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Abstract
Selecting the most appropriate bit for any hole section is the key feature in achieving superior drilling performance. This feature has an enormous influence on the optimized performance of drilling and also economically affects the process greatly. The most important factor to correctly select a bit is formation physical characteristics, mainly rock compressive strength. Different sources can be used to obtain rock strength along the wellbore, such as logs, cutting and rock mechanical test, or even drilling data. The use of empirical correlations based on logs is often the only way to estimate strength in many situations due to the absence of cores for laboratory tests. In this study, one of the Iranian oil fields was selected to be analyzed. The goal was calculating the compressive strength from sonic log parameters. In order to develop an exponential correlation, log, drilling data, and backward simulation were used. Log data included sonic, neutron, density, and gamma ray logs. The correlation developed is applicable to determine the quantity of compressive strength of underground formations in absence of cores.
NOMENCLATURE
| a, b | = | bit constant, hr.rpm.inch/ft |
| ac, bc, cc | = | lithology-dependent constant |
| as, bs | = | rock strength lithology constant |
| c | = | bit constant, hr.lbf.gal/ft.lb.cp.inch |
| C0, UCS | = | uniaxial compressive strength, psi |
| db | = | bit diameter, inches |
| E | = | Young’s modulus, psi |
| fc(Pe) | = | chip hold-down function (dimensionless) |
| Fjm | = | modified jet impact force, lbf |
| N | = | rotary speed, rpm |
| Pe | = | effective differential pressure, psi |
| R, ROP | = | rate of penetration, ft/hr |
| S | = | compressive strength, psi |
| Vclay | = | clay content (dimensionless) |
| Vp | = | compressive wave velocity, ft/sec |
| W, WOB | = | weight on bit, lb |
| Wf | = | wear function (dimensionless) |
| ΔBG | = | change in bit tooth wear (dimensionless) |
| &phis; | = | porosity (dimensionless) |
| γf | = | fluid density, ppg |
| μf | = | fluid viscosity, cp |
| ν | = | Poisson’s ratio (dimensionless) |
| ρ | = | density, pcf |