Optimizing Shale Reservoir Characterization With Spectroscopy Evaluation While Drilling

Abstract

Abstract

The unparalleled growth of unconventional shale plays has greatly changed the practice of petrophysics; from the analytical workflows to the laboratory analysis techniques to the tools utilized for logging. Data such as kerogen volume, total organic carbon (TOC), and clay mineralogy have proven to be reservoir characteristics fundamental to the shale oil and gas reservoir petrophysical analysis. Typically, this data is derived in the laboratory on vertical well cuttings and core samples with Pyrolysis, Fourier Transform Infrared Spectroscopy (FTIR), X-ray Flourescence (XRF), X-ray diffraction (XRD). These vertical well data points are then extrapolated through the horizontal well section and typically only correlated with total gamma ray. This methodology makes the assumption of mostly homogeneity in the shale reservoir and negligibly accounts for heterogeneity with total gamma ray, which is normally measured by a standard Measurement While Drilling (MWD) gamma ray sonde sensor.

The Spectral Gamma Ray Logging While Drilling (LWD) tool measures the Potassium (K), Uranium (U), and Thorium (Th) concentrations of the formation along the vertical and horizontal wellbore and provides this data in real-time while drilling. When calibrated to the laboratory data points of the vertical offset or pilot well, the Uranium content provides a reliable TOC and Kerogen volume, which not only provides the heterogeneity reservoir characteristics for an accurate unconventional petrophysical analysis, but can be used to place the wellbore in the reservoir “sweet-spot.” Potassium and Thorium concentrations historically have been used for qualitative mineral clay typing based on the Th/K ratio. By identifying clay minerals from cuttings with FTIR, XRF, and XRD, the Th/K ratio is calibrated to a specific formation or litho-facies to provide accurate clay mineral identification. This calibration is then used to qualitatively identify clay minerals in horizontal wells, which can be done while drilling with a LWD Spectral Gamma Ray tool. Additionally, variations of Thorium and Potassium spectroscopy measurements can aid in the well bore placement by identifying facies markers that the total gamma ray measurement cannot resolve.

The real time while drilling acquisition of Uranium, Potassium, and Thorium concentrations calibrated to geochemistry measurements from cuttings provide an unconventional petrophysical analysis and enhance the horizontal well placement.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016