Estimating and optimizing wellbore pressure during primary cementing in low pressure and leakage formations

Abstract

In most realistic situations, casing eccentricity and various fluids injected during primary cementing lead to a significant difference in fluid flow and circulating pressure, compared with a single fluid flowing in the concentric annulus. Ignoring the effect of eccentricity and fluid rheological model on the wellbore circulating pressure results in an inaccurate evaluation and leads to formation leakage, especially for low pressure and leakage formations. To overcome this challenge, firstly, rheological models of various fluids were analyzed using experimental data, including drilling fluid, spacer, and cement slurry. Secondly, the modified correlations of frictional pressure losses for Herschel-Bulkley and Power Law fluids in the eccentric annulus were developed, using the simulation data of computational fluid dynamics software (CFD) and considering the effects of flow behavior index, pipe diameter ratio, and casing eccentricity on the frictional pressure losses. Finally, based on the developed mathematical models, the optimal method of primary cementing and evaluation results were presented, analyzed, and tested in the field with great success. The results demonstrated that the drilling fluid and spacer fluid had the highest goodness of fit for the Herschel-Bulkley model, and the cementing slurry was suitable for the Power Law model. The ratio coefficient of pressure loss in the eccentric to concentric annulus decreased with a decrease of hole size and an increase of casing eccentricity. A 50 percent reduction in frictional pressure loss was evaluated when the casing lies on the borehole wall. Comparing the calculations of the developed correlation with the published correlation in the literature reveals a more accurate evaluation of friction pressure losses in the analyzed case. Combined with the realistic situation of a test well, the density, flow rate, and returning depth of the lead cement slurry were recommended as 1.5 g/cm3, 0.5 m3/min and 4100 m, respectively, according to the leakage pressure of 65.6 MPa that was calculated by the developed mathematical model. Most importantly, the developed method in this paper can accurately predict the variation characteristics of wellbore pressure during the primary cementing process and provide effective guidance in managing downhole pressure in low pressure and leakage formations by real-time adjustment of the most effective and controllable operation parameters.