A Novel Pulsed Neutron LWD Geochemical Logging Tool with Sigma and Direct Carbon Measurements

Abstract A novel 6.75-in. logging-while-drilling (LWD) geochemical tool has been developed for accurate lithology, mineralogy, well- placement, and geosteering applications in complex reservoirs. This LWD tool utilizes capture and inelastic gamma rays induced by a pulsed neutron generator to provide real-time formation elemental and mineralogical concentrations, sigma, and direct carbon measurements. These measurements provide essential information for an accurate petrophysical interpretation and aid timely decision- making for superior drilling performance. The new LWD tool is based on an advanced 14-MeV pulsed neutron generator and a high-resolution Lanthanum Bromide (LaBr3) gamma-ray scintillator coupled with a ruggedized photomultiplier tube (PMT). The generator-sensor package is situated inside a superalloy drill collar with a boron sleeve over the sensor section. The raw data is stored in memory and processed in real time. The inelastic and capture energy spectra are fitted to elemental standards and the weight concentrations are derived from the yields through oxide closure and special calibrations. The results are processed into minerals and lithologies. A dual-exponential fit to the time decay spectra determines sigma. The tool was characterized and tested using approximately 30 natural and artificial lab formations as well as temperature, pressure, shock, and vibration testing facilities. The tool elemental standards and sensitivities were derived using a combination of lab measurements and Monte Carlo simulation. Drilling tests were conducted at a test rig using a bottomhole assembly (BHA) including a rotary steerable system (RSS) and a typical triple combo to drill 2,900-ft through the Glenn Rose Limestone, Pearsall Shale, Hosston- Travis Dolomite, and the Cotton Valley Sandstone formations. Several sections were wiped multiple times with the same and different tools to assess real-world measurement repeatability. The tool answers were also compared to a full suite of open-hole wireline logs which were logged in the same well shortly after the drilling was completed. In addition, 48 cores from this well were obtained with wireline side-wall coring runs and were analyzed with laboratory standard procedures. The analysis included mineralogy, major elements, trace elements, and total carbon. This paper presents results from laboratory formations and logs recorded in the test well to illustrate tool performance. The new tool features a large detector volume and a precisely controlled neutron generator, coupled with high-speed electronics and advanced controls. The development process included comprehensive characterization and testing that included mock-up and prototype tools, a thorough computer modeling and simulation effort, and development of precise data-processing algorithms. The result is the first LWD geochemical logging tool to report directly measured carbon from the inelastic energy spectrum.