Changes

[[file:M91Figure162.JPG|thumb|400px|{{figure number|2}}Problems can be encountered with landing a horizontal well if the target zone is too high or too low compared to what is predicted.<ref name=Shepherd_2009>Shepherd, Mike, 2009, Types of wells, ''in'' M. Shepherd, Oil field production geology, AAPG Memoir 91, p. 231-297.</ref>]]

[[file:M91Figure162.JPG|thumb|400px|{{figure number|2}}Problems can be encountered with landing a horizontal well if the target zone is too high or too low compared to what is predicted.<ref name=Shepherd_2009>Shepherd, Mike, 2009, Types of wells, ''in'' M. Shepherd, Oil field production geology, AAPG Memoir 91, p. 231-297.</ref>]]

One problem in drilling a horizontal well is in locating the kick off point at about the right distance above the reservoir ([[:file:M91Figure162.JPG|Figure 2]]). The kick off point will be planned for a specific depth above the prognosed target zone depth, such that there will be enough room to turn the well around, so as to enter the target at a near horizontal angle. If the target zone comes in high on prediction, the chances are that the well will be drilled all the way through the reservoir before being able to turn round quickly enough to establish a horizontal trajectory. If the target zone is deeper than expected, then quite a long distance of well can be drilled at a very high angle before the reservoir is entered. Given the normal uncertainty on establishing the depth of a target zone from the seismic method, it is common for a pilot hole to be drilled first to get this information directly. Pilot holes may be vertical, although it is better to deviate the pilot hole in the direction of the horizontal well path, and closer to the planned entry point for the horizontal section. If a horizontal well is planned near an appraisal well, then this can be used as a proxy for a pilot hole.

One problem in drilling a horizontal well is in locating the kick off point at about the right distance above the reservoir ([[:file:M91Figure162.JPG|Figure 2]]). The kick off point will be planned for a specific depth above the prognosed target zone depth, such that there will be enough room to turn the well around, so as to enter the target at a near horizontal angle. If the target zone comes in high on prediction, the chances are that the well will be drilled all the way through the reservoir before being able to turn round quickly enough to establish a horizontal trajectory. If the target zone is deeper than expected, then quite a long distance of well can be drilled at a very high angle before the reservoir is entered. Given the normal uncertainty on establishing the depth of a target zone from the [[seismic data]], it is common for a pilot hole to be drilled first to get this information directly. Pilot holes may be vertical, although it is better to deviate the pilot hole in the direction of the horizontal well path, and closer to the planned entry point for the horizontal section. If a horizontal well is planned near an appraisal well, then this can be used as a proxy for a pilot hole.

A horizontal well can be drilled geometrically where there is a reasonable confidence in the expected reservoir geometry. The targets are defined at the entry point and at total depth, and the well is drilled according to a set geometrical plan between them.

A horizontal well can be drilled geometrically where there is a reasonable confidence in the expected reservoir geometry. The targets are defined at the entry point and at total depth, and the well is drilled according to a set geometrical plan between them.

The alternative is to geosteer a horizontal well, particularly where there is less confidence in predicting the reservoir geology. Geosteering involves using geological information obtained as the well is being drilled to try and keep the well path within the target. This can involve the use of real-time log data but may also include input while drilling from well-site biostratigraphy or from examination of drill cuttings if the lithologies at the top and base of the reservoir are distinctive.

The alternative is to geosteer a horizontal well, particularly where there is less confidence in predicting the reservoir geology. Geosteering involves using geological information obtained as the well is being drilled to try and keep the well path within the target. This can involve the use of real-time log data but may also include input while drilling from well-site [[Biostratigraphic correlation and age determination|biostratigraphy]] or from examination of [[Mudlogging: drill cuttings analysis|drill cuttings]] if the lithologies at the top and base of the reservoir are distinctive.

The main technique in geosteering involves the use of a real-time log data display while the horizontal well is being drilled. The downhole log data can be directly transmitted to a computer screen in the geologist's office from the well site. This allows the geologist to establish which part of the reservoir is being drilled through and then decide where the well should be steered to next. This is done by comparing the real-time logs with data from nearby wells. Log responses in horizontal wells can look different from that in conventional wells.<ref name=Meehan_1994>Meehan, D. N., 1994, [https://www.onepetro.org/journal-paper/SPE-29242-PA Geological steering of horizontal wells]: Journal of Petroleum Technology, SPE 29242, v. 46, no. 1, p. 3-12.</ref> A catalog of expected log responses, as they would appear in a horizontal trajectory, can be created by computer modeling. If the geologist thinks that the well is above the target zone, they will ask the directional driller at the rig site to steer down; if the geologist believes they are below the target, they will ask the driller to steer up.

The main technique in geosteering involves the use of a real-time log data display while the horizontal well is being drilled. The downhole log data can be directly transmitted to a computer screen in the geologist's office from the well site. This allows the geologist to establish which part of the reservoir is being drilled through and then decide where the well should be steered to next. This is done by comparing the real-time logs with data from nearby wells. Log responses in horizontal wells can look different from that in conventional wells.<ref name=Meehan_1994>Meehan, D. N., 1994, [https://www.onepetro.org/journal-paper/SPE-29242-PA Geological steering of horizontal wells]: Journal of Petroleum Technology, SPE 29242, v. 46, no. 1, p. 3-12.</ref> A catalog of expected log responses, as they would appear in a horizontal trajectory, can be created by computer modeling. If the geologist thinks that the well is above the target zone, they will ask the directional driller at the rig site to steer down; if the geologist believes they are below the target, they will ask the driller to steer up.