How to Determine True Acid Diffusion Coefficient to Optimize Formation Damage Treatment? Available to Purchase

Diffusion Coefficient (D_e) is a very important parameter for the formation damage treatment job design. Commonly a Rotating Disk Apparatus (RDA) is used to obtain acid D_e and reaction rate. There are two major limitations of conventional lab practices (1) The D_e is obtained at 1000 psi (or below) system pressure, which is too low to represent realistic reservoir conditions. (2)The D_e obtained from lab experiments are based on a fresh acid, which has been ignoring the impact of reaction products, especially CO₂. Hence the results of D_e cannot represent the real reaction kinetics between carbonate rock and injected HCl during the acid treatment.

In this paper, we investigate the effects of D_e of HCl acid as it reacts with both calcite and dolomite. A modified RDA is used to obtain the true spent acid reaction kinetics at reservoir conditions. In order to provide accurate reaction kinetics data that is more representative of the true reservoir conditions, a true spent acid concept is adopted here in this study. The true spent acid is formulated at reservoir condition in various spending stages. In addition, different mineralogy including calcite and dolomite are fully studied. The reaction products of CO₂ and counter ions are taken into consideration at different acid spending stages.

The experimental results show that the D_e of the HCl acid is much lower at high pressure than low pressure at the same concentration due to the impact of CO₂ produced by the HCl-carbonate reaction. For example, at 150 °F, the diffusion coefficient of 15% HCl at 3,000 psi reduced 50% of its original value when at 1,000 psi of 15% HCl. The diffusion coefficient of the fresh acid is much higher than the diffusion coefficient of the same acid strength of true spent acid due to the presented count ion and CO₂ in the true spent acid. The D_e of HCl/calcite is two orders of magnitude higher than D_e of HCl/dolomite at the same conditions (acid concentration, temperature).

Conventional experimental results significantly overestimate the D_e, dissolution rate and reaction rate. When the D_e implemented in a job (formation damage treatment) design, those defective data could result in dramatic errors in determining the treatment fluid schedule, rate, and volume. In order to properly design the formation damage treatment jobs, the D_e of acid acquired from high-pressure and various spent conditions should be used.