Over the years, sandstone acidizing has experienced low success rate. This makes it essential to develop a systematic method for selecting the optimum hydrofluoric acid (HF) concentration, which can achieve the highest possible permeability enhancement. Field experience and experimental studies show the significance of HF acid concentration in controlling the outcome of the treatment: adding too much HF acid will result in secondary and tertiary reactions damaging the reservoir while the use of low concentrations of HF will result in insufficient dissolution. The objective of this study is to investigate the effect of HF acid concentration and design a systematic method for designing sandstone-acidizing treatments.

Experimental work was designed to investigate the effect of the HF acid concentration, and identify the optimum concentration for different sandstone formations. Two types of sandstone cores were used in this study: Grey Berea which is characterized by low carbonate content (3 wt%), and Bandera which has a high carbonate content (16 wt%) and high content of troublesome minerals (10 wt% illite). Coreflood experiments were performed on 6 in. cores with 1.5 in. diameter. Temperatures of 280 and 325°F were used for the experiments to address the moderate and high temperature applications. HF acid concentrations varied from 0 to 3 wt% HF. Optimum concentrations were selected based on the ratio of permeability improvement. Inductively coupled plasma (ICP) was used to measure the concentrations of key ions in core effluent.

The experimental results proved the relationship between the acid concentration and mineral content. At the temperature of 280°F, Grey Berea cores, with clay content of 8 wt% and 3 wt% feldspars, the HF concentration of 1.5 wt% was able to achieve the best results. For the Bandera cores, which have a clay content of 14 wt% (10 wt% illite, 1 wt% chlorite, 2 wt% kaolinite) and 12 wt% feldspars, the acid that achieved the best results contained 1 wt% HF. When temperatures were increased to 325°F, permeability enhancement was significantly decreased while the optimum concentration remained fairly at the same HF concentration. This proves that the optimum acid concentration is independent on the temperature, but for high temperatures (>300°F) retarded acids should be considered.

This work introduces the concept of designing acid treatments as a function of the mineral content. The optimum concentration of the HF was proven to adopt an inverse relationship with the mineral concentration. The proposed relationship can be used to determine the optimum acid concentration required for sandstone stimulation.

You can access this article if you purchase or spend a download.