Effect of Physico-Mechanical Characteristics of Some Basement Complex Rocks on their Drillability in North Central Nigeria

The work examined the effect of physical and mechanical characteristics of some basement complex rock types on their drillability in north central Nigeria. The work was carried out using the field data and rock samples collected from the four locations of four different rock types, namely; City Gate (Porphyritic Granodiorite); Garaku (Granite); Gawaraka (Biotite Granite); and Madalla (Hornblende Granite). The physical properties (hardness, density and Water Absorption), mechanical properties (Uniaxial Compressive Strength (UCS), Point Load and Brittleness) and the drilling rate index (DRI) were determined in the laboratory using the appropriate standards. The results of the investigation showed that physico-mechanical characteristics of rocks have influence on the drillability. City Gate (Porphyritic Granodiorite) that has the lowest hardness of 45, lowest density of 2.661kg/m3 and highest water absorption value of 0.4%; was found to have the highest DRI value of 61. However, Madalla with the highest hardness of 52, has the highest density of 2.831kg/m3 and lowest water absorption value of 0.25%; has the lowest DRI value of 32. City gate and Madalla are classified as having high and low DRI respectively. This same trend was observed in the mechanical properties. City Gate (Porphyritic Granodiorite) with the highest DRI value was found to have the UCS value of 88.6MPa, point load value of 4.68MPA, with brittleness value of 78.23 while Madalla with lowest DRI value has UCS, point load and brittleness values of 126.6MPA, 6.97MPA and 49.78% respectively.