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.

You can access this article if you purchase or spend a download.