RHEOLOGICAL INVESTIGATION AND DEVELOPMENT OF HIGH-PRESSURE AND HIGH-TEMPERATURE DRILLING FLUID

Due to the global increase in crude oil and gas consumption, the drilling of deeper high-pressure and high-temperature (HPHT) wells has been carried out extensively in recent years. The drilling of these wells is challenging due to the lack of reliable measurement and the deterioration of drilling fluid under HPHT conditions. The present study aims to develop an accurate shear rate model for the time-independent high yield stress drilling fluids using the Fann 35 viscometer readings. The true yield stress, an input parameter for the proposed shear rate equation, is also determined accurately with the proposed two-capillary method, based on capillary rise dynamics. The shear rates obtained using the proposed model are compared with the conventional shear rate model and are used to predict the model parameters of power-law, Herschel-Bulkley, and Robertson-Stiff rheological models. The accuracy of the model parameter prediction obtained in the present study and conventional shear rate is judged by validating the experimentally obtained pressure loss data. The mean square error in pressure drop is found to be minimum, indicating the efficacy of the proposed shear rate model. Two drilling fluids based on polyanionic cellulose grafted copolymer involving bentonite and functionalized fly ash (poly-ben and poly-fnFA) have also been formulated to obtain the desired thermal rheological, filtration, and lubrication behavior under HPHT conditions. The proposed shear rate model has been evaluated using the developed HPHT drilling fluids and has been used to predict pressure drop under surface downhole conditions. The pressure- and temperature-dependent shear viscosity of drilling fluids have been determined at HPHT conditions using the fundamental principle of momentum transport. Input parameters for the HPHT shear viscosity model are (i) thermal decomposition temperature, (ii) isothermal compressibility, and (iii) reference shear viscosity of drilling fluid at the surface conditions. Similarly, a pressure- and temperature-dependent density model has also been presented to estimate density at HPHT conditions. The viscosity and density model has also been experimentally validated and adequately predicts properties at downhole conditions. The above viscosity and density models have been used to predict pressure drop under downhole conditions at laminar and turbulent flow regimes.