Estimating formation leakage pressure using a coupled model of circulating temperature-pressure in an eccentric annulus

Abstract

The eccentric annulus represents the majority of real-life situations and is attributed to a continuous change of hole-deviation angle and azimuth angle with well depth, leading to a difference in fluid flow and heat transfer mechanisms, compared with a concentric annulus. Errors resulting from ignoring the effect of eccentricity on wellbore circulating pressure and equivalent circulating density can lead to formation leakage. Based on a close relationship between circulating pressure and leakage pressure, a practical approach to predicting formation leakage pressure was established in the eccentric annulus, after running the casing, through the gradual increase of the flow rate and back pressure at the wellhead. Based on the conservation principle of energy and mass, and considering the collective effect of lost circulation and an eccentric annulus on wellbore temperature and pressure, a transient heat transfer model and a fluid flow model were developed and solved to estimate leakage pressure accurately. The numerical solution results were presented, analyzed and tested using a practical example. The results indicated that, with increasing flow rate, annulus pressure increases and temperature decreases. Temperature and pressure in the wellbore have a significant influence on the properties of the rheological parameters of Herschel-Bulkley fluid. Additionally, pressure loss and the equivalent circulating density (ECD) in the annulus were further elevated by setting back pressure at the wellhead, compared with an increase of flow rate. Most relevant is that the validity of the established method was verified through both monitored and calculated data.