Self-Generating Mud Acid
Authors: Templeton C.C. , Richardson E.A. , Karnes G.T. , Lybarger J.H. 
Published: December 2007

Since regular mud acid can dissolve clay only within a few inches of the wellbore, removal of deeper permeability damage caused by clay requires in-situ generation of hydrofluoric acid. Such a retarded acidization can be based on an aqueous solution of ammonium fluoride and an alkyl carboxylic ester. This technique has resulted in improved stimulation in oil wells in the East Bay, La., area.

Introduction
In-situ generation of hydrochloric acid for carbonate-formation acidizing was developed by Dilgren in 1965, but no similar scheme has been reported previously for generating hydrofluoric acid to dissolve clay from sandstone formations. An in-depth hydrofluoric acid treatment would be most applicable to wells in a sandstone reservoir containing natural clays, where the normal production of water along with oil disperses the clay into fines that migrate toward the borehole and produce permeability damage. The desirability of removing such damage permeability damage. The desirability of removing such damage caused by natural clay was expressed in 1970 by Gatewood et al., who estimated that such damage might extend to a depth of 3 ft into the formation. They calculated that the productivity of a well treated with 1,000 gal of 3-percent HF/12-percent HC1 per foot of zone would be improved significantly by retarding the rate of reaction tenfold. Further, Smith and Hendrickson have stated that such clay must be deposited uniformly in the flow channels for a large part of the HF to dissolve clay rather than the silica walls of the pores.Smith and Hendrickson have made extensive core tests injecting HF-HC1 into Berea sandstone that contained 5.7 percent uniformly distributed clay. In these tests, a marked decrease in permeability