Four Keys to Geosteering Success
Four critical factors must be determined to steer a well accurately. We will describe each factor below.
Geometric Position of the Wellbore
This includes positioning with respect to TVD and X/Y. The directional MWD tool is the most common source for this data. This information places the well in space, but says nothing about where the well is located stratigraphically, relative to the target. TVD has no stratigraphic significance whatsoever.
Stratigraphic Position of the Wellbore
The stratigraphic position of the wellbore is THE single most important piece of information needed for steering the well. After all, the only reason for drilling a horizontal well is to increase wellbore exposure to the reservoir -if the position of the well and its placement within the stratigraphy is not known with some degree of certainty, then what purpose is achieved by drilling a horizontal well?
Apparent formation Dip Rate
The second most important piece of information for accurately steering a well is the rate of apparent dip. This information is used to refine the inclination of the bit to help the drillstring to stay in the target. Any interpretation method that is unable to provide apparent formation dip rate will be insufficient to adequately steer a horizontal well.
Location throw and direction of faults
The ability to identify faults accurately and quickly will make steering much easier. As previously mentioned, most formations, (even those areas where no faults are recognized), are often found to be cut by small faults that fall below the resolution of the seismic data. These small faults nonetheless may be large enough to complicate geosteering, or may affect the producing properties of a horizontal wellbore.
Putting It All Together
There is no “Black Box” that will magically gather the above four keys to geosteering. Interpretation of horizontal data is always required. Experience is also essential to steering a well. It is very important for the novice to mentor under someone else that has steered a number of wells. If such experience is not within the company, then it is best to find a consultant who has the experience. It will be money well spent in the long run.
In addition to experience, a successful Geosteering Team Leader must be able to visualize the interaction between the wellbore and the geology, and must possess good pattern-recognition or correlation skills. (Some people simply do not have these prerequisite skills wired into their brain, and find geosteering to be extremely difficult.) These skills are so important that it must be said again. Horizontal data must be interpreted and correlated -so it is imperative that skilled geoscientists be used to steer a well.
All other evaluations of the data can be made after the steering decision is made, and the exact petrophysical characteristics of the reservoir can be determined at some other time. If too much data has to be analyzed in the process of arriving at a steering decision, then time will be wasted, and the critical steering decision may come too late.
