Technological advances in both equipment and computer software have enabled the implementation of new approaches in generation of mineralogical datasets at petroleum wellsites. These datasets are currently utilized by hydraulic fracturing engineers to assist in designing optimized fracture stage intervals in horizontal wellbores, rather than using evenly spaced intervals between treatment stages. Mineralogical data is generated by downhole wireline logging tools, and on drill cuttings, conventional wholecores, and rotary sidewall coreplugs utilizing a variety of analytical instrumentation techniques. This paper documents a study undertaken to assess mineralogical datasets generated on comparable samples, focused on evaluating analytical limitations and variances, toward obtaining consistent mineralogical results.
Instrumentation typically used to generate these datasets include x -ray diffraction (XRD), x-ray fluorescence (XRF), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), fourier transform infrared spectroscopy (FTIR), and inductively coupled plasma techniques (ICP mass spectroscopy or ICP optical emission spectroscopy). Variables introduced into the analysis in addition to the different analytical techniques include sample types and sizes, sampling methods, sample preparation, drilling mud contaminants, lithological heterogeneity, and depth correlations between cuttings, cores, and wireline measurements.
As the study evolved it became clear that wellsite sampling and preparation protocols needed to be properly defined to assure sample quality and accurate sample depth reference. Analytical equipment limitations must be fully understood, as should differences in measurement technologies both in the laboratory and at the wellsite. Equipment destined for wellsite analysis was evaluated in a controlled laboratory environment using reference mineral standards and standard mixtures to understand testing limitations and refine mineral phase calculations. Mineral terminologies, classifications, compositions, and the resulting databases were reviewed for consistency. Multiple cuttings and core sample sets from conventional sandstones, carbonates, and current mudrock plays such as the Eagle Ford and Marcellus shales were sub-divided and analyzed to allow direct comparisons of generated datasets. Complementary testing was conducted to confirm data quality.
This study yielded increased confidence in wellsite and laboratory analyses, including caveats where necessary, procedural guidelines for each analytical technique, and verification of deliverables appropriate to unconventional mudstone reservoirs. Example datasets, graphical comparisons, and report formats are included. The resultant wellsite datasets, in tandem with additional wellsite analytics, enhance confidence in optimized fracture stage interval decisions.