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Traditionally, geoscientists determined their drilling objectives based on seismic, geologic, and reservoir models, displayed most recently in 3D visualization packages. The locations of these targets are passed along to well planners and drilling engineers, who create a proposed drilling path within the context of a spreadsheet: the geologic context is often lost resulting in geometric plans that may not be optimally placed in either the reservoir section or through the overburden, if not enough information has been passed along. These geometric plans may also fail to fully accommodate the geologic risks and uncertainties. By keeping the geologic model integrated in the planning and drilling phases, information pertinent to the success of the job can be exchanged easily and quickly, and, as an added bonus, does not rely on interpretations of written data: The geoscientist and the well planner can work using the same data (from both sides) in the same context; the rigsite geologist and the directional driller are able to update their position relative to the earth model, check the validity of the earth model, and make appropriate adjustments to geometrically steer the well to its optimal location.
In this theoretical example, a very large (1000m x 500m x 175m) geologic target has been specified, along with two intermediate targets, intended to keep the drilling path within the target zone (Figure 1). Although a wellpath can be easily created that intersects each of these targets, it is not necessarily apparent, without the geologic information, if the wellpath has indeed remained within the target zone (Figure 2). By looking in 3D at the curtain section along the wellpath (rather than simply a plot of the vertical section), the first pass shows that the path dips below the target zone, intersecting the lower boundary at the fault intersection—a potentially undesirable position (Figure 3).
Knowing that, the first target can be entered at a higher inclination, allowing less directional drilling and the wellpath to stay within the target zone—all changed in a matter of moments, rather than waiting for a proposal and redesign request to pass between the geoscientist and the well planner. (Figure 4)
Even with the better path, however, the positional uncertainty needs to be taken into account (Figure 5, 6). While the geoscientist may feel that a target is so large it cannot be missed, that might not be the case. Geologic targets should be ″eroded″ to determine a driller′s target: this new eroded target is the geologic target volume ″minus″ the positional-uncertainty ″volume″ (based on the defined survey programme). The remaining target is what is left for the driller to intersect. Since the eroded target size is dependent on the accuracy of the tool type and model, if the survey programme designed uses tools with small positional uncertainty models (i.e., more accurate tools), the driller′s target should be quite large, and the likelihood of being able to hit the target is high (given the confidence level set for the positional uncertainty) (Figure 5). If, however, the survey programme uses tools with poor or large positional uncertainty so that the positional uncertainty increases, the driller′s target can be reduced so significantly or even completely disappear, resulting in an unacceptable probability of success, even with such an initially large reservoir target.
Additional information, such as angles at which the drill bit will intersect a fault given a specified plan (Figure 7) or the distance from a hazard such as a fault or lease line, can also be more easily determined both visually and numerically by including the geologic model during the planning phases of a drilling project. While drilling, horizon information can be easily incorporated back into the earth model (e.g., by shifting horizons to match log information) to keep a consistent updated view of the project (Figure 8).
In the end, both sides benefit when an integrated approach (9, 10, and 11) is carried from the planning phase to the drilling phase. Planning can be performed quicker with less iterations necessary; planning can be performed against alternate earth models quickly and efficiently; affects of positional uncertainty are easily viewed; earth models can be updated with drilling information at the rigsite, and geometric well steering within the context of the earth model can be optimized.