Changes

==Sidetrack wells==

==Sidetrack wells==

A typical operation is to sidetrack a well. This is where a well has already been drilled or partly drilled and there is a need to exit out of one side of the well to a different target. A sidetrack may be required if there is an object stuck in the original hole, which cannot be fished out. In producing fields, an existing well may be sidetracked if there is no further use for that well, e.g., the oil well has watered out. A window will be cut in the casing of the original well by a special milling assembly, and drilling will then proceed out of the window toward a new target.

A typical operation is to [[sidetrack]] a well. This is where a well has already been drilled or partly drilled and there is a need to exit out of one side of the well to a different target. A sidetrack may be required if there is an object stuck in the original hole, which cannot be fished out. In producing fields, an existing well may be sidetracked if there is no further use for that well, e.g., the oil well has watered out. A window will be cut in the casing of the original well by a special milling assembly, and drilling will then proceed out of the window toward a new target.

==Horizontal wells==

==Horizontal wells==

Horizontal wells are wells where the reservoir section is drilled at a high angle, typically with a trajectory to keep the well within a specific reservoir interval or hydrocarbon zone. In a strict sense, these wells are rarely perfectly horizontal, but they tend to be near horizontal mostly, generally at an angle greater than 80° from vertical.

Horizontal wells are wells where the reservoir section is drilled at a high angle, typically with a trajectory to keep the well within a specific reservoir interval or hydrocarbon zone. In a strict sense, these wells are rarely perfectly horizontal, but they tend to be near horizontal mostly, generally at an angle greater than 80° from vertical.

Horizontal wells are drilled in a specific configuration. The tangent section of the well is drilled along a deviated well path to just above the reservoir section, to what is known as the kick off point. From the kick off point, the well is drilled at an increasingly higher angle, arcing around toward an angle close to horizontal. The point at which the well enters (or lands on) the reservoir is called the entry point. From there on, the well continues at a near-horizontal orientation with the intention of keeping it substantially within the reservoir target until the desired length of horizontal penetration is reached ([[:file:M91Figure161.JPG|Figure 1]]).

Horizontal wells are drilled in a specific configuration. The tangent section of the well is drilled along a deviated well path to just above the reservoir section, to what is known as the [[Kickoff point (KOP)|kick off point]]. From the kick off point, the well is drilled at an increasingly higher angle, arcing around toward an angle close to horizontal. The point at which the well enters (or lands on) the reservoir is called the entry point. From there on, the well continues at a near-horizontal orientation with the intention of keeping it substantially within the reservoir target until the desired length of horizontal penetration is reached ([[:file:M91Figure161.JPG|Figure 1]]).

[[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>]]

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 [[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 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.

At very high angles, if the top reservoir is 15 m (49 ft) deeper than predicted, the target will be penetrated much later than planned, or maybe missed altogether ([[:file:M91Figure162.JPG|Figure 2]]). Sometimes, after tracking the target interval, the well may then cross an unexpected subseismic [[fault]] and exit out of the target zone. It may not be clear which stratigraphic interval has been found on the other side of the fault. The geologist monitoring the well may not know if the target is above or below the well path. Another problem that can occur is that the predicted formation [[dip]] angle is wrong by a few degrees. In this instance, the well will quickly exit out of the top or base of a thin target. It can take a long section of the drilled interval before it can be steered back into the target horizon again ([[:file:M91Figure163.JPG|Figure 3]]).

At very high angles, if the top reservoir is 15 m (49 ft) deeper than predicted, the target will be penetrated much later than planned, or maybe missed altogether ([[:file:M91Figure162.JPG|Figure 2]]). Sometimes, after tracking the target interval, the well may then cross an unexpected subseismic [[fault]] and exit out of the target zone. It may not be clear which stratigraphic interval has been found on the other side of the fault. The geologist monitoring the well may not know if the target is above or below the well path. Another problem that can occur is that the predicted formation [[dip]] angle is wrong by a few degrees. In this instance, the well will quickly exit out of the top or base of a thin target. It can take a long section of the drilled interval before it can be steered back into the target horizon again ([[:file:M91Figure163.JPG|Figure 3]]).

Some geologists refer to the steering efficiency of a horizontal well; the percentage of the total well length within the target zone beyond the entry point. Modern logging-while-drilling [[Basic_open_hole_tools#Resistivity|resistivity logs]] used in geosteering assemblies have some degree of look-ahead capability to try and maximize the steering efficiency. The current created by the tool can have a sufficient depth of penetration to detect if the drilling assembly is converging on a bed boundary. This can give enough warning to allow the well to be steered away from the bed boundary.

