Shales account for more than 75% of the formations drilled worldwide, and fixed-cutter bits are used to drill most of the footage. Cutter/rock interactions after the initial failure of rock are shown to be a major source of drilling inefficiency. Analytical and numerical modeling was implemented to understand these interactions and their controlling factors, but a comprehensive model does not exist. This is mainly because of the complex nature of the problem that depends on several factors that include rock characteristics, pressure-and-temperature environment, bit design, and drilling-fluid properties.
Having such a large range of factors requires a cross-disciplinary approach to tackle the problem. This paper is focused on fabric analysis of the rock cuttings. Recent developments in fabric-analysis techniques allow the study of the size, shape, composition, and spatial arrangement of particles and matrix constituents in fine-grained rocks. This has led to an increased understanding of compaction phenomena, shear strength, porosity, permeability, fracturing, electrical propagation, and seismic properties of the rock. Despite this, the changes in rock fabric during interaction with the drill bit are not well-understood. This work takes advantage of those techniques by analyzing the shale cuttings at the macro-, micro-, and nanolevels to understand how shales break and deform under confining pressure to better understand drill-bit/rock interactions.
Cuttings recovered from a well in Tuscaloosa, Louisiana, drilled with fixed-cutter bits were analyzed in multiple scales: macro, micro, and nano. Shallower and deeper sections were drilled with water-based mud and oil-based mud (OBM), respectively. Samples were gathered from seven depth intervals ranging from 13,500 to 21,320 ft. The microscale analysis was performed with a X-ray computed-tomography scanning technique, whereas nanoscale analysis was performed with a scanning electron microscope (SEM). Shale-cuttings fabric was characterized by images produced by energy-X-ray-descriptive spectroscopy (EDS) and backscattered-electron microscopy of ion-milled samples.
Cuttings were generally formed in the shape of layered ribbons in which the mud-facing side is uneven and serrated whereas the cutter side is smooth and has a darkened clay film. The size of the ribbons and thickness of the layers were larger in areas drilled with OBM. Cuttings accumulation in the form of a ball attached to some of the ribbons from drilling in OBM provided evidence that cutter balling can occur during field drilling operations. SEM-EDS analysis of cuttings showed significant accumulation of barite, a component in the drilling fluid, on the external surface of the serrated sides of the ribbons. In addition, scattered barite zones were found inside the cuttings. X-ray diffraction analyses indicated a mixed mineral assemblage dominated by quartz and smectite with minor illite, kaolinite, chlorite, mixed-layered materials, and traces of calcite and pyrite. It was hypothesized that the absence or scattered appearance of barite in some zones of produced cuttings, particularly the cutter side of the ribbon and the cutter ball, may relate to higher deformation of cuttings at those zones. In addition, the mechanism of cutter balling was explained with an analogy with metal-cutting theories. This was supported by comparison between the geometry of shale cuttings from this field and copper cuttings from single-cutter experiments.
Structural analysis of cuttings from actual field drilling reinforced the relevance of the observations made during laboratory experiments. It also provided unique insights, observations, and incentives for additional investigation of how cuttings are formed and what influences dysfunctions or inefficiencies. This is a significant step in understanding shale/cutter interactions that severely affect the bit penetration rate, especially under high confining pressure.