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Core orientation is the process by which the original ''in situ'' position or orientation of a core cylinder is determined. Typically, a mark, groove, or line is placed on the surface of the core and the ''in situ'' azimuth of the marking is determined with respect to geographic north.

Core orientation is the process by which the original ''in situ'' position or orientation of a core cylinder is determined. Typically, a mark, groove, or line is placed on the surface of the core and the ''in situ'' [[azimuth]] of the marking is determined with respect to geographic north.

Cores are oriented to facilitate measurement of directional properties in the rock. Most routinely, orientation is used to measure large scale features such as bedding, cross-bedding, fractures, flow textures, and stylolites.<ref name=pt03r40>Pettijohn, F. J., P. E. Potter, and R. Siever, 1973, Sand and sandstone: New York, Springer-Verlag, 618 p.</ref> In recent years, oriented core has been used to establish the directions of downhole stress and strain fields.<ref name=pt03r46>Teufel, L. W., C. M. Hart, A. R. Sattler, and J. A. Clark, 1984, Determination of hydraulic fracture azimuth by geophysical, geological, and oriented core methods at the multi-well experiment site, Rifle, Colorado: Sandia National Laboratories Paper SAND 84-0380, Society of Petroleum Engineers Paper No. 13226, 15 p.</ref><ref name=Smith el al._1985>Smith, M. B., N.-K. Ren, G. G. Sorrells, and L. W. Teufel, 1985, A comprehensive fracture diagnostics experiment, Part II, Comparison of seven fracture azimuth measurements: Society of Petroleum Engineers Paper No. 13894, 16 p.</ref><ref name=pt03r31>Lacy, L. L., 1984, Comparison of hydraulic fracture orientation techniques: Society of Petroleum Engineers Paper No. 13225, 12 p.</ref>

Cores are oriented to facilitate measurement of directional properties in the rock. Most routinely, orientation is used to measure large scale features such as bedding, cross-bedding, fractures, flow textures, and stylolites.<ref name=pt03r40>Pettijohn, F. J., P. E. Potter, and R. Siever, 1973, Sand and sandstone: New York, Springer-Verlag, 618 p.</ref> In recent years, oriented core has been used to establish the directions of downhole stress and strain fields.<ref name=pt03r46>Teufel, L. W., C. M. Hart, A. R. Sattler, and J. A. Clark, 1984, Determination of hydraulic fracture [[azimuth]] by geophysical, geological, and oriented core methods at the multi-well experiment site, Rifle, Colorado: Sandia National Laboratories Paper SAND 84-0380, Society of Petroleum Engineers Paper No. 13226, 15 p.</ref><ref name=Smith el al._1985>Smith, M. B., N.-K. Ren, G. G. Sorrells, and L. W. Teufel, 1985, A comprehensive fracture diagnostics experiment, Part II, Comparison of seven fracture azimuth measurements: Society of Petroleum Engineers Paper No. 13894, 16 p.</ref><ref name=pt03r31>Lacy, L. L., 1984, Comparison of hydraulic fracture orientation techniques: Society of Petroleum Engineers Paper No. 13225, 12 p.</ref>

Core orientation techniques fall broadly into two categories: mechanical and core-based.

Core orientation techniques fall broadly into two categories: mechanical and core-based.

The compass, camera, and timer system is mounted at the top of the inner core barrel, inside a nonmagnetic collar. The battery-driven timer is set prior to running in the hole to expose a frame of film automatically at intervals of 1 to 8 min. Each frame records the compass and the position of a reference mark. As the coring assembly is made up, a lug on the compass is aligned with a ''reference scribe'' (one of usually three knives set in a scribe shoe at the base of the inner core barrel) ([[:file:core-orientation_fig2.png|Figure 2]]). As the core is cut and enters the mouth of the barrel, grooves are cut in the surface of the core by the scribe knives. Angles between the scribe knives vary between coring companies, but the reference scribe is usually offset from the secondary knives by oblique angles on the order of 130° to 150°. A preferred arrangement is an asymmetric scribe shoe in which the angles from the reference scribe to each of the secondary scribes differs ([[:file:core-orientation_fig2.png|Figure 2]]). Such an arrangement allows up-down directions on the core to be distinguished easily.

