Investigating the Use of Chelating Agents for Clay Dissolution and Sandstone Acidizing Purposes

Mud acid, a mixture of HCl and HF, has been frequently used for stimulating sandstone reservoirs. However, using HCl in such environments can be problematic, especially at higher temperatures. Some of the most common problems are the following: clay sensitivity, secondary/tertiary reactions, and precipitation of salts and corrosion. To combat these problems mixtures of HF have been developed along with organic acids and chelating agents such as citric acid, acetic acid, EDTA, HEDTA, GLDA etc. Compared to HCl, these chelating agents offer lower corrosion, no mineral sensitivity issues, stability at high temperatures (? 200 ?F) and bio-degradability.

This thesis explores the use of two chelating agents, citric acid and a newly developed sodium salt of L-Glutamic acid N,N-Di Acetic Acid (Na-GLDA). Experiments were conducted to find out the aluminosilicates dissolution and chelation capabilities of these chelating agents. The first set of experiments were clay dissolution experiments, conducted using different concentrations of citric acid (1 wt%, 3 wt%, and 5 wt%) added to regular 9:1 mud acid. This was done to study and analyze its clay dissolution properties, as well as its chelation abilities to reduce precipitation. For comparison purposes, experiments were also completed using regular 9:1 mud acid to compare its results to that of using citric acid along with 9:1 mud acid. The results suggest that using 1 wt% citric acid along with 9:1 mud acid provided the best results, both in terms of clay dissolution as well as reducing precipitate formation.

The next set of experiments investigated the use of Na-GLDA along with HF for sandstone acidizing purposes. First, compatibility experiments were conducted to find out the optimum acid mixture between Na-GLDA and HF that causes no incompatibilities. Following the compatibility test, coreflood experiments were run on Bandera and Berea cores using the optimum acid mixture formulation found in the preceding experiment. Coreflood results showed the good chelation ability of Na-GLDA to iron, calcium and magnesium. But very low concentrations of any aluminosilicates were found in the ICP samples indicating either the lack of dissolution of aluminosilicates or the precipitation of aluminosilicates within the core.

In conclusion, the experimental results suggest that adding 1 wt% citric acid to 9:1 mud acid provides better dissolution and precipitation results. But factoring in the cost of citric acid makes it a financially unfavorable formulation, especially since regular 9:1 mud acid performed almost as well as 9:1 mud acid with 1 wt% citric acid added to it. Also, the newly developed Na-GLDA is compatible with HF at certain concentrations of both. The optimum acid mixture formulation was found to be 20 wt% Na-GLDA + 1 wt% HF. Coreflood results show that Na-GLDA added to HF is able to keep cations such as iron, calcium and magnesium in solution at higher temperatures, but it is unable to properly dissolve and chelate to aluminosilicates and its damaging salts.