Some geologists refer to the steering efficiency of a horizontal well; the percentage of the total well length within the target zone beyond the entry point. Modern [[Logging while drilling (LWD)|logging-while-drilling]] [[Basic_open_hole_tools#Resistivity|resistivity logs]] used in geosteering assemblies have some degree of look-ahead capability to try and maximize the steering efficiency. The current created by the tool can have a sufficient depth of penetration to detect if the drilling assembly is converging on a bed boundary. This can give enough warning to allow the well to be steered away from the bed boundary.

Despite these problems, horizontal wells often end up as the best producers in a field. There are many reasons for drilling a horizontal well as opposed to a conventional well. They can produce considerable volumes of incremental reserves from what would otherwise be an underperforming area of the reservoir. Although they are more expensive to drill and are more prone to failure, horizontal wells often produce at several times the rate of an equivalent conventional well in the same reservoir. For example, experience in the heavy oil belt of Venezuela has shown that flow rates are increased significantly by producing from horizontal wells, yet they cost only 1.5 times more than vertical wells.<ref name=Hamiltonetal_2003>Hamilton, D. S., R. Barba, M. H. Holtz, J. Yeh, M. Rodriguez, M. Sanchez, P. Calderon, and J. Castillo, 2003, [http://archives.datapages.com/data/specpubs/method14/me14ch08/me14ch08.htm Horizontal-well drilling in the heavy-oil belt, eastern Venezuela Basin: A postmortem of drilling experiences], ''in'' T. R. Carr, P. Mason, and C. T. Feazel, eds., Horizontal wells: Focus on the reservoir: [http://store.aapg.org/detail.aspx?id=525 AAPG Methods in Exploration 14], p. 127-141.</ref> In the Widuri and adjacent fields, offshore Sumatra, 15% of the producers are horizontal wells, yet these provide 30% of the oil production volume.<ref name=Carteretal_1998>Carter, D. C., W. Kortlang, M. Smelcer, and J. C. Troncoso, 1998, An integrated approach to horizontal well design and planning in Widuri field, offshore southeast Sumatra, Indonesia: Proceedings of the Indonesian Petroleum Association, 26th Annual Convention, May 1998, v. 2, p. 135-162.</ref>

Despite these problems, horizontal wells often end up as the best producers in a field. There are many reasons for drilling a horizontal well as opposed to a conventional well. They can produce considerable volumes of incremental reserves from what would otherwise be an underperforming area of the reservoir. Although they are more expensive to drill and are more prone to failure, horizontal wells often produce at several times the rate of an equivalent conventional well in the same reservoir. For example, experience in the heavy oil belt of Venezuela has shown that flow rates are increased significantly by producing from horizontal wells, yet they cost only 1.5 times more than vertical wells.<ref name=Hamiltonetal_2003>Hamilton, D. S., R. Barba, M. H. Holtz, J. Yeh, M. Rodriguez, M. Sanchez, P. Calderon, and J. Castillo, 2003, [http://archives.datapages.com/data/specpubs/method14/me14ch08/me14ch08.htm Horizontal-well drilling in the heavy-oil belt, eastern Venezuela Basin: A postmortem of drilling experiences], ''in'' T. R. Carr, P. Mason, and C. T. Feazel, eds., Horizontal wells: Focus on the reservoir: [http://store.aapg.org/detail.aspx?id=525 AAPG Methods in Exploration 14], p. 127-141.</ref> In the Widuri and adjacent fields, offshore Sumatra, 15% of the producers are horizontal wells, yet these provide 30% of the oil production volume.<ref name=Carteretal_1998>Carter, D. C., W. Kortlang, M. Smelcer, and J. C. Troncoso, 1998, An integrated approach to horizontal well design and planning in Widuri field, offshore southeast Sumatra, Indonesia: Proceedings of the Indonesian Petroleum Association, 26th Annual Convention, May 1998, v. 2, p. 135-162.</ref>

* Low-permeability reservoirs. Where an interval shows low permeabilities, horizontal wells can make up for this by maximizing the contact length with the reservoir. This means that low-permeability rocks such as chalk can produce at economic rates that would be marginal to uneconomic with conventional wells.

* Low-permeability reservoirs. Where an interval shows low permeabilities, horizontal wells can make up for this by maximizing the contact length with the reservoir. This means that low-permeability rocks such as chalk can produce at economic rates that would be marginal to uneconomic with conventional wells.

* Reservoirs prone to [[Production_problems#Water-gas_coning|coning]]. Because of the lower drawdown, horizontal wells may be less prone to water or gas coning behavior. For example, horizontal wells have been drilled in the [[Widuri field]], offshore Sumatra, so as to minimize water coning. High vertical permeabilities and viscous oil are factors likely to promote coning behavior in the vertical wells in the field.<ref name=Carteretal_1998 />

* Reservoirs prone to [[Production_problems#Water-gas_coning|coning]]. Because of the lower drawdown, horizontal wells may be less prone to water or gas coning behavior. For example, horizontal wells have been drilled in the [[Widuri field]], offshore Sumatra, so as to minimize water coning. High vertical permeabilities and viscous oil are factors likely to promote coning behavior in the vertical wells in the field.<ref name=Carteretal_1998 />

* Similarly, individual horizontal wells produce more oil in heavy oil reservoirs because the lower pressure drawdown tends to keep water and gas away from the well longer. For example, a total of 110 horizontal wells had been drilled prior to 2002 in the [[Hamaca field]] in Venezuela's Orinoco heavy oil belt. The development plan is to ultimately drill over 1000 horizontal laterals to produce the 8–10&deg; [[API gravity]] oil.<ref name=Tankersleyandwaite_2002>Tankersley, T. H., and M. W. Waite, 2002, [https://www.onepetro.org/conference-paper/SPE-78957-MS Reservoir modeling for horizontal exploitation of a giant heavy oil field-Challenges and lessons learned]: Presented at the SPE International Thermal Operations and Heavy Oil Symposium and International Horizontal Well Technology Conference, November 4-7, 2002, Calgary, Canada, SPE Paper 78957, 6p.</ref>

* Similarly, individual horizontal wells produce more oil in heavy oil reservoirs because the lower pressure drawdown tends to keep water and gas away from the well longer. For example, a total of 110 horizontal wells had been drilled prior to 2002 in the [[Hamaca field]] in Venezuela's Orinoco heavy oil belt. The development plan is to ultimately drill over 1000 horizontal [[lateral|laterals]] to produce the 8–10&deg; [[API gravity]] oil.<ref name=Tankersleyandwaite_2002>Tankersley, T. H., and M. W. Waite, 2002, [https://www.onepetro.org/conference-paper/SPE-78957-MS Reservoir modeling for horizontal exploitation of a giant heavy oil field-Challenges and lessons learned]: Presented at the SPE International Thermal Operations and Heavy Oil Symposium and International Horizontal Well Technology Conference, November 4-7, 2002, Calgary, Canada, SPE Paper 78957, 6p.</ref>

* Oil rims, thin oil columns typically lying below a gas cap, can be targeted with horizontal wells. The reduced drawdown minimizes the chances of coning water up from the water leg or drawing gas down from the gas cap.

* Oil rims, thin oil columns typically lying below a gas cap, can be targeted with horizontal wells. The reduced drawdown minimizes the chances of coning water up from the water leg or drawing gas down from the gas cap.

[[file:M91Figure165.JPG|thumb|400px|{{figure number|5}}Multilateral wells in the Tern field, UK North Sea. From Black et al.<ref name=Blacketal=1999>Black, R. C., H. J. Poelen, M. J. Roberts, and S. E. Roddy, 1999, Tern field development: A marriage of new technologies for business benefit, ''in'' A. J. Fleet and S. A. R. Boldy, eds., Petroleum geology of northwest Europe: Proceedings of the 5th Conference, Geological Society, London, p. 1063-1073.</ref> Reprinted with permission from the Geological Society.]]

[[file:M91Figure165.JPG|thumb|400px|{{figure number|5}}Multilateral wells in the Tern field, UK North Sea. From Black et al.<ref name=Blacketal=1999>Black, R. C., H. J. Poelen, M. J. Roberts, and S. E. Roddy, 1999, Tern field development: A marriage of new technologies for business benefit, ''in'' A. J. Fleet and S. A. R. Boldy, eds., Petroleum geology of northwest Europe: Proceedings of the 5th Conference, Geological Society, London, p. 1063-1073.</ref> Reprinted with permission from the Geological Society.]]

Multilateral wells are wells that have more than one branch radiating from the main borehole ([[:file:M91Figure165.JPG|Figure 5]]). Each branch can drain a separate part of the reservoir and produce into a common single wellbore. The advantage of multilateral wells is that, for the same number of drainage points, they can be somewhat cheaper than if separate wells had been drilled.

[[Multilateral well]]s are wells that have more than one branch radiating from the main borehole ([[:file:M91Figure165.JPG|Figure 5]]). Each branch can drain a separate part of the reservoir and produce into a common single wellbore. The advantage of multilateral wells is that, for the same number of drainage points, they can be somewhat cheaper than if separate wells had been drilled.

==Coiled tubing drilling==

==Coiled tubing drilling==