(90d) Effect of Crude Oil on the Reaction Kinetics of Diesel Emulsified Acid with Calcite at High-Pressure and High-Temperature HPHT

Emulsified acids have been used as an effective well stimulation fluid since 1933. It is primarily an emulsion solution consisting of two immiscible liquids, one of them is dispersed (inner or dispersed phase) in the other liquid (continuous or outer phase) in the form of droplets. It used in oil fields because it has several advantages. Firstly, its used to minimize the danger of corrosion of the tubular system including a casing, tubing, etc. Secondly, it has good reaction retardation properties that lead to deep penetration into the formation and finally the high viscosity of the emulsified acid allows it to sweep the targeted region and distribute uniformly. In order to model the carbonate acidizing treatment, there is an important experimental parameter that should be estimated which is the diffusion coefficient of H⁺ (De) to the surface of the reservoir rock. The diffusion coefficient is calculated by measuring the reaction rate at different RPM using rotating disk apparatus RDA. Almost, all the diffusion coefficients values calculated from RDA experiments for calcite with diesel emulsified was done through water-saturated disk at lower pressure that is not enough to keep CO2 in solution. However, this work, study the effect of crude oil saturation as well as HPHT condition on the diffusivity of H⁺ to the surface of the limestone, which mimics the real reservoir condition where emulsified acids are used.

In this work, two sets of reaction kinetic experiment were conducted using rotating disk apparatus RDA. The first set of oil-saturated disks, and the second for dry calcite ones. Emulsified acid was prepared using acid to diesel ratio of 70:30, emulsifier concentration and corrosion inhibitor were used in 1 vol.% and 0.3 vol. % respectively. Compositional analysis for diesel was done. Limestone core samples from local outcrop were used after checking its purity using XRD and XRF analysis. A small disk of 1.5 in. the diameter and 0.75 in. the length was prepared. The saturation by crude oil was made by soaking the disks in crude oil for 48 hours at 65^ₒC. All the rotating desk experiments were conducted at 3000psi and 135^ₒC. The disk rotational speed varied between (200-1500 rpm). Effluent samples of 5 ml were taken every two minutes and then analyzed through Inductively coupled plasma mass spectrometry (ICP-MS). The dissolution rate was measured at each RPM an

From the lab results a couple of two conclusions were drawn, firstly the Diffusion coefficient of H⁺(De) value at HPHT is much lower than those obtained at low-pressure low- temperature LPLT and the extrapolation of these De values at lower downhole conditions for deep reservoirs could lead to severe acidizing and hydraulic fracturing design inaccuracy. Secondly, two De values were observed for oil saturated limestone reaction kinetics, one for lower RPM where crude oil saturation effect is highly impacting the diffusion and leading to lower reaction, whereas at higher ones the (De) value is almost close to the (De) value for dry calcite rock.

To sum up, from lab results and for better acidizing modeling It’s recommended to use diffusion coefficient (De) calculated using disks saturated with reservoir fluids at downhole conditions.