Abstract

The introduction of at-bit gamma ray and inclination measurements in a Logging-While-Drilling (LWD) tool string greatly enhances the geosteering capabilities of the Bottom Hole Assembly (BHA). The measurements are made in a short sub positioned immediately above the drill bit. The gamma ray sensor is located 11 inches above the bit, the inclination 22 inches. These tools are now routinely used in Coalbed Methane (CBM) drilling, where the horizontal section within a particular coal seam can be greatly extended. CBM is not the only application of the at-bit measurements as they are used in both conventional and unconventional horizontal drilling to significantly extend the horizontal section. At-bit measurements allow for greater directional control and confidence in geosteering complex sections. The tools are used in traditional reservoirs to steer into one or more particular sand(s), in shale-gas plays such as the Barnett shale, and in fractured carbonate sequences.

Geosteering requires a software package capable of pre-well modeling, displaying the measured information, and interactively adapting the model to the real-time data. Offset well data or pilot hole data are used together with the well plan for a horizontal or extended reach well to predict the log response of the different logging tools. At the same time, different models can be generated to predict what the log response will be when drilling out of zone, either at the top or the bottom, when faults (often sub-seismic) are encountered or when dramatic facies changes occur. While drilling, the models are continuously updated and compared to the real-time data. A more detailed model of the formation and the structures is made while drilling, which allows real-time decisions to modify the well path to stay in the intended zone.

The at-bit measurement tools stay in two-way contact with the Measurement-While-Drilling (MWD) tool. Any other LWD tool can then be added to the bottom hole assembly, such as resistivity, density-neutron and sonic tools. A particular application of the sonic data is in the identification of natural fractures. A waveform processing technique adapted from surface seismic processing to LWD or wireline waveforms (Instantaneous Waveform Characterization) is used to highlight the presence of open natural fractures, specifically in carbonate reservoirs. This information can be used in designing completions where multiple acid fracturing stages can be optimally spaced.