High inclination wells (above 70°) and specially horizontal wells have specific difficulties for obtaining accurate surveys. Two main problems arise with high inclination surveying:
- getting wireline survey tools down;
- vertical position uncertainty.
Another application with horizontal wells where surveying is a problem is with dual horizontal wells. With dual horizontal wells, two closely spaced parallel wells are drilled. In addition to the two above problems, magnetic interference from the casing of the first well while drilling the second well will preclude the use of conventional magnetic surveying techniques. Special parallel well surveying/tracking techniques have been developed to overcome this problem.
1. Getting wireline survey tools down
There have always been difficulties in getting wireline survey tools down in high inclination wells. Traditional solutions have been to:
·ensure that the tool running gear is appropriate, with sinker bars, roller or wheeled centralisers;
·ensure that the casing has been cleaned out with a bit and scraper run and the mud is in good condition;
·displace the well to a lighter and less viscous fluid (e.g. sea water, brine or completion fluid) prior to running the survey;
·run drillpipe and gently pump the wireline tool down through it;
·run the tool on a drillpipe logging system, so the survey is made as the drillpipe is run into and pulled out of the hole.
One problem in high angle holes is detecting if the tool has stopped moving down the hole as this is not often clear from the surface wireline tension. To help detect tool movement, a downhole tension measurement in the cable head is useful. A Casing Collar Locator (CCL) can be run, but these often have a minimum and maximum speed between which they need to be run to give a good response. As the tool slows in high inclination sections, the CCL may stop responding before the tool stops. If a CCL log is required, it may also limit the maximum running speed of the survey tool.
New methods now include 'stiff wireline'. Although special semi-rigid electric line is in prototype form it is not yet available commercially. The most usual 'stiff wireline' is coiled tubing with a conductor wireline threaded through the bore. The disadvantage is the size and cost of a coiled tubing unit, however, it may be the only way to get a survey or log.
2. Vertical position uncertainty
In conventional surveying the lateral position uncertainty is more important than the vertical position uncertainty. However, for horizontal wells the vertical position uncertainty may become greater than target tolerances set or even the reservoir thickness. The vertical position uncertainty is made up of a combination of tool and system uncertainties. System uncertainties make up the major part of the vertical depth uncertainty.
For horizontal wells geosteering may be used in combination with directional surveys to reduce vertical uncertainty by using a geological marker or to control the well trajectory by following a gas-oil contact.
For MWD tools, a major source of vertical uncertainty comes from the deflection of the drillstring between stabilisers. Deflection may be of the order of 0.3°. The exact magnitude of the deflection depends on the bottom hole assembly configuration, but in general the inclination uncertainty from deflection is fairly constant for the complete build-up section and reduces towards horizontal.
Where a well requires vertical control from the MWD surveys, the effect of deflection, can be reduced or corrected for by:
·deflection computation and correction;
·spacing of sensor at deflection neutral point;
·running of additional magnetic or gyroscopic single or multi-shot, magnetic or gyroscopic wireline steering survey tools.
In addition, drillpipe tally uncertainties and drillpipe stretch can be significant sources of vertical position uncertainty. Specially pulling the bit of bottom before taking a survey may introduce a vertical position uncertainty.
Care should be taken to calculate the exact along-hole depth of a survey station.
3. Dual horizontal/multi-lateral wells
In some enhanced oil recovery techniques using steam flooding, two parallel horizontal wells are drilled in close proximity, one higher than the other. The upper well is used to inject steam, while the lower well produces the heated oil and condensed steam. The success of this recovery method requires a consistent separation between the wells which cannot be obtained using normal survey techniques.
Two techniques have been developed to maintain the separation while continuously drilling. These techniques can also be used for drilling of multi-lateral wells where a completion of the neighbouring well may cause magnetic interference while drilling the next well.
Parallel well tracking technique
The technique is based on magnetostatic homing-in technology. The casing joints of the horizontal section of the first well are artificially magnetised prior to running in the hole. This creates a unique magnetic field pattern that is used as a reference for monitoring the trajectory of the second well. This field pattern is detected with conventional MWD tools so the second well can be drilled relative to the first.
The advantages of the method are:
·uses standard MWD survey tools that can transmit raw sensor data;
·no access is required in the first wellbore while drilling the second well;
·no homing-in or special tools are required.
The Sperry-Sun and Vector Magnetics Inc. method
Sperry-Sun, in conjunction with Vector Magnetics, Inc., have developed a Magnetic Guidance Tool (MGT) for twinning wells. The MGT and associated software provide distance and directional information between two wells. The MGT tool is an electromagnetic field source, which when energised superimposes a magnetic field of known strength and orientation on the Earth's local magnetic field. The magnetic fields are detected by a Sperry-Sun MWD tool.
The procedure is to drill the lower well first with standard directional drilling and survey procedures. The build-up section of the second well is also drilled with standard techniques. The MGT tool on wireline is then pumped down the lower well to guide the drilling of the upper well. A steerable BHA with a modified Sperry-Sun MWD is used to drill the upper well. The MWD is used to detect the magnetic signal from the MGT and as the upper well is drilled, the MGT is pumped further along the lower well. The distance and direction between the wells can then be used to steer the top well to maintain the separation between the wells.