Borehole Deviation

Basic Concept

Though boreholes are typically intended to be straight and vertical, drilling direction is subject to variations with depth. Drilling direction can be influenced by drilling techniques as well as subsurface-material properties (e.g., formation density, hardness). The deviation (or inclination) of a borehole is its angle from vertical and can affect the results of other logs that assume borehole verticality. Additionally, when a borehole is intentionally drilled at an incline, the deviation tool can verify the construction.

Borehole deviation logging continuously measures and records borehole inclination and direction (i.e., azimuth) with depth. Because both inclination and direction vary over the length of the well, borehole deviation is rarely consistent. The borehole deviation log provides a detailed assessment of the well path, which itself may be useful as a standalone log. However, the deviation log is often used to correct feature orientation and depth information recorded by other borehole geophysical logs (Twining, 2016).

Theory

Borehole deviation logging has encompassed many technologies since the inception of the method. Initially, a cable-suspended camera was used to photograph a compass and plumb bob at selected depths, which were determined by cable length. Other survey types included the use of piezo-pendulum sensors, potentiometer-pendulum sensors, and electrolyte fluid level sensors. Modern deviation tools are typically equipped with an accelerometer and magnetometer pair plus or minus a gyroscope (US Bureau of Reclamation, 2001).

Triple-axis accelerometers are electromechanical devices that typically use capacitive-plate pairs to measure the static and/or dynamic forces of acceleration that act on three orthogonal directions. The capacitors are designed so that their relative motion and the resulting change in capacitance are related to the acceleration caused by the local gravitational field. The directional measurements of gravitational acceleration allow for the determination of the accelerometer orientation, which is used to calculate the local borehole inclination angle.

Triple-axis fluxgate magnetometers essentially function as electronic compasses that operate by considering the magnetic-field strength measured in three orthogonal directions. Each of the three fluxgate sensors outputs a signal that is influenced by Earth’s magnetic field to a degree that depends on its orientation relative to the field. When combined with accelerometer information, the sensor-induced voltage signals aid the determination of the inclination direction of the borehole relative to magnetic north (Senthilmurugan and others, 2020).

In boreholes that have steel casing or contain magnetic minerals, magnetometer data are inaccurate and can be supplemented using electric-gyroscope technology. A gyroscope maintains its orientation in space due to forces created by an internal wheel that is mounted on and spins about an axel. Gyroscopic-based deviation tools can measure the probe orientation in space relative to either a user-defined direction or geographic (i.e., true) north by considering the rotation of Earth.

Applications

Generally, the measurement-point depth within a well is determined by the length of wire within the well that has passed a certain surface reference point. Thus, a borehole log is initially depicted within a vertical well that has a depth equal to the measured length of wire. Because vertical wells rarely exist, log accuracy often depends on borehole deviation data, which can be plotted in three-dimensional block diagrams or two-dimensional rose plots.

Borehole deviation data can be used to correct logs by determining the true vertical depths (TVD) to the features of interest identified with other methods. Deviation information is also used to correct the orientation of planar features determined with borehole imaging tools. The deviation log is also very useful in aquifer modeling, as incorrect reference elevations could result in errors up to several feet on potentiometric maps (Twining, 2016).

Generally, a vertically drilled well is be essential for its intended purposes, a deviated well may prevent testing, logging, and/or installing casing and pumps. In special cases, deviation logging occurs concurrently with well construction to ensure proper completion. Additionally, a borehole may need to be drilled at a specific inclined angle to intersect a target of interest or avoid obstructions. However, borehole deviation logging most commonly aids the following:

Determination of true vertical depth of features identified with other methods
Correction of feature orientation and bed thickness measurements
Two- or three-dimensional mapping of borehole direction, inclination, and drift

Examples/Case studies

Acuña, J., Palm, B., and Hill, P., 2008, Characterization of Boreholes: Results from a U-pipe Borehole Heat Exchanger Installation, in Proceedings, 9th IEA Heat Pump Conference: Zurich, Switzerland, International Energy Agency, p. 4-19.

Abstract: Heat exchange with the bedrock for ground source heat pumps is commonly done with the help of U-pipe energy collectors in vertical boreholes. At the moment, there exist many uncertainties about how efficient the heat transfer between the rock and the collector is. For a complete performance analysis of these systems, a 260 m deep water filled borehole is characterized, by measuring the borehole deviation, the ground water flow and the undisturbed ground temperature. Significant attention is devoted to detailed temperature measurements along the borehole depth during operation providing a complete description of the temperature variations in time both for the secondary working fluid and for the ground water. The results show a deviated borehole from the vertical direction without any relevant ground water flow. The undisturbed ground temperature gradient varies from negative to positive at approximately half of the borehole depth. The transient response of the borehole during the heat pump start up is illustrated and it is observed that there does not exist any thermal short circuiting between the down and up-going pipes when the system is in operation.

