Insights into the Relationship between Equivalent Circulation Density and Drilling Fluid Rheological Properties

Abstract

Equivalent circulation density (ECD) management is a key factor for the successfulness of the drilling operations, especially when dealing with narrow mud-weight windows. Poor management of ECD can result in unsafe and/or inefficient drilling as well as an increase in drilling cost due to associated non-productive time (NPT). Different parameters can affect the ECD directly or indirectly including, but not limited to, wellbore geometry, cuttings, hole cleaning efficiency, flow rate, and rheological properties of the drilling fluid. However, the magnitude of the effect of each parameter is not well understood. In this paper, a comprehensive statistical analysis using the correlation coefficient was conducted using real field data to investigate the effect of three controllable factors – solid contents (SC), yield point (Yp), and plastic viscosity (PV) – on ECD. Data from over 1000 wells drilled in thirteen oilfields in Iraq were collected, tested for outliers, and examined for normality to appropriately select the method of the analysis. The results showed that all three parameters have a direct relationship with ECD, or in other words, increasing the values of these parameters will lead to increasing ECD values. However, the plastic viscosity effect showed the highest magnitude, or strongest relationship to ECD, while the yield point resulted in the lowest effect, or weakest relationship with ECD from among these three parameters. Therefore, it is crucial to pay close attention to the magnitude of PV and alter it accordingly. Understanding the relationship between the drilling fluid rheological properties and ECD is vital for better ECD management in order to minimize the non-productive time and cost associated with ECD-related drilling problems.