Calculations of Equivalent Circulating Densities (ECDs) in Underbalanced Drilling Using Landmark WELLPLAN

Underbalanced drilling (UBD) is a drilling operation where the equivalent mud weight is maintained below the open hole pore pressure. In meeting the world’s rising energy demand in a sustainable manner, this technique is one of the best solutions to access challenging reservoirs and improve the recoverable reserves. However, performing underbalanced operation is not an easy task and any failure in it will cause severe damage, in most of the time, worst than conventional drilling. This project aimed to tackle one of the main challenges in underbalanced drilling which is obtaining reliable prediction of the downhole pressure or commonly expressed in terms of Equivalent Circulating Densities (ECDs). Dependable ECDs prediction method is very important as the operation window in UBD is usually very small. The key to acquire ECDs of aerated fluid accurately, which is calculations of frictional pressure loss of two-phase flow in the annular section was studied and presented in this paper. Due to the existence of gas phase in its composition, the prediction of ECDs is much more complex and might not be perfectly done by existing drilling hydraulics simulators. By using water and air as both phases in the aerated fluid, a set of experimental pressure drop data was used to make comparisons against frictional pressure loss calculated with Beggs and Brill method and results obtained from Landmark WELLPLAN software. The accuracy of both Beggs and Brill method and Landmark WELLPLAN software were discussed by taking into considerations effects of slip, flow pattern and inclination of well towards ECDs of drilling fluid. Results showed that WELLPLAN underestimated ECDs in all inclination sections while Beggs and Brill method is accurate for horizontal well section but less accurate for inclined sections.