Changes

* 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 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 8–10deg 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.

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 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.

==Designer wells==

==Designer wells==