Most directional survey tools take surveys at discrete directional survey stations, with manual input of the wireline depth. The standard instrumentation on a wireline unit is the Gearhart Owen wireline speed and length measuring system with a Martin Decker 0-4,550 kg (0-10,000 lbs) weight indicator to measure the cable tension. Typical accuracies are about 0.002 AHD/AHD at best.
The wireline logging companies have more sophisticated measuring heads, usually with tangential wheels and electronic encoders that can provide depth inputs directly into the logging unit computer.
Some North Seeking gyro tools and Inertial Navigation Systems which directional survey in continuous mode require continuous input of wireline depth. The most common system used by directional survey companies are from Kerr Measurements Inc.
Depth measurements
Kerr wireline directional survey depth measurement systems
Kerr Measurements Inc. have a number of different measuring systems for slickline and braided cable wirelines. These systems comprise magnetic mark systems and continuous electronic counter systems.
Magnetic directional survey mark system
For the full system to operate, the wireline needs to have been measured and magnetic marks placed every 100 ft or 25 m as required, at a reference tension of 1000 lbs. The quoted accuracy of the system is 0.001 AHD/AHD for a wireline that has been correctly measured and magnetically marked. This method of measurement is efficient for conductor wireline operations in newly drilled wells or open holes (the magnetic marks deteriorate as the wireline is run in cased or completed wells). The magnetic mark system is therefore used as a back-up system for continuous electronic counter system.
Continuous electronic counter
In slickline and other applications where the magnetic mark system is not practical or available, a electronic counter should be used. Typical accuracies will be about 0.0005 AHD/AHD.
Casing collar locator
A CCL detects the collars of casing and drillpipe by use of a magnetic field. The advantage of a CCL is that the resolution of the measurement is independent of wireline depth. However, an accurate casing tally is required. Specially where wireline depth inaccuracies are expected to be large (high inclination, deep wells) it is recommended to use a CCL. The accuracy of a CCL is a composition of the accuracy of the casing tally and the accuracy of determining the position of the casing joint. The latter which is approximately 0.5 metres. Typical overall accuracy is 0.0005 AHD/AHD.
Wireline depth uncertainties
Wireline depth uncertainties depend heavily on borehole inclination and running conditions. For highly inclined boreholes wireline overrun may occur during the inrun, while running out high tensions in the wireline are observed. Wireline depth uncertainties can be divided into a systematic depth uncertainty, inrun/outrun depth difference at surface and a depth uncertainty due to stretch at the start of the outrun.
Directional survey tool misalignment
Tool misalignment comprises misalignment due to imperfections of the running gear and misalignment due to deflection of the BHA. Imperfection of the running gear is toolface dependent as with changes in tool orientation the tool misalignment will rotate along with the tool. BHA deflection is toolface independent. The deflection is oriented with gravity irrespective of tool orientation.