Diffusion coefficient (De) is an important parameter used in both matrix acidizing and acid fracturing job design. There are two major limitations of conventional lab practices. Firstly, De is obtained at 1000 psi (or below) system pressure, which is too low to represent realistic reservoir conditions. Secondly, De obtained from conventional lab practice is based on fresh acid, which largely ignores the impact of reaction products, especially CO2.
Herein, we investigate the effects of both pressure and reaction products on De of HCl acid reacting with dolomite using fresh acid (15 wt. %) and various spent acid concentrations (12.5, 10 and 7.5 wt. %). A customized rotating disk apparatus was used to obtain the reaction kinetics data at pressure conditions of 1000 and 3000 psi at various disk rotational speeds (250 – 1250 rpm) at 150 °F. Samples of the reacted acid were collected and analyzed using Atomic Absorption Spectroscopy. In addition, the disks were scanned using X-ray micro computed tomography to characterize the 3D image of the disks.
Our experimental results showed that at equal HCl concentration levels, De was significantly decreased at elevated pressures, due to the impact of CO2 produced by the HCl-carbonate reaction. There was approximately 23% decrease in the diffusion coefficient value when compared with 1000 psi results for HCl/dolomite reaction. On the other hand, when the fresh acid was spent to 12.5 wt. %, there was a slight decrease in the diffusion coefficient results because of the slow reaction rate of dolomite. Moreover, HCl/dolomite reaction was mass transfer limited at low disk rotational speeds and surface reaction limited at high disk rotational speeds. In addition, diffusion coefficient was significantly affected by rock porosity due to variation in surface reaction area which caused increase in dissolution rates as observed by Micro XCT images of the reacted disk.
Rotating Disk experimentation have been used for many years to describe acid reactivity. Literature suggests very few have been conducted under sufficient pressure to represent reservoir conditions. Thus, in order to properly design the acid treatment jobs, the De of acid acquired from high-pressure and various spent acid conditions should be used to provide more reliable parameters estimation such as acid injection rate, volume and pumping schedule leading to more accurate production forecast.