MagCUBE™. Improve Positioning Accuracy in Directional Drilling

Directional drilling maximizes the extraction of oil and gas while minimizing the cost. The Measurement While Drilling (MWD) describes directional surveying and drilling-related measurements. A cost-effective survey in MWD is to measure the distance and two angles: the dip measured by a triaxial accelerometer and the azimuth by a triaxial magnetometer.

The magnetic azimuth of the bottom hole assembly (BHA) is inferred from the vector component measurements of the MWD survey tool. To convert the magnetic azimuth into the true (geographic) azimuth, one needs the magnetic declination provided by a geomagnetic reference model at the drilling site and time. This reference model should include all three sources for accuracy: the main, crustal, and disturbance fields. The more accurate the reference model, the more accurate the true azimuth of the BHA. The traditional or standard MWD survey uses a global geomagnetic reference model that is described by a mathematical technique named spherical harmonic expansion. The spatial resolution of such a model is defined by the spherical harmonic degree (SHD). A global model has a limited spatial resolution and accuracy. The resolution and accuracy can be improved by the inclusion of the local short-wavelength crustal magnetic field. For wellbore surveying, this inclusion is called In Field Referencing (IFR) survey.

Magnetic declinations, along a profile of 140000 ft long at 10000 ft below sea level, in the Powder River Basin, Wyoming. Three global models contain the main field only (SHD13), the main field, and the long-wavelength crustal field (SHD133 and SHD720). The global models are associated with the right vertical axis in degrees. The local crustal field is relative to the global model SHD720 and uses the left vertical axis in degrees. The local crustal field or IFR has significant effects.

Any measurements contain errors. Errors from different sources are statistically independent and cumulative, and errors propagate in proportion to how far the drill bit is from the origin. A combined effect of errors in the dip, azimuth, and distance is described by the ellipse of uncertainty (EOU). In practice, wells are drilled at high angles and long distances for increased productivity. In such a well, the area of EOU (green cone) using a standard MWD survey may grow to exceed the dimensions of the geological target (red rectangle), thus increasing the drilling risks and potential costs of missing the target. To reduce the size of EOU, several corrections are applied to the downhole tool readings. A correction using an IFR model is called IFR correction and is a major correction in wellbore surveying. By utilizing IFR, it is capable to position the wellbore (blue cone) inside the sweet spot of the reservoir as well as minimizing the directional control which reduced drilling time and created significant saving. In directional drilling, a wanted orientation accuracy is about 0.1°.

In addition to providing declination, the geomagnetic reference model further provides information on the dip and strength of the ambient field. This information is not directly required for drilling since the dip of the well can be inferred from the accelerometer in the MWD tool. The dip and total field measurements provided by the geomagnetic reference or IFR model, therefore, constitute redundant information that can be used to meet two additional objectives: (1) Validation of the magnetic MWD tool readings and (2) correction for magnetic drill string interference. Magnetic interference correction is another major correction in wellbore surveying.

The MWD engineer applies IFR, magnetic interference, and other corrections; then calculates the tool orientation from the corrected measurements, and finally advises the drilling engineer to adjust the drilling towards the desired direction.

We have developed a technique named MagCUBE that converts surface-observed scalar total magnetic intensity anomalies into a subsurface vector crustal magnetic field distribution. We build an IFR model using high-resolution magnetic data. We estimate the uncertainties in the main, crustal, and disturbance fields. A combination of these efforts results in improved accuracy of wellbore positioning. We have also pre-built MagCUBE IFR models of onshore US basins, for immediate availability. 

MagCUBE has been successful in improving accuracy and reducing costs in directional drilling projects onshore and offshore around the world. This advanced technology is available globally, and exclusively, from Xcalibur Multiphysics.