Evaluation of novel microemulsion-based (O/W) drilling fluid with nonionic surfactant and shale interaction mechanisms

Abstract

The increasing production of hydrocarbons from unconventional reservoirs, particularly water-sensitive formations, has led to the emergence of technologies necessary to maintain wellbore stability. The drilling fluid must always be adequate to the needs presented by such formations since serious problems may be generated due to inadequate drilling fluid planning. Microemulsions appear in this scenario with advantages over the traditional fluids already used by the industry due to their thermodynamic stability, ease of preparation, and is environmentally acceptable. This work aimed to evaluate the inhibition capacity of microemulsion-based drilling fluids comprised of nonionic surfactants and pine oil, environmentally friendly components, and never before assessed in the laboratory. Initially, a shale sample from a sedimentary basin in Northeastern Brazil was characterized using XRF, XRD, TGA, and DTA. The microemulsions, base of the drilling fluids, were formulated from a ternary phase diagram, in which a water/glycerol solution (1:1 by mass) was used as the aqueous phase, pine vegetable oil as the oil phase, and two nonionic surfactants, Ultranex NP100 and Alkest Tween 80 were used separately in the formulated systems. Potassium citrate (INIB1) and potassium sulfate (INIB2) were also used to inhibit the swelling of reactive formations. In order to evaluate the behavior of drilling fluids, rheology tests, filtrate loss, filtercake thickness, and permeability were considered. Finally, drilling fluid systems were tested for shale inhibition and dispersion capability. In addition to systems composed of INIB1 and INIB2, potassium chloride (KCl) (INIB3) was evaluated, and a system without a swelling inhibitor (SI). It was observed that the shale samples presented clay minerals from the smectite group, which represent a more significant interaction with water for swelling of hydratable formations. Drilling fluids showed rheological behavior following the Herschel-Bulkley model. There is a variability of interactions in the fluids concerning the concentrations of surfactant, oil phase, and swelling inhibitor, with more significant responses associated with increased surfactant concentration in the microemulsion. The drilling fluids showed a significant swelling inhibition, especially the fluid formulated from the surfactant Ultranex NP100 and INIB2, with a 6.9% swelling variation. Excellent results concerning dispersibility were also achieved, with the NP100_INIB1 fluid being the lowest dispersion of cuttings (0.075% and 0.215%). In general, the microemulsion-based drilling fluids efficiently inhibited swelling of the shale sample, showing good rheological and filtration behavior.