Drilling process is fundamentally controlled and influenced by the properties of the penetrated formation. The focus of various studies is directed mainly on the optimal design of drill bit and drilling operations related to the (expected) geological situation of a safe drilling process.
Out of interest the question "Is it possible to extract any lithologic information from drilling data?" also arises.
The drilling process at the bit represents a complicated rock mechanical process. The drill bit acts as a rotating cutter, controlled mainly by weight on bit (WOB), bit size, number of revolutions per time (RPM), which controls speed of the cutting process. We define a "cutting force Fc" as a combination of these parameters and investigate the relationship between Fc and rate of penetration (ROP). For the correlation with lithology of penetrated formations, two methods are applied on test wells:
crossplots Fc versus ROP with discrimination for dominant lithology.
histograms of the ratio of two variables for dominant lithology.
Drilling data from two basins are analyzed (in both cases wells are nearly vertical):
Vienna Basin: Dominant lithologies are sandstones (varying degree of cementation), shale/marls, limestones, and dolomites.
Williston Basin: Dominant lithologies in the analysed section are sandstone shale, dolomitic limestone, and limestone with anhydrite.
Data points from the rocks with similar lithology in the crossplot are situated in a cloud - different rocks show different cloud position. Particularly between clastic (sand, silt, shale) and carbonate (limestone, dolomite) rocks, a clear separation is visible.
The implementation of lines for a constant ratio Fc/ROP in the crossplot separate the different lithologies. Therefore, the statistical distribution of this ratio S = Fc/ROP for the dominant lithologies was analyzed by histograms. For the two test wells, histograms separate the different lithologies and confirm the information content.
The drilling process is controlled by rock type; the analyse of drill-process data can be used for a lithological discrimination and especially for detecting changing lithology (boundaries) during drilling process. For two test wells, the discrimination could be demonstrated by the crossplot and histogram technique. The exact position of discriminator magnitudes (center of data clouds in the crossplots, peaks in the histograms) is specific for the considered field and may be controlled by more drilling parameters.