Changes

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]]).