Investigations on the influence of rock and machine characteristics on coring rates in vertical and mildly inclined exploration drilling in complex coal bearing rock formation

Resource characterization and modelling are important in exploration wherein cores of rock are extracted from the surface. Such cores not only help to assess the quantity and quality of the mineral or ore but also provide means to establish an excavation and ground control regime for different types of mining. Prediction of coring rate assumes critical importance for planning the coal mines which have fragile nature of ground posing challenges in strata control and excavation performance. Coring rate or associated rock factors characterize the rockmass to help in this regard. In line with the above need, the geological and machine performance data of two representative boreholes drilled at 90 degrees and 80 degrees in a complex coal bearing strata were collected covering drill machine variables, penetration rates and physico-mechanical properties of the rock cores in laboratory. The penetration rate showed significant deviation up to a depth of 250 m from the surface in both the holes. The variation in penetration rate was 28% in basalt and 38% in mudstone, 16% in sandy shale with coal, 10% in sandy shale and - 2% in medium to coarse grained sandstone. In order to model the penetration rate, a rock designation factor was devised in terms of the depth, tensile strength and density of the rockmass. The rock designation factor along with other machine variables were analyzed initially using PCA enabled Gaussian progression method which helped in model refinement and selection of best variables. Subsequently, response surface analysis of selected variables viz., rock designation factor, torque, drill fluid pressure and RPM, using multivariate non-linear ANOVA with backpropagation method to develop a penetration rate model. Drill angle and rock type were retained as categorial factors. The model thus devised was evaluated using various techniques and resulted in RMSE of 8.1, MAE of 5.59, MSE of 65.61 with predicted R-2 of 0.927. The intercepts for different rocks analyzed were thus enumerated. The model developed can be used for defining penetration rates for the type of drill investigated. Explicit surface response with variations in different variables could thus be defined along with the relative sensitivity of the variables entering into the interaction with perturbation method. Also, a variation of 50 mm/min in penetration rates in 90 degrees and 80 degrees drilling is accounted by the model. The penetration rate prediction model is sensitive to the rock designation factor, RPM, drill fluid pressure, angle of drilling and torque in decreasing order. It is observed that the best penetration rate can be achieved at an average RPM of 436 and minimum drill fluid pressure of 6 kg/cm(,)(2) indicating that higher drill fluid pressures may not favor higher penetration rates.