Formulating Sag-Resistant, Low-Gravity Solids-Free Invert Emulsion Fluids

Abstract

Abstract Invert-emulsion fluid (IEF) systems formulated without organophilic clay have successfully addressed issues of barite sag and reservoir productivity. While ‘clay-free’ IEFs provide robust properties, additional materials in the form of low-gravity solids (LGS), like sized calcium carbonate or clay-type materials are required to bolster the rheological properties and suspension character of the system. This increases the plastic viscosity of the fluid, thereby resulting in a lower rate of penetration (ROP) and higher equivalent circulating densities (ECD). In the absence of LGS, the loss of suspension, especially in an inclined well, can lead to sag and cause a density gradient along the fluid column that may fracture the formation. This paper presents a solution towards sag in an IEF that is free of LGS viz. fine sized calcium carbonate or clay-type material, and is formulated with a novel sag control additive to suspend barite. The additive was shown to prevent sag in 9-ppg, 12-ppg, and 16-ppg clay-free IEFs in a static-aging cell held vertical at 90° and then inclined at 45°. The additive provided sag control in different base oils with less than 5-mL oil separation. Extended sag testing with the sag-control additive at 250°F for 24 hours and at 150°F for 60 hours gave minimal oil separation and no barite sag. High temperature-high pressure (HTHP) testing on 9-ppg and 12-ppg demonstrates a flat rheological profile in the presence of this additive. Contamination testing of IEFs containing this additive shows that the suspension characteristics remain unaffected and the mud properties are tolerant to the presence of contaminants. The additive is North Sea compliant with a biodegradability of 71.4% in 42 days and LC50 of >10 g/mL for aquatic organisms. Experimental data demonstrating both the environmental and anti-sag performance of the novel sag control agent is presented and compared to fluids without the sag control agent.