Bulant, P., Eisner, L., Pšenčík, I., and Calvez, J., 2007, Importance of borehole deviation surveys for monitoring of hydraulic fracturing treatments: Geophysical Prospecting, v. 55, no. 6, p. 891-899, doi:10.1111/j.1365-2478.2007.00654.x.

Abstract: During seismic monitoring of hydraulic fracturing treatment, it is very common to ignore the deviations of the monitoring or treatment wells from their assumed positions. For example, a well is assumed to be perfectly vertical, but in fact, it deviates from verticality. This can lead to significant errors in the observed azimuth and other parameters of the monitored fracture‐system geometry derived from microseismic event locations. For common hydraulic fracturing geometries, a 2° deviation uncertainty on the positions of the monitoring or treatment well survey can cause a more than 20° uncertainty of the inverted fracture azimuths. Furthermore, if the positions of both the injection point and the receiver array are not known accurately and the velocity model is adjusted to locate perforations on the assumed positions, several‐millisecond discrepancies between measured and modeled SH‐P traveltime differences may appear along the receiver array. These traveltime discrepancies may then be misinterpreted as an effect of anisotropy, and the use of such anisotropic model may lead to the mislocation of the detected fracture system. The uncertainty of the relative positions between the monitoring and treatment wells can have a cumulative, nonlinear effect on inverted fracture parameters. We show that incorporation of borehole deviation surveys allows reasonably accurate positioning of the microseismic events. In this study, we concentrate on the effects of horizontal uncertainties of receiver and perforation positions. Understanding them is sufficient for treatment of vertical wells, and also necessary for horizontal wells.

Mastin, L., 1988, Effect of borehole deviation on breakout orientations: Journal of Geophysical Research: Solid Earth, v. 93, no. B8, p. 9187-9195, doi:10.1029/JB093iB08p09187.

Abstract: Well bore breakouts are zones of spalling and fracture that form on opposite sides of a well bore and tend to change the cross‐sectional shape of the borehole from circular to roughly elliptical. In vertical holes drilled in areas where one principal stress (Sv ) is vertical, breakouts tend to form at opposite ends of the borehole diameter parallel to the least compressive horizontal principal stress direction (Sh ). This paper uses an analytical elastic solution for stress at the wall of a borehole to analyze the rotation of breakout orientations away from the direction of Sh as the borehole deviates from the direction of the vertical principal stress. The calculated orientations of breakouts in deviated boreholes depend on the type of faulting regime in which the well was drilled (i.e., normal, strike‐slip, or thrust), on the deviation angle ø of the borehole axis from vertical, on the angle θ between the horizontal projection of the borehole axis and the direction of Sh , and on the relative magnitudes of the three principal stresses (Sh ; SH , the greatest compressive horizontal stress; and Sv ). In the strike‐slip faulting regime, regardless of the values of θ, Sv , SH , and Sh , a borehole must deviate at least 35° from vertical before the horizontal projection of the breakout orientation differs by more than 10° from Sh . In the normal and thrust faulting regimes, however, the borehole deviation angle øcrit required to rotate projected breakout orientations by 10° from Sh approaches zero as the value of Sh approaches that of SH . In the thrust faulting regime, only about 3% of all combinations of θ, Sv , SH , and Sh will produce øcrit values less than 10°, while about 12% of all such combinations will yield øcrit < 10° in the normal faulting regime. About 12% and 33% of such combinations will yield øcrit < 20° in the thrust and normal faulting regimes, respectively. Careful study of changes in breakout orientation as a function of borehole deviation may improve the resolution of inferred stress directions when studying breakouts in deviated boreholes in these two faulting regimes.

Morris, W.A., Ugalde, H., and Milkereit, B., 2008, Borehole Magnetics: Magnetostratigraphy: An example from UNAM‐7, Chicxulub impact crater: Society of Exploration Geophysicists Technical Program Expanded Abstracts 2008, p. 716-720, doi:0.1190/1.3063748.

Abstract: Magnetic reversal boundaries define global chronostratigraphic surfaces. As such, rapid identification of these boundaries is useful for constructing chronologically correct correlations and for defining fault displacements. Borehole deviation surveys which use a tri‐axial fluxgate sensor to provide geographic orientation also contain a record of the variation of the vector magnetic field versus depth. When the crustal component of the observed vector magnetic field is dominated by remanence minor fluctuations in the orientation of the observed vector reflect the underlying magnetostratigraphy. UNAM‐7 which was drilled on the periphery of the Chixculub impact crater, Yucatan peninsula, Mexico was logged using a standard deviation probe. Processing of this data revealed a magneto‐stratigraphic reversal history that is compatible with the limited paleomagnetic database, the petrophysical data, and sedimentation rates as defined by results from other boreholes.

