Changes

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 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.

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, 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., [http://store.aapg.org/detail.aspx?id=525 Horizontal wells: Focus on the reservoir]: 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.