Formation evaluation has been historically done ‘after the fact’, by means of wireline logs run at either an intermediate casing point, or when the well had been TD'ed. While this procedure is clearly inefficient in unexplored areas, there is also significant room for improvement in even relatively developed areas, if only one were able to measure and analyze formation attributes just after drill bit penetration. Ruggedization and miniaturization of the electronics have yielded tools that can be placed just behind the drill bit: increasingly efficient data transmission protocols permit ever more data to be examined, in real time. Saudi Aramco utilizes this technology for both real-time formation evaluation, and well-bore trajectory optimization.
The basic Logged-While-Drilling (LWD) service includes gamma ray (GR), density-neutron porosities (Rhob-NPhi) and resistivities at a variety of depths of investigation, transmitted up-hole at sufficient resolution to allow real-time formation evaluation. With this information one is in a position to quantitatively monitor local reservoir quality, and to make recommendations with regard to wellbore trajectory (deflect up or down, etc) and wellbore length (drill ahead, TD early, etc). One is not, however, able to address reservoir geometry questions (bed dip), which also have a direct bearing on the ‘drill ahead question’.
Modern LWD services include azimuthal gamma ray, density and resistivity measurements, which ‘look’ up, down and to the side. As the wellbore intersects various reservoir features, well-documented sinusoidal patterns present themselves visually in the data, and when coupled with real-time directional survey measurements allow one to deduce the local reservoir geometry. Formation evaluation personnel are literally ‘in the driver’s seat’ and can steer the wellbore either through or along (depending upon the circumstances) the feature of interest. One can furthermore extrapolate this local geometry (as opposed to field averages) geometry over to the next drilling location, and therefore compound the value. These capabilities are illustrated with actual data sets, in a development well environment.
The field was discovered and delineated in the late-60's and early 70's, but due to its remote location not developed till the late-90's. About 35 vertical wells were drilled and cored during the delineation phase, and initial formation evaluation algorithms developed. We thus had, going into the development phase, an overview of what to expect.
Development drilling was in two phases:
Additional vertical wells, which were cored and served to further define the field boundaries and interpretation techniques.
Horizontal wells which were generally located and drilled to allow efficient reservoir depletion. Upon occasion, these wellbores were deliberately steered so as to produce both a commercial wellbore, and to further delineate the reservoir (by deflecting them upwards near the toe of the well, so as to intersect the Top Formation).
In the case of horizontal wells, gravity is not sufficient to pull the tools into the outer reaches of the well, so that pipe-conveyance was used in conjunction with routine wireline line logs. Logging-While-Drilling (LWD) options were also investigated, and as we gained favorable experience with this technique, began to rely upon it more. Modern LWD tools provide both the basic petrophysical measurements, and borehole images. We repeatedly found these images to be beneficial in the development well environment.