Selecting a suitable fluid loss control additive is paramount when engineering the invert emulsion drilling fluids typically preferred for drilling under extreme temperatures. However, choosing an appropriate fluid loss control agent is not a straightforward decision as most commercially available additives come with some distinctive environmental, technical, and/or economic limitations. Until recently, these constraints further exacerbated the complexities inherent in formulating a drilling fluid to withstand elevated bottomhole temperature (BHT), thereby prompting research to develop a single agent that could safely and economically satisfy the fluid loss control requirements of high-temperature (HT) well construction.

Investigation has identified naturally occurring quebracho modified with fatty acid-derived amines as the most environmentally and technically viable option for cost-effective fluid loss control in HT wells. Field trials in the North Sea, Egypt, Thailand, and elsewhere have verified the capacity of the thermally stable additive to reduce fluid loss appreciably and minimize the risks of formation damage while delivering a comparatively benign environmental profile.

Fluid loss control in hot wells

With respect to borehole and thermal stability, corrosion inhibition, fluid loss control, and the deposition of a high-quality filter cake, invert-emulsion drilling fluids deliver clear advantages over their water-base counterparts in HT and similarly demanding applications. With tightening environmental restrictions on conventional oil-base drilling fluids, operators increasingly rely on the more environmentally acceptable synthetic-base muds (SBM) for demanding applications. However, SBM are not entirely immune to regulatory scrutiny, as environmental restrictions also extend to the additives used to control fluid loss.

Comparative overall rheological properties and HP/HT FL of a 14.2-lb/gal SBM using either the new ATQ additive or a polymer fluid loss control agent The respective properties were tested at 149°C (300°F). (Images courtesy of Schlumberger)

Superior fluid loss control, and in turn filter cake quality, is crucial because of the influence this property has on myriad aspects of well construction, including wellbore stability, the completion process, downhole losses, differential sticking, and formation damage. For invert emulsion drilling fluids, formation damage results when solids invade the formation or there is an interaction with surfactants in the filtrate that comprises the base oil and oil-soluble emulsifiers, wetting agents, rheology modifiers, and the fluid loss control additives. Filtrate loss control hinges on the deposition of a high-quality filter cake, which is essential to preventing the creation of emulsions with reservoir fluids or any fluid interaction with the formation.

Until now, the selection of a non-damaging fluid loss control additive largely boiled down to a choice among gilsonite, amine-treated lignite, and synthetic organo-soluble gel resin, each of which presents distinct issues with regard to environmental compliance, high comparative costs, or technical inadequacy under extremely high temperatures.

Those constraints opened the door for the newly developed nonasphaltic fluid loss control agent, which is synthesized by reacting nontoxic quebracho with fatty amines at an elevated temperature. The result is a fine, reddish-brown powder that is completely soluble-to-dispersible in synthetic and other invert emulsion drilling fluids.

Results of Acute Sediment Toxicity Tests have shown the new additive is well below the median lethal concentration (LC50) threshold for drilling fluids used in the marine environment.

HT testing protocol

Test methodologies were conducted in accordance with API RP 13B-2, which establishes evaluation procedures for the typical drilling fluid properties of plastic viscosity (PV), yield point (YP), 10-minute and 10-second gel strengths, electrical stability (ES), and HP/HT fluid loss (high-temperature, high-pressure). The quebracho-based fluid loss control agent was tested against gilsonite, amine-treat lignite, and other additives in a variety of nonaqueous fluid formulations covering a wide range of applications. The additive properties were evaluated under assorted base oils, oil/water ratios (O/W), mud weights, and temperatures.

Rheological properties and HP/HT FL test at 177°C (350°F) of a 19-lb/gal mineral oil-base drilling fluid using the new additive in a deep gas well trial. The formulation was tested after aging at 232°C (450°F).

To determine the effects of mud weight and O/W ratios on fluid loss, investigators examined the additive in drilling fluid densities from 10 lb/gal to 16 lb/gal. At concentrations of 8 lb/bbl, the quebracho-based additive was tested with an O/W ratio of 80:20 for the 10- and 12-lb/gal drilling fluids, increasing to 90:10 for the 14- and 16-lb/gal formulations.

Once the samples were heat-aged, the respective rheologies were measured at 49°C (120°F), while the HP/HT FL was measured at 149°C (300°F) at 500 psi differential pressure. These and other results showed that when compared to commercially available fluid loss additives, the quebracho-based candidate delivered obvious advantages in HT to ultra-HT applications. Specifically, in 24-hour static aging, the additive remained stable at 252°C to 300°C (485°F to 575°F) with minimal fluid loss.

In evaluating the likelihood of formation damage, researchers conducted a series of comparative return permeability tests on high-permeability sandstone and Berea core samples. The tests confirmed the nondamaging characteristics of the additive with return permeabilities approaching 100% on both the sandstone and Berea samples. Test results were attributed to the deposition of a high-quality filter cake and minimal fluid invasion.

Field results

A number of field trials using a range of synthetic and mineral oil-base drilling fluids confirmed the thermal stability and capacity of the new additive to prevent formation damage. A deep gas well was identified to test the stability of the additive in a 19-lb/gal (2.2) mineral oil-base drilling fluid at up to 450°F (232°C) BHT. The results showed fluid formulation containing the additive remaining stable to 450°F with excellent fluid loss control.

Return permeability 232°C (400°F) test results on sandstone.

In Egypt, meanwhile, the additive was used in a 17.2 lb/gal SBM, where its performance was compared against a commercially available polymeric fluid loss control agent. After heat aging, the respective rheologies were measured at 66°C (150°F) and HP/HT FL at 177°C (350°F ) at a differential pressure of 500 psi. The quebracho-based additive was shown to maintain HT stability with overall higher performance than the polymeric agent.

A test in a North Sea well focused on reducing filtration loss in the production interval to assess formation damage risks. The BHT was measured at 135°C (275°F), and the HP/HT filtration loss evaluated on conventional filter paper and a 5-micron aloxite disc. The additive was incorporated in an SBM where it demonstrated its ability to readily control filtration behavior.

Analysis of lab and field data results have validated the new quebracho-based additive as an environmentally sound, technically proficient, and cost-effective option for HT fluid loss control.

(This article was adapted from AADE-11-NTCE-29 “Non-Asphaltic, Fluid-Loss-Control Agent for High-Temperature Applications in Synthetic-Based Invert Emulsion Drilling Fluids.” presented at the 2011 AADE Fluids Conference and Exhibition in Houston)