Reaction kinetics of fresh & spent acid in dolomite rock at high pressures

Abstract

Carbonate matrix acidizing extends a well’s effective drainage radius by dissolving rock and forming wormholes near the wellbore. Wormholing during matrix acidizing of carbonate reservoirs is controlled by the fluid injection rate and acid diffusion coefficient (De) which dictates the speed and profile of the wormholes. Injection rate is easily obtained from the job execution whereas the diffusion coefficient is unknown parameter of the fluid and reaction conditions. Acid diffusion coefficient data used in modelling of wormholing processes are commonly obtained at 1000 psi system pressure, which is too low to represent realistic reservoir conditions. Moreover, the conventional wormhole models use the diffusion coefficient of fresh acid to predict the wormholing process which overestimates the dissolution rate as it is not representative of the wormhole penetration deeper into the formation. In order to properly quantify the acid penetration inside the formation, the diffusion coefficient of acid acquired from high pressure reservoir conditions and representing the level of acid spending at the tip should be employed. In this research, the effects of diffusion coefficients of HCl acid as it reacts with pure dolomite rock disks were investigated. A rotating disk apparatus was used to obtain the reaction kinetics data at high pressure conditions at various disk rotational speeds (250 – 1250 rpm) at a temperature of 65ºC. 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. The first set of experiments were conducted using 15 wt. % HCl concentration whereas the second set of experiments were conducted at various spent acid concentrations (12.5, 10 & 7.5 wt. %). It was concluded that dissolution rates and diffusion coefficient decreased at high pressure condition since CO2 was more soluble in aqueous solution and tends to buffer the diffusion of hydrogen ions from the bulk to the rock surface. 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 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. The new set of kinetics data can be implemented in acidizing job design model which can provide more reliable parameters estimation such as acid injection rate, volume, pumping schedule leading to more accurate production forecast.