The compass, camera, and timer system is mounted at the top of the inner core barrel, inside a nonmagnetic collar. The battery-driven timer is set prior to running in the hole to expose a frame of film automatically at intervals of 1 to 8 min. Each frame records the compass and the position of a reference mark. As the coring assembly is made up, a lug on the compass is aligned with a ''reference scribe'' (one of usually three knives set in a scribe shoe at the base of the inner core barrel) ([[:file:core-orientation_fig2.png|Figure 2]]). As the core is cut and enters the mouth of the barrel, grooves are cut in the surface of the core by the scribe knives. Angles between the scribe knives vary between coring companies, but the reference scribe is usually offset from the secondary knives by oblique angles on the order of 130° to 150°. A preferred arrangement is an asymmetric scribe shoe in which the angles from the reference scribe to each of the secondary scribes differs ([[:file:core-orientation_fig2.png|Figure 2]]). Such an arrangement allows up-down directions on the core to be distinguished easily.

At agreed upon depth intervals, pumps and rotation are shut down for a period of several minutes to allow for a vibration-free film frame to be snapped. The process is repeated at intervals until the core barrel is tripped out. At the surface, the film is retrieved and developed. The azimuth of the compass lug, and hence of the reference groove, is then determined for the depths at which vibration-free shots were taken. Results can be available at the wellsite within hours of core retrieval. A recent refinement of this system uses a modified [[measurement while drilling]] (MWD) unit to replace the camera and timer within the nonmagnetic collar. The unit is powered by a battery pack and records azimuth data continuously. The recorded data are retrieved from the unit when the barrel is tripped out. This system has the advantage that it is not necessary to shut down pumps and rotation to record data.

At agreed upon depth intervals, pumps and rotation are shut down for a period of several minutes to allow for a vibration-free film frame to be snapped. The process is repeated at intervals until the core barrel is tripped out. At the surface, the film is retrieved and developed. The [[azimuth]] of the compass lug, and hence of the reference groove, is then determined for the depths at which vibration-free shots were taken. Results can be available at the wellsite within hours of core retrieval. A recent refinement of this system uses a modified [[measurement while drilling]] (MWD) unit to replace the camera and timer within the nonmagnetic collar. The unit is powered by a battery pack and records azimuth data continuously. The recorded data are retrieved from the unit when the barrel is tripped out. This system has the advantage that it is not necessary to shut down pumps and rotation to record data.

==Core-based orientation techniques==

==Core-based orientation techniques==

Numerous studies have been done to determine the accuracy of different orientation techniques. Nelson et al.<ref name=pt03r37>Nelson, R. A., L. C. Lenox, and B. J. Ward, 1987, [http://archives.datapages.com/data/bulletns/1986-87/data/pg/0071/0004/0350/0357.htm Oriented core—its use, error, and uncertainty]: AAPG Bulletin, v. 71, n. 4, p. 357–367.</ref> provides a particularly comprehensive review of error sources for mechanical orientation and comparisons of accuracy with other techniques. Core orientation techniques are capable of achieving, under optimum conditions, accuracies of ≤5°. Sources and severity of errors depend on many factors, some of which are listed here:

Numerous studies have been done to determine the accuracy of different orientation techniques. Nelson et al.<ref name=pt03r37>Nelson, R. A., L. C. Lenox, and B. J. Ward, 1987, [http://archives.datapages.com/data/bulletns/1986-87/data/pg/0071/0004/0350/0357.htm Oriented core—its use, error, and uncertainty]: AAPG Bulletin, v. 71, n. 4, p. 357–367.</ref> provides a particularly comprehensive review of error sources for mechanical orientation and comparisons of accuracy with other techniques. Core orientation techniques are capable of achieving, under optimum conditions, accuracies of ≤5°. Sources and severity of errors depend on many factors, some of which are listed here:

* At high latitudes, most techniques are prone to failure for similar reasons: the inclination of the ambient earth's field makes an accurate measurement of azimuth difficult.

* At high latitudes, most techniques are prone to failure for similar reasons: the inclination of the ambient earth's field makes an accurate measurement of [[azimuth]] difficult.

* High angle deviation of a borehole may exacerbate or reduce errors, depending on the direction of deviation. In holes approaching horizontal, orientation of cores to geographic north loses its meaning; up-down core directions then become important.

* High angle deviation of a borehole may exacerbate or reduce errors, depending on the direction of deviation. In holes approaching horizontal, orientation of cores to geographic north loses its meaning; up-down core directions then become important.

* During mechanical core orientation, film or batteries can fail under high temperature conditions (generally >[[temperature::240&deg;F]]) or film may run out if downhole time is extended. Tandem systems are recommended to reduce the chance of failure.

* During mechanical core orientation, film or batteries can fail under high temperature conditions (generally >[[temperature::240&deg;F]]) or film may run out if downhole time is extended. Tandem systems are recommended to reduce the chance of failure.