Examination and sensitivity analysis of friction factors as indicators of hold cleaning condition during drilling operations

Abstract

Rotation of the drill string equipped with the drill bit at the bottom leads to the progress of drilling process as a certain level of force is applied to the contact point of the drill bit and the formation. Rock penetration generates drill cuttings which are removed by the fluid flow from the bottom to the surface through the annulus around the drill string. In an optimized drilling process the rate of cuttings generation is almost equal to the rate of cuttings transformation to the surface. There are several parameters which need to be controlled during drilling such as bottom hole pressure and rate of penetration, therefore in most of the cases, maintaining a complete cuttings removal process may be challenging. Cuttings accumulation in the annulus can be the source of more severe problems like mechanical drill pipe sticking. A real-time hole cleaning monitoring system is expected to activate the pre-designed alarms for the drilling team and prevent further problems. The increase in the cuttings concentration in the annulus causes the increase in the value of mechanical and viscous friction factors. Some of the measured drilling parameters such as torque and hook load are a direct function of these friction factors. To take a step toward designing a real-time hole cleaning monitoring system during drilling operation, an estimator is designed in this study to extract the value of friction factors out of torque, hook load, pump pressure and down-hole pressure while drilling. The estimator for mechanical friction factor is a nonlinear estimator which calculates the friction value iteratively and filters the measurement noise. The annular pressure is a linear function of viscous friction factor hence a linear estimator is introduced to extract the value of the viscous friction factor in the presence of the measurement noise. Results indicate that the design estimators are capable of extracting the value of mechanical and viscous friction factors out of noisy measurements as indicators of well bore condition in terms of hole cleaning. Both of the estimators are model based estimators therefore selected models for the indicator parameters are introduced in this research as well. The results of the estimators are a function of the intensity of the relation between the measured drilling parameters and friction factors. The intensity of this dependency is a function of adjustable drilling parameters such as drilling fluid flow rate. The existence of uncertainty associated with the drilling parameters such as the Buoyancy factor, II necessitates the investigation of the degree of dependency of torque, hook load, pump pressure and bottom hole pressure to the friction factors. Therefore sensitivity and uncertainty analysis using the developed models have been carried out in this research.