Development of rock bitwear models for selected rocks in Nigeria using dominant rock properties

This work deals with the development of wear rate models for some selected rocks in Nigeria. Rock samples obtained were tested for chemical composition, point load strength index, uniaxial compressive strength, and porosity in the laboratory. These properties were correlated with wear rate. The results of statistical correlation matrix revealed that silica content, point load strength index, uniaxial compressive strength, porosity, Equivalent Quartz Content (EQC), rebounds hardness value and circularity factor are dominant rock properties affecting bit wear having high coefficient of correlation. These rock properties were used for development of regression model to predict wear rate for each of the selected rocks. The regression equation for predicting bit wear rate of porphyritic hornblende-granite is WG = 0.135 – 0.001 Is50 – 0.002EQC – 0.007n, the regression model has multiple correlation coefficient of R2 = 0.992 (99.2%), which means that 99.2% of the variation in wear rate can be attributed to variation in point load strength Index (Is50), Equivalent Quartz Content (EQC) and porosity (n) of porphyritic hornblende-granite. The results of comparison between field measured wear rate and simulated wear rate followed the same trend and the coefficient of correlation between wear rate (dependent variable) and regression standard predicted value is R2 = 0.99, which suggest that the models are valid.

Evaluation of Tri-Cone Bit Performance on Limestone Formation

This study examines the characteristics of limestone formation as well as the performance of different tri-cone bits in limestone in Ewekoro. Rock samples were collected from different layers of limestone encountered in the quarry during drilling operation. These samples were tested in the laboratory for uniaxial compressive strength and the tensile strength. Also, the chemical compositions of the samples were determined using the X-Ray Fluorescence (XRF) Spectrometer and the results were used in the determination of the Equivalent Quartz Content (EQC). The length of the insert buttons on the surface of the drill bits were measured using digital vernier calliper at regular intervals. In addition, the tooth and bearing wear rates as well as the penetration rate were determined on the field. The results of the uniaxial compressive strength varied from 86.5MPa (medium strength) to 112 MPa (high strength). The Equivalent Quartz Content (EQC) of the rock samples varied from 17.37% to 36.676% while the Rock Abrasivity Index (RAI) varied from 15.03 to 43.317. The results of the drilling variables showed that higher wear rate was experienced when milled tooth tri-cone bit was used for drilling than using insert tri-cone bit for drilling limestone formation, hence the tri-cone bits performed optimally well in all the formations except in glauconite (GLAB) where its performance was low. However, the economic analysis indicated that the insert tri-cone bit drilling cost per metre varied from N660/m to N673/m while the milled tooth tri-cone bit drilling cost per metre varied from N684/m to N710/m. This will act as a data base for selection of drill bits and drilling equipment in limestone quarries.