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

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.

Influence of modified starches composition on their performance as fluid loss additives in invert-emulsion drilling fluids

“…This suggests that the chemical modification contributed to maintain the stability of the emulsions even after the aging process. The stability of an emulsion can decrease slightly with high temperature because the number of collisions between the water droplets dispersed in the system increases [39]. The modified starch provided greater stability for the water/oil emulsion after the insertion of the fatty chain (C-12 or C-18) in its molecule, with the ES values being directly related to the degree of chemical modification attained in the reactions.…”

“…Gel strength is directly related to the emulsion's capacity to carry the gravel and other solids produced by the drill bit by keeping them in suspension in the fluid [6,39,42,43]. The values measured for the initial gel (IG) and final gel (FG) properties contributed for gel strength's (GS) results bellow 0.96 Pa, indicating the fluids investigated do not have a progressive tendency to form gels.…”

Experimental Investigation of the Rheological Behavior of an Oil-Based Drilling Fluid with Rheology Modifier and Oil Wetter Additives

“…Similarly, it can plummet severalfold because of increasing the temperature of the bottomhole from ambient conditions. Such a change in viscosity can have significant consequences on the equivalent circulating density (ECD), the stability of the wellbore, and the suspension properties [ 26 ]. The effect of temperature reduction on the rheological properties of Jatropha curcas oil, groundnut oil and diesel oil was studied by Anawe et al [ 27 ].…”

Invert Drilling Fluids with High Internal Phase Content

“…Invert drilling fluids also contribute to increasing the drilling speed, extending the lifetime of the drill bit and reducing the adhesion force of the drill string to the filter cake on the borehole wall. These are just some of the advantages of invert drilling fluids that are taken into account when choosing the type of drilling fluid in difficult geological and technical conditions [17][18][19].…”