The majority of petrophysical data has been gained from downhole logging and laboratory core measurement. Drill cuttings are easily available for all drilled wells and can provide reservoir data throughout all phases of field development. They can also provide continuous petrophysical measurements for both vertical and horizontal wells. Many different rock properties can be extracted from drill cuttings; however, quantitative petrophysical measurements from source rock drill cuttings remain a challenge. This paper presents an optimized NMR-based workflow to measure the porosities and densities of drill cuttings from unconventional source rock reservoirs. The workflow includes sample cleaning, particle size selection, saturation, NMR, and mass measurements. We will also discuss some inherent problems in petrophysical measurement of drill cuttings generated by the PDC drill bit.
In this workflow, the first crucial step in obtaining quantitative data from drill cuttings is to clean the cuttings of mud contamination. We found that simultaneous ultrasonic bathing and shaking of the samples in a mesh basket submerged in diesel was very efficient to remove drilling mud additives. The process also automatically sieved to select the desired particle size for analysis, which was essential for accurate results. The sonication process saturated the drill cuttings to a certain degree. Further saturation was carried out using pressure.
Saturated samples were then subjected to a series of mass and NMR measurements. For unconventional source rocks, NMR allows for separation of liquid signals from inside and outside the drill cutting particles. In combination with mass measurements, both in air and in diesel, the NMR technique provided accurate porosity and density data of the drill cuttings samples.
The workflow measured porosity and density of real drill cuttings and crushed particulate samples. The results on crushed samples were in good agreement with other accepted lab techniques. Measurement errors on drill cuttings generated by the PDC drill bits were investigated.
The workflow was applied to drill cuttings from a horizontal source rock well. The measured porosity from the cuttings changes significantly along the measured depth of the tested horizontal well, indicating that the studied horizontal well was drilled through a very heterogeneous formation. The result is consistent with the large formation heterogeneity measured on acquired whole cores from the same reservoir.