Advancements in Weight Material Sag Evaluation: A New Perspective with Advanced Laboratory Equipment
Authors: Jaimes Juan Pablo , Farooq Omer , Bravo Oscar , Croy Seth , Nenjerama Yeukayi 
Published: February 2024

In this paper, we introduce innovative laboratory equipment, the high-temperature, high-pressure sag cell (HTHP-SC), designed specifically to measure the static settling of weight material under actual downhole conditions. high-pressure testing to assess drilling fluids’ weight-material suspension is not new in the drilling fluids industry. However, the HTHP-SC is an advancement in the field of drilling fluid testing equipment because its design improves measurement accuracy, and because it is compatible with current cementing consistometers and curing chambers. This compatibility enhances its utility for evaluating drilling fluid performance, especially high-performance invert emulsion fluids under field pressure and temperature regimes.

Accurate measurement and thorough fluid property evaluation are of paramount importance to customize the design of the drilling fluids to ensure their performance under particular drilling envelopes. Therefore, simulating actual downhole conditions in the laboratory becomes instrumental in improving the prediction accuracy of fluid performance under actual drilling scenarios. This approach is invaluable as it enables a better understanding of the dynamics involved, helping to optimize drilling fluid properties and additives for improved outcomes.

It is established that the types of additives used to formulate drilling fluids and the resulting fluid properties play a decisive role in weight-material sag performance of invert emulsion fluids. Failing to recognize this could result in a flawed fluid design, and ineffective fluid treatments during drilling operations that could potentially lead to fluid issues, impairing, in turn, the drilling performance. For this reason, a comparative analysis of low-pressure and high-pressure tests conducted up to a 7-day period is presented in this paper. The results demonstrate that low-pressure density measurements can greatly exaggerate settling compared to testing at actual well pressures and bottomhole temperatures, a behavior already recognized in the drilling fluids industry. However, with the attempt to shed more clarity on the reason for these measurements, we show in this paper that these significant differences in density measurements cannot simply be related to an increase in fluid viscosity caused by higher pressure, or a lesser expansion of the base oil, but principally to the type of rheology modifiers used. This insight is essential for selecting the appropriate chemistry for the rheology modifier(s) to achieve the desired fluid performance, based upon the drilling conditions, and to avoid overtreatments with these additives, which could lead to higher than desired equivalent circulating densities (ECD), surge and swab pressures, and pressures to break circulation. Furthermore, we present and discuss the dynamic sag performance of the fluids tested and compare it with the static performance.

In conclusion, we present low-pressure static and dynamic sag measurements, and high-pressure static results using a newly designed piece of laboratory equipment aiming to contribute to the understanding of weight-material sag under static and dynamic conditions when varying the type of rheological modifiers. We observed that the choice of rheology modifier is crucial for static sag behavior, with less distinct effects on dynamic sag tendencies. A combination of a clay-based modifier and an organoclay-based additive showed superior performance compared to fluids containing a copolymer latex and a polyamide. The findings from this study attempt to improve operational efficiency and safety in drilling operations. The HTHP-SC represents a step forward in the quest for an accurate assessment of the static weight-material sag potential, particularly in the context of high-performance invert emulsion fluids (HPIEF), or the so-called low-ECD fluids.