating magnetic field. The permanent magnet magnetic field is also variable with bit rotation.
Chinese Patent No. CN101799558B proposed using two induction magnetometers, to form a synchronized three component magnetic field measurements of space in two locations. An induction magnetometer measures the rotation of the rotating magnetic field generated by the magnet. Another induction magnetometer to measure the Earth's magnetic field, together with the accelerometer to establishing the Earth's gravity and terrestrial magnetic field coordinates. However, this method is not high precision.
U.S. Pat. No. 5,589,775 and Lee proposed a dipole magnetic field distribution to calculate distance, but the dipole magnetic field distribution is interfered by a drill bit and mud motor and permeability of the target well, will affect the calculate accuracy of distance.
Patent U.S. Pat. No. 5,485,089 proposed using a movable solenoid to generate the magnetic field in the drilled well. Patent CN101852078B proposed using two solenoids to generate the magnetic field in the drilled well, and measured with MWD fluxgate. But MWD fluxgate is far from the drill bit and measuring data is too limited, As well as MWD cannot performance under normal circumstances.
There is a need to overcome the disadvantages of the prior art magnetic field measurements, to avoid the accumulation of errors and to improve the accuracy of measurement between the drill bit and target relative distance.
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