In this topic, we present a brief list of principles to guide you through planning your GeoSteering project and in actually carrying out your plan.
Geosteering Principles
Geosteering is primarily a stratigraphic problem, with the goal of exposing the greatest length of horizontal wellbore to the targeted horizon. (If this is NOT the case, then why go to the expense of drilling a horizontal wellbore in the first place?) Therefore, the planning and the drilling of a horizontal well should concentrate PRIMARILY upon the stratigraphy of the target.
This is accomplished through focusing upon True Stratigraphic Position rather than True Vertical Depth. When the position of the wellbore is described in reference to a particular stratigraphic marker, it will be much easier for the Geosteering Team to guide the wellbore to its optimum position within the target reservoir.
Pre-Drill Planning
Pre-drill planning is a crucial part of geosteering. The primary objective of this stage is to arrive at a thorough understanding of the target reservoir, with emphasis on the structure and stratigraphy. These plans will help ensure that the appropriate tools for analysis will be used and the appropriate personnel resources are prepared to steer the well. Ultimately, if a well is planned correctly, the company’s objectives will be met even if unexpected events occur.
Preliminary planning should include two stages: Optimal Planning and Uncertainty Planning.
Optimal Plan
The Optimal Plan is devised to enable the wellbore to meet all objectives. The plan should be stated in terms of stratigraphic and engineering parameters, and should include:
- the depth to KOP and its stratigraphic position (which has priority over depth),
- the depth to target,
- the stratigraphic description of the target reservoir,
- the minimum reservoir exposure required to meet objectives,
- the maximum and minimum inclination, and
- the expected apparent formation dip.
Uncertainty Plan
Uncertainty Planning is performed to predict and describe the expected range of variance from the Optimal Plan. Expect uncertainty to occur, both in the mechanics of directional drilling and in the stratigraphy. The Uncertainty Plan should serve as the basis for creating contingency plans that define decision points and set forth the best options before they occur in the drilling phase.
For example, the Geosteering Team should plan for sidetrack or bailout points, depending upon whether reservoir exposure objectives are met at the point where a fault cuts the wellbore.
- If the reservoir exposure objectives are already met and a fault is cut that is too large to steer back into target, then the bailout contingency would dictate that the well be completed at that point.
- If, on the other hand the reservoir exposure objectives have not been met, then an open hole sidetrack may be the best option for landing the target across the fault.
If contingency plans are developed before the well spuds, then valuable time will be saved when such a situation is encountered while drilling.
