Abstract
The success of matrix acidizing treatments, whether in carbonate or sandstone formations, depends significantly on the selected acid or acid mixtures. Limitations are applied on all existing acidizing fluids including hydrochloric acid and organic acids. These limitations include: low dissolving power, product solubility, stability, biodegradability, and the inevitable cost of additives necessary to mitigate corrosion problems. This work proposes a new mixture of lactic and gluconic acids which offers favorable technical characteristics and excellent health and environmental profile. After formulated, the acid was tested and optimized for the maximum calcium product solubility. The new acid is noncorrosive, nonvolatile, nontoxic, and can be used at a higher pH with significant sequestering power, and it is readily biodegradable (98 % at 48 h). The solubility of calcium salt of this acid is approximately 400g/l (compared with 300 g/l for calcium acetate, 166 g/l for calcium formate, and 79 g/l for calcium lactate). Interestingly, sodium salt of the acid mixture was reported as a corrosion inhibitor for steel alloys.
The objectives of the work are to: (1) examine the dissolving capacity and reactivity of the proposed acid through solubility and reaction rate studies over a temperature range of 80-300°F using the rotating disk reactor, (2) investigate the effectiveness of the new acid to create dominant wormholes and determine the optimum injection conditions in calcite cores.
Acid capacity reactions with Pink Desert limestone powder showed that 1:1 of 1 M lactic:gluconic acid mixtures was the optimum molar ratio that resulted in dissolving the maximum calcium amount for the reaction at 25°C and 500 rpm, while the reaction of lactic acid alone at the same acid concentration showed a white precipitation of calcium lactate in the collected samples. Reaction rate experiments on the rotating disk reactor showed that the rate of reaction of the proposed acid at 1:1 molar ratio is confined by the reaction rate of the two individual acids (lactic and gluconic acids). However, the reaction of lactic acids resulted in white precipitates on the surface of the rock disks used in the experiments. Coreflood study showed the ability of the new acid mixture to stimulate Indiana limestone cores at various injection rates, acid concentrations, and over temperature range between 150 and 300°F. The results also confirmed that 1:1 molar ratio of the two acids is the optimum for the minimum acid pore volume required to breakthrough. 20 wt% of the proposed acid was the optimum acid concentration associated with the minimum acid pore volume. Above this concentration, little impact was noted and the reduction in the pore volume leveled off.
