Changes

The industry standard for many years has been a mechanical orientation technique utilizing a specially designed core barrel and nonmagnetic drill collar with a compass, camera, battery pack, and timer.<ref name=pt03r41>Rowley, D. S., Burk, C. A., Manual, T., Kempe, W. F., 1971, Oriented cores: Christensen Diamond Products Paper, 15 p.</ref> A typical assembly is shown in [[:file:core-orientation_fig1.png|Figure 1]].

The industry standard for many years has been a mechanical orientation technique utilizing a specially designed core barrel and nonmagnetic drill collar with a compass, camera, battery pack, and timer.<ref name=pt03r41>Rowley, D. S., Burk, C. A., Manual, T., Kempe, W. F., 1971, Oriented cores: Christensen Diamond Products Paper, 15 p.</ref> A typical assembly is shown in [[:file:core-orientation_fig1.png|Figure 1]].

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 (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.