Modeling Acid Stimulation of the Halfway Formation, Canada, Using a Geochemical Computer Model
Authors: Haskin H. K. , Moore C. H. , Ortoleva P. J. 
Published: December 2007

ABSTRACT
Geochemical reaction-transport modeling has grown out of the need to understand and predict rock-water interactions and their effect on matrix porosity. These models can also be powerful tools for understanding and controlling rock-water interactions in a reservoir during well completion, well stimulation, or oil recovery. This paper reports on the use of REACTRAN, a new reaction-transport numerical model, to model laboratory acid stimulation of cores from the Halfway Formation, Canada. Not only is there good agreement between model predictions and laboratory observations, but REACTRAN provides a likely explanation for previously unexplained permeability declines after injection of 15% HCl.
Details of the laboratory study are given in the report entitled Reservoir Sensitivity and Completion Procedures Project published in 1983.1 The Halfway formation in British Columbia and Alberta consists of fine to medium-grained quartz-rich sandstones cemented with dolomite and anhydrite. Hydrochloric acid stimulation is a widely used remedy against the migration of dolomite fines.
The report describes the mineralogy (quartz, feldspar, dolomite, and anhydrite), the injected fluid composition (1–15% HCl), and the acid velocity.1 The literature provides thermodynamic equilibrium constants for the aqueous phase chemical reactions and the mineral-water reactions that we specified for this system. Only the dolomite and the anhydrite dissolution rate constants were unknown. The one value for each mineral that best described all of the experiments was used in these simulations.
REACTRAN brings the fast aqueous phase reactions to equilibrium, calculates a new fluid composition at each point in space, and then allows minerals to grow or to dissolve in accordance with their kinetic rate laws. Time is advanced and the process is repeated to yield the temporal evolution of the spatial distribution of mineral and of fluid composition.
Laboratory studies of core plugs treated with 1%, 2%, or 3% HCl showed dolomite dissolution near the inlet and permeability increases. No acid was produced from the core plugs. REACTRAN predicts similar dolomite dissolution, porosity increases, and no acid production from the cores. Although extensive dolomite dissolution was observed in the core plugs which received 15% HCl, unexplained permeability declines not due to fines migration were observed in the laboratory experiments. REACTRAN provides a possible explanation. Depending on the choice of the anhydrite rate constant, anhydrite may dissolve in the first few millimeters of the core plug and precipitate farther along the core plug causing a small drop in porosity in one region. If this anhydrite were depositing in pore throats it could cause the permeability declines seen, but not explained in the laboratory study. According to REACTRAN, this problem can be handled by addition of CaCl2 to the HCl to suppress the anhydrite dissolution. Validation of the anhydrite rate constant used in this modeling can be done by repeating these experiments, taking special care to look for changes in anhydrite as well as dolomite. That work is in progress.