Geologic Steering
In many drilling programs, it is easy to distinguish between planning the well trajectory and actually following it. The directional profile is clearly defined in the well plan, and then the well is drilled more or less along this pre-assigned course – a process known as geometric steering.
Geometric steering served the industry for many years, and remains an important aspect of directional drilling. With a bent-housing positive displacement motor, a well-designed bottomhole assembly, and MWD capabilities, it is possible to minimize wellbore deviation and follow a planned course simply by changing the tool face orientation as needed.
But even the most precise geometric steering capabilities may not be adequate to reach some pay zones. Many horizontal and extended-reach wells target small, thin, heterogeneous, highly fractured or steeply dipping reservoirs. Such reservoirs are all too easy to miss – a pre-assigned trajectory that is even a few feet too shallow, too deep or too far to the right or left can result in an unsuccessful well.
Fortunately, there is an alternative approach to reaching difficult drilling targets and maximizing reservoir exposure: a process known as Geologic steering, or geosteering.
Geologic steering combines directional drilling technology with geological and geophysical information and real-time formation evaluation capabilities to guide wells as they approach prospective pay zones, to stay within target intervals, to avoid zone boundaries or fluid contacts and to execute successful well completions.
Figure 1, for example, depicts a horizontal well drilled in the North Sea to exploit a thin oil column below a gas cap.
Based on extensive reservoir simulation studies, the well’s target was the oil-water contact. Using real-time resistivity data, the bit was steered up or down as needed, and the well was maintained at the desired position.
(For more information on geologic steering tools and methods, refer to the topic Geosteering: Fundamentals, Planning and Implementation, which may be found in the Geology and Drilling).
