Experimental investigation of the acid-oil emulsion stability influenced by operational conditions and oil properties

Improper design of acidizing may cause new induced formation damages in the form of stable acid-oil emulsion, which would hinder fluid flow inside the pore passages of reservoirs. Furthermore, unwanted reduction of injected acid capability to resolve precipitations and rock should also be expected because the acid droplets are trapped in crude oils. There are not enough experimental studies and data on the effect of operating conditions and crude oil properties on the stability of acid-oil emulsions. This study focused on the formation of emulsion and its stability as well as acid-induced sludge formation for various crudes affected by mixing intensity (as a representation of acid injection rate to the reservoir), acid volume percent or AMR (as a representation of acid volume in the porous medium), temperature and crude oil viscosity. The experimental results showed that there exists a direct relation between emulsion stability and the mixing speed, which depends on crude oil properties such as viscosity and asphaltic compounds. Three crude samples of A, B and C from an oil reservoir were tested. Samples A and B showed 20 and 4 % higher emulsion stability at a mixing speed of 1500 rpm compared to that at 500 rpm. Higher shear rates produce larger interfacial areas and maybe easier and faster adsorption of various protonated asphaltenes and precipitates. Furthermore, changes in temperature from 30 to 85 °C caused 37 and 63 % increase in emulsion stability, probably because of higher droplets and molecules kinetic energy. AMR had a reverse impact in such a way that changing from 0.2 to 0.8 resulted in 87 % reduction in emulsion stability for both samples of A and B. However, the emulsion stability of sample C had not been changed by AMR, temperature and mixing shear rate. It seems that little sensitivity existed in the case of less viscous sample (C). It can therefore be inferred that crude oil properties are imperative during emulsion formation. More experiments were performed for diluted crude B by a mixture of toluene and heptane, keeping colloidal instability index intact, for reducing its viscosity from 56 to 2.5 cP at ambient temperature. Finally, a reduction of 17 % was observed in emulsion stability at 85 °C, AMR of 0.5 and mixing speed of 1500 rpm. The findings of this study facilitated a better understanding of operating conditions which influence the stability of HCl-oil emulsions during acidizing jobs.