Eisner, L., Bulant, P., and Calvez, J., 2006, Borehole deviation surveys are necessary for hydraulic fracture monitoring: Society of Exploration Geophysicists Technical Program Expanded Abstracts 2006, p. 359-363, doi:10.1190/1.2370276.

Abstract: Not performing accurate borehole deviation surveys for hydraulic fracture monitoring (HFM) and neglecting the effects of the borehole trajectory results in significant errors in the calculated fracture azimuth and other parameters. For common HFM geometries, a 5° deviation uncertainty of monitoring or treatment wells can cause more than 40° uncertainty in inverted fracture azimuths. Furthermore, if the positions of injection point and receiver array are not known accurately and the velocity model is artificially adjusted to locate fracture on an assumed injection point, several milliseconds discrepancy between measured and modelled P‐to‐S‐wave travel‐times may appear at utmost receivers of the receiver array. This travel‐time discrepancy may then be misinterpreted as VTI anisotropy. In the case of HFM, the uncertainty of the relative positions between the monitoring and treatment wells can have a cumulative, non‐linear effect on inverted fracture parameters.

Twining, B.V., 2016, Borehole deviation and correction factor data for selected wells in the eastern Snake River Plain aquifer at and near the Idaho National Laboratory, Idaho: U.S. Geological Survey Scientific Investigations Report 2016-5163, 23 p., doi:10.3133/sir20165163.

Abstract: The U.S. Geological Survey (USGS), in cooperation with the U.S. Department of Energy, has maintained a water-level monitoring program at the Idaho National Laboratory (INL) since 1949. The purpose of the program is to systematically measure and report water-level data to assess the eastern Snake River Plain aquifer and long term changes in groundwater recharge, discharge, movement, and storage. Water-level data are commonly used to generate potentiometric maps and used to infer increases and (or) decreases in the regional groundwater system. Well deviation is one component of water-level data that is often overlooked and is the result of the well construction and the well not being plumb. Depending on measured slant angle, where well deviation generally increases linearly with increasing slant angle, well deviation can suggest artificial anomalies in the water table. To remove the effects of well deviation, the USGS INL Project Office applies a correction factor to water-level data when a well deviation survey indicates a change in the reference elevation of greater than or equal to 0.2 ft. Borehole well deviation survey data were considered for 177 wells completed within the eastern Snake River Plain aquifer, but not all wells had deviation survey data available. As of 2016, USGS INL Project Office database includes: 57 wells with gyroscopic survey data; 100 wells with magnetic deviation survey data; 11 wells with erroneous gyroscopic data that were excluded; and, 68 wells with no deviation survey data available. Of the 57 wells with gyroscopic deviation surveys, correction factors for 16 wells ranged from 0.20 to 6.07 ft and inclination angles (SANG) ranged from 1.6 to 16.0 degrees. Of the 100 wells with magnetic deviation surveys, a correction factor for 21 wells ranged from 0.20 to 5.78 ft and SANG ranged from 1.0 to 13.8 degrees, not including the wells that did not meet the correction factor criteria of greater than or equal to 0.20 ft. Forty-seven wells had gyroscopic and magnetic deviation survey data for the same well. Datasets for both survey types were compared for the same well to determine whether magnetic survey data were consistent with gyroscopic survey data. Of those 47 wells, 96 percent showed similar correction factor estimates (≤ 0.20 ft) for both magnetic and gyroscopic well deviation surveys. A linear comparison of correction factor estimates for both magnetic and gyroscopic deviation well surveys for all 47 wells indicate good linear correlation, represented by an r-squared of 0.88. The correction factor difference between the gyroscopic and magnetic surveys for 45 of 47 wells ranged from 0.00 to 0.18 ft, not including USGS 57 and USGS 125. Wells USGS 57 and USGS 125 show a correction factor difference of 2.16 and 0.36 ft, respectively; however, review of the data files suggest erroneous SANG data for both magnetic deviation well surveys. The difference in magnetic and gyroscopic well deviation SANG measurements, for all wells, ranged from 0.0 to 0.9 degrees. These data indicate good agreement between SANG data measured using the magnetic deviation survey methods and SANG data measured using gyroscopic deviation survey methods, even for surveys collected years apart.

References

Mastin, L., 1988, Effect of borehole deviation on breakout orientations: Journal of Geophysical Research: Solid Earth, v. 93, no. B8, p. 9187-9195, doi:10.1029/JB093iB08p09187.

Senthilmurugan, S., Ganesh, V., and Prabhu, A., 2020, Design and Implementation of Three axis Fluxgate Magnetometer and its Applications: International Journal of Innovation Technology and Exploring Engineering, v. 9, no. 4, p. 960-965, doi:10.35940/ijitee.D1186.029420

U.S. Bureau of Reclamation, 2001, Borehole Geophysical and Wireline Surveys, in Engineering

Geology Field Manual - Second Edition: Washington D.C, U.S. Department of the Interior, Bureau of Reclamation, v. 2, p. 37-81.