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Graphical abstract Abstract This work aims to investigate the changes in the shear and oscillation responses of an ethoxylated alkylamine emulsion with the addition of acetic acid and establishes their relationships with the emulsion phase inversion. The initial emulsion behaves as a non-Newtonian fluid exhibiting shear thinning with a viscoelastic gel structure (at rest). Upon the addition of acetic acid, the emulsion first transforms to a structureless (at rest) non-Newtonian fluid with lower viscosity and worse shear thinning compared with the initial state and then converts back to a non-Newtonian fluid with a higher gel strength, higher elasticity (at rest) and better shear thinning than the initial state as the phase inversion proceeds from an oil-continuous stage to a bicontinuous stage. This regained non-Newtonian character can be maintained throughout the whole bicontinuous stage and can even extend to the beginning of the water-continuous stage which is obtained from the bicontinuous stage by increasing the acetic acid content; however, it is ultimately replaced by a structureless (at rest) non-Newtonian fluid that exhibits lower viscosity and worse shear thinning compared with the initial state as the acetic acid content increases. The establishment of relationships among the interfacial moduli and deformation, geometrical factors (i.e., interfacial curvature, droplet shape and size and positional structure) and the liquid crystal phase behaviors help to elucidate the mechanisms that control the rheological transitions of emulsions. Based on these results, a novel water-washable viscoelastic emulsion drilling fluid is proposed. [ABSTRACT FROM AUTHOR]

Reversible invert emulsion muds (RIEMs) offer unique advantages over other oil-based muds for the completion of a well due to the pH-responsiveness of reversible emulsifiers. This paper presents new insights into the effects of pH-responsiveness on the completion performance of RIEMs and the mechanisms involved. At pH 4 and 6, RIEMs are easy to remove, but the formation damage is relatively severe. At pH 10, the removal of RIEMs is relatively difficult but can be facilitated by intense and repeated flushes. Once the mud has been removed, there is little formation damage. At pH 8, the mud is very difficult to remove, and the greatest amount of formation damage is observed. The spontaneous curvature of the surfactant layer changes at pH 4 and 6, resulting in the emulsion inversion and thereby enabling the removal of RIEMs by the solvent effect. These changes do not occur at pH 8 and 10. More emulsifiers adsorb to the rock surface at pH 4, 6, and 8, resulting in a strongly oil-wet condition and high formation damage. The desorption of emulsifiers occurs at pH 10, causing a water-wet condition and low formation damage. The variety in chemical states accounts for the changes in the adsorption behavior of the emulsifiers. [ABSTRACT FROM AUTHOR]

Highlights: [•] Two phase inversion pathways in ethoxylated alkylamine emulsion are proposed. [•] Two pathways have the same threshold and different destinations. [•] Phase inversion is induced by the addition of acetic acid, but not exclusively. [•] Phase inversion mechanisms rely on changes in the spontaneous curvature. [•] Structural transition images help in understanding phase inversion mechanisms. [Copyright &y& Elsevier]