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[[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 LWD 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 [[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.

* Thin reservoirs. A conventional well will intersect a relatively thin section of the reservoir, whereas a horizontal well can run the length of the reservoir and produce much more hydrocarbons.<ref name=Fayersetal_1995>Fayers, F. J., S. Arbabi, and K. Aziz, 1995, [http://pg.geoscienceworld.org/content/1/1/13.full.pdf Challenges in reservoir engineering from prospects for horizontal wells]: Petroleum Geoscience, v. 1, p. 13-23.</ref>

* Thin reservoirs. A conventional well will intersect a relatively thin section of the reservoir, whereas a horizontal well can run the length of the reservoir and produce much more hydrocarbons.<ref name=Fayersetal_1995>Fayers, F. J., S. Arbabi, and K. Aziz, 1995, [http://pg.geoscienceworld.org/content/1/1/13.full.pdf Challenges in reservoir engineering from prospects for horizontal wells]: Petroleum Geoscience, v. 1, p. 13-23.</ref>

* Horizontal wells can target long, narrow macroforms such as channel fill sandstones.

* Horizontal wells can target long, narrow macroforms such as channel fill sandstones.

* Fractured reservoirs. A horizontal well has a much greater chance of intersecting vertical or steeply dipping natural fractures compared to conventional wells. This can be a particularly effective way of producing fractured reservoirs with very low matrix permeabilities.<ref name=Majorandholtz_1997>Major, R. P., and M. H. Holtz, 1997, [http://archives.datapages.com/data/bulletns/1997/07jul/1063/1063.htm Identifying fracture orientation in a mature carbonate platform reservoir]: AAPG Bulletin, v. 81, no. 7, p. 1063-1069.</ref>

* Fractured reservoirs. A horizontal well has a much greater chance of intersecting vertical or steeply dipping natural [[fracture]]s compared to conventional wells. This can be a particularly effective way of producing fractured reservoirs with very low [http://www.britannica.com/EBchecked/topic/369508/matrix matrix] [[Permeability|permeabilities]].<ref name=Majorandholtz_1997>Major, R. P., and M. H. Holtz, 1997, [http://archives.datapages.com/data/bulletns/1997/07jul/1063/1063.htm Identifying fracture orientation in a mature carbonate platform reservoir]: AAPG Bulletin, v. 81, no. 7, p. 1063-1069.</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 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 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.

In certain parts of the world, horizontal wells are the well type of preference, whereas conventional wells are much less common. This is true of the Danish North Sea, where chalk is the main reservoir interval, and also in parts of the Middle East such as Qatar, Abu Dhabi, and Oman.<ref name=Nurmi_1996>Nurmi, R., Horizontal highlights: Middle East Well Evaluation Review, no. 16, p. 8-25.</ref>

In certain parts of the world, horizontal wells are the well type of preference, whereas conventional wells are much less common. This is true of the Danish North Sea, where [[chalk]] is the main reservoir interval, and also in parts of the Middle East such as Qatar, Abu Dhabi, and Oman.<ref name=Nurmi_1996>Nurmi, R., Horizontal highlights: Middle East Well Evaluation Review, no. 16, p. 8-25.</ref>

There are situations where it is not advantageous to drill horizontal wells. In reservoirs where there is a very low Kv/Kh because of small-scale bedding-parallel baffles, bedding-parallel horizontal wells are not effective.<ref name=Haldorsenetal_1987>Haldorsen, H. H., D. M. Chang, and S. H. Begg, 1987, Discontinuous vertical permeability barriers: A challenge to engineer and geologists, ''in'' J. Kleppe, E. W. Berg, A. T. Buller, O. Hjelmeland, and O. Torsaeter, eds., North Sea oil and gas reservirs I: London, Graham & Trotman, p. 127-151.</ref> Numerous baffles parallel to the wellbore will severely restrict the contactable drainage volume. It is better to drill strongly layered reservoirs like these with slanted instead of horizontal wells. Some subsurface professionals will advise against drilling horizontal wells if it can be more practical to drill a slant well. These are less risky to drill, and there is a better chance of establishing which part of the reservoir stratigraphy has been penetrated by the well. Slant wells may be a better option for drilling injection wells where it is important to ensure waterflood support to a specific reservoir interval.

There are situations where it is not advantageous to drill horizontal wells. In reservoirs where there is a very low Kv/Kh (vertical permeability/horizontal permeability) because of small-scale bedding-parallel baffles, bedding-parallel horizontal wells are not effective.<ref name=Haldorsenetal_1987>Haldorsen, H. H., D. M. Chang, and S. H. Begg, 1987, Discontinuous vertical permeability barriers: A challenge to engineer and geologists, ''in'' J. Kleppe, E. W. Berg, A. T. Buller, O. Hjelmeland, and O. Torsaeter, eds., North Sea oil and gas reservirs I: London, Graham & Trotman, p. 127-151.</ref> Numerous baffles parallel to the wellbore will severely restrict the contactable drainage volume. It is better to drill strongly layered reservoirs like these with slanted instead of horizontal wells. Some subsurface professionals will advise against drilling horizontal wells if it can be more practical to drill a slant well. These are less risky to drill, and there is a better chance of establishing which part of the reservoir stratigraphy has been penetrated by the well. Slant wells may be a better option for drilling injection wells where it is important to ensure [[Waterflooding|waterflood]] support to a specific reservoir interval.

==Designer wells==

==Designer wells==