A rotating disk instrument was used to measure acid dissolution rates, reaction rates, reaction order and activation energy of reservoir rock from a deep dolomitic gas reservoir in Saudi Arabia. These values are required for acid fracturing simulations. Results of more than 50 experiments are reported in this paper. Measurements are made from room temperature to 85 ?C at rotational speeds of 100 to 1000 rpm and acid concentrations of 0.05 to 5N HCl (0.2 to 17 wt%). The results show how acid dissolution rates change as the reservoir rock varied from 3 to 100 wt% dolomite. Factors affecting the measured parameters are discussed in detail. It was found that the reactivity of the rock varied from values expected for pure calcite marble to those expected for pure dolomite marble. At grain densities near 2.72 g/cm3 (expected for pure calcite), rock dissolution rates varied by more than an order of magnitude due to rock mineralogy. At grain densities near 2.83 g/cm3 (expected for pure dolomite) rock dissolution rates were higher than that observed with pure dolomitic marble. Reaction rates depended on mineralogy and the presence of trace components such as clays.
An accurate knowledge of acid reaction rates of deep gas reservoirs can contribute to the success of matrix and acid fracture treatments. Many studies of acid stimulation treatments of formation K, a deep, dolomitic gas reservoir in Saudi Arabia, have been published(1)(2)(3). This is the first study of acid reaction rates and reaction coefficients of this important formation.
The rotating disk instrument is widely used in the petroleum industry for kinetic studies of the reaction of acidic fluids and chelating agents with reactive rock (4–10). This system allows the determination of rock dissolution rate, reaction rate constants, reaction order, and diffusion coefficients (4)(11).
Lund et al. studied the dissolution of both calcite (5) and dolomite (6)with the rotating disk instrument. Their work showed that at 25 °C, the dissolution of calcite is mass transfer limited even at high disk rotational speeds, while at −15.6 °C, both mass transfer and surface reaction rates limit the dissolution rate. In contrast, Lund et al. (6) showed that the dissolution of dolomite was surface reaction rate limited at 25 °C even at low disk rotational speeds. As the temperature was increased to 100 °C, the dissolution process approached diffusion limitation even at relatively high rotational speeds (6).
The mass transfer limited and the surface reaction limited regimes are important features of the rotating disk instrument. Fluid-solid reactions can be described by the sequence of acid diffusion to the interface, surface reaction and diffusion of reactants from the interface. The slowest step can be considered the rate-determining step. If the slowest step is the diffusion of reactants and products to and from the surface, then the reaction is mass transfer limited. If the slowest step is the surface reaction itself, then the reaction is surface reaction limited. In the rotating disk instrument, both of these regimes can occur.