Spectral Gamma-Ray Measurement While Drilling

With the significant growth in the volume of unconventional wells over the past decade there is a need for an economical and reliable method for formation evaluation in horizontal wells. To lower drilling program costs, the majority of these wells are drilled with only measurement-while-drilling (MWD) gamma-ray logs. This limits available data to make critical well-placement decisions while drilling or for postdrilling well evaluation. A low-cost spectral gamma-ray measurement has recently been developed for use in logging-while-drilling (LWD) applications. Applications range from geosteering and bed-boundary detection to formation evaluation for clay typing and for empirical correlations to estimate total organic carbon (TOC). The spectral gamma-ray measurement is incorporated into an MWD collar, thereby avoiding the need for additional collars in the bottomhole assembly (BHA). This improves system reliability and reduces distance behind bit in a shorter BHA. The measurement quality, which is designed to match that of the wireline spectral gamma-ray tool, is ensured by employing two large NaI detectors that provide high counting rates and spectral definition, thereby enabling a measurement with very good statistical precision. Highly accurate characterization formations for potassium (K), uranium (U), and thorium (Th) were constructed. Tool responses acquired in these standard formations, as well as in the spectral gamma-spectroscopy pits at the University of Houston, allowed a highly accurate characterization of the measurement. Advanced features include robust sourceless gain regulation, real-time full-spectrum analysis, and automatic borehole potassium correction. To further enhance geosteering, the design may also be configured with a single azimuthal total gamma-ray detector and a single spectral gamma-ray detector. This combination provides bedding layers and composition data in real time. This novel system, which available in 43/4- or 63/4 in. MWD collar sizes, makes accurate measurements of formation potassium, uranium, and thorium concentrations in an economical and reliable manner. Log examples, laboratory data, computer modeling results, and comparisons with wireline spectral gamma-ray data are presented.