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[[file:M91Figure161.JPG|thumb|400px|{{figure number|1|}}Horizontal wells are drilled at a high angle, generally greater than 80&deg;, with the intent of keeping the well within a specific reservoir interval or hydrocarbon zone.<ref name=Shepherd_2009>Shepherd, Mike, 2009, [http://archives.datapages.com/data/specpubs/memoir91/CHAPTER28/CHAPTER28.HTM Types of wells], ''in'' M. Shepherd, Oil field production geology, [http://store.aapg.org/detail.aspx?id=788 AAPG Memoir 91], p. 231-297.</ref>]]

[[file:M91Figure161.JPG|thumb|400px|{{figure number|1|}}Horizontal wells are drilled at a high angle, generally greater than 80&deg;, with the intent of keeping the well within a specific reservoir interval or hydrocarbon zone.<ref name=Shepherd_2009>Shepherd, Mike, 2009, [http://archives.datapages.com/data/specpubs/memoir91/CHAPTER28/CHAPTER28.HTM Types of wells], ''in'' M. Shepherd, Oil field production geology, [http://store.aapg.org/detail.aspx?id=788 AAPG Memoir 91], p. 231-297.</ref>]]

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&deg; from vertical.

[[Horizontal well]]s 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&deg; 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 [[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]]).

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]]).

Frequently, when drilling new well locations, the geology will turn out quite different from what was expected and this reflects the nature of reservoir uncertainty. Even so, the outcome from the vertical penetration of a reservoir interval is a lot more predictable than when a horizontal well is drilled. Random geological uncertainties that will have a relatively trivial effect on the drilling outcome of a vertical well can cause serious problems with a horizontal well operation.

Frequently, when drilling new well locations, the geology will turn out quite different from what was expected and this reflects the nature of reservoir uncertainty. Even so, the outcome from the vertical penetration of a reservoir interval is a lot more predictable than when a horizontal well is drilled. Random geological uncertainties that will have a relatively trivial effect on the drilling outcome of a vertical well can cause serious problems with a horizontal well operation.

[[file:M91Figure163.JPG|thumb|400px|{{figure number|3}}A horizontal well will be geosteered through a target zone by assuming the bed dip. If the assumed dip is wrong, the well may exit the target zone. Problems also occur if the well crosses an unexpected fault.<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:M91Figure163.JPG|thumb|400px|{{figure number|3}}A horizontal well will be geosteered through a target zone by assuming the bed [[dip]]. If the assumed dip is wrong, the well may exit the target zone. Problems also occur if the well crosses an unexpected fault.<ref name=Shepherd_2009>Shepherd, Mike, 2009, Types of wells, ''in'' M. Shepherd, Oil field production geology, AAPG Memoir 91, p. 231-297.</ref>]]

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 (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.

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>

Reservoirs tend to be much longer and wider laterally compared to their thickness, so a horizontal well is more likely to be in significantly greater contact with a given length of reservoir than a vertical well. Another feature of a horizontal well is that, for a given flow rate, a longer well needs less pressure drawdown to produce at that rate.

Reservoirs tend to be much longer and wider laterally compared to their thickness, so a horizontal well is more likely to be in significantly greater contact with a given length of reservoir than a vertical well. Another feature of a horizontal well is that, for a given flow rate, a longer well needs less pressure drawdown to produce at that rate.