Designing Low-ECD Drilling Fluids for Superior Hydraulics and Sag Performance: A Rheological Approach

Abstract

Typical organophilic clay-free invert-emulsion fluids (IEFs) are known to offer improved performance compared to conventional organophilic clay-based IEFs. However, for both the typical clay-free IEFs and conventional clay-based IEFs, high-temperature stability continues to remain a challenge from aequivalent circulating density (ECD) management and cuttings transport standpoint. This paper discusses the latest generation of clay-free IEFs, termed as "low-ECD clay-free IEFs,"that consist of specific combinations of weighting agents, colloidal particles, and polymers. The composition of these low-ECD fluids leads to different rheological behavior compared to typical clay-free IEFs, especially at high temperatures (>200°F). This paper demonstrates that the peculiar rheological behavior of the low-ECD clay-free IEFs results in superior hydraulics and sag performance. Understanding the rheology also helps effectively design these fluids. Experiments were conducted to study the rheology of several low-ECD and typical clay-free IEFs at high-pressure/high-temperature (HP/HT) conditions using rotational viscometers. Fluids were studied across a temperature range of 40 to350°F and up to 12,000-psi pressure. The data were used to develop models for predicting the rheology of various fluids as a function of temperature and pressure. Using the rheological models, hydraulics performance of the low-ECD and typical clay-free IEFs was simulated in a high-temperature well. The simulations evaluatedthe ECD and cuttings carrying capacity of these fluids. Sag behavior of the fluids in the well was also investigated. The rheological study showed that typical clay-free IEFs thin with temperature (i.e., viscosity decreases with increasing temperature). In contrast, the low-ECD clay-free IEFs showed increased viscosity with temperature (i.e., temperature thickening). Temperature thickening was especially prominent at lower viscometer rotational speeds (i.e., lower shear rates). The temperature thickening helps the fluid retain its ability to suspend cuttings, barite, and other particulates. The contrast in the rheology behavior was used to demonstrate the ultimate advantage of the low-ECD clay-free IEFs—the desired cuttings transport and sag performance in a well can be achieved at significantly lower wellbore pressures (or lower ECDs) compared to typical fluids. Wellbore case studies confirmed the accuracy of the rheology and hydraulics models for low-ECD fluids and the fluids’ superior performance at high temperatures. The low-ECD clay-free IEFs can be accurately designed to provide superior wellbore pressure management and wellbore stability. This work is crucial for narrow-margin and high-temperature wells.

Low Equivalent Circulating Density Organoclay-Free Invert Emulsion Drilling Fluids

“…The removal of these low gravity solids (LGS) with the polymeric additives would help to reduce and optimize the overall rheology of the fluid. This reduced rheology would in turn also reduce the strong surge and swab effects which would help in better ECD management [16][17][18]. The use of polymeric additives in the organoclay-free invert emulsion fluids (OCIEFs) would help to form a gel structure required to achieve the necessary viscosity control.…”