Abstract

Though the drilling fluid rheology and wellbore hydraulics are essential to hole cleaning, wellbore stability and drilling economics are governed, few published hydraulic audit studies of individual wells have been performed in Libyan desert environments. In this study, a complete rheological and hydraulic characterization of the water-based mud (WBM) system utilized in the 12¼ in. open-hole section of Well A-201/4 that was drilled to the total depth of 7,969 ft true vertical depth (TVD) has been conducted. The field measurements were made in accordance with API RP 13B-1 and the Bingham Plastic and Power Law models were fitted to the data obtained from the rotational viscometer. The fluid velocity, effective viscosity, Reynolds number, flow regime and the frictional pressure loss obtained for all seven circulation intervals (API RP 13D) were calculated along with the equivalent circulating density, bit hydraulic parameters and cuttings transport ratio. The measured mud viscosity was close to a Bingham Plastic model with R² = 0.9999 and exhibited a plastic viscosity of 13 cp, yield point of 19 lbf/100 ft² and a pipe-flow behaviour index of 0.492. Due to only three viscometer speeds being recorded, the Herschel-Bulkley model was not determined. All the drill string flow was fully turbulent (Re = 3,966–10,872), while all the annulus flow was laminar (Re = 77–169). The hydraulic program has two main deficiencies. First, the annular velocity in the drill pipe / open-hole interval was only 62 ft/min, compared to the minimum API recommended of 100-120 ft/min over the longest interval in the well, which represents a 38 per cent fault. Second, and not previously reported, the nine nozzles that were 16/32 in. diameter caused a slight loss of head of only 24.5 psi, which gave 0.038 hp/in² hydraulic horsepower per square inch, about 1.5 per cent of the minimum accepted. The calculated standpipe pressure is 787 psi, which is 31 per cent of the standpipe pump installed, with 1,713 psi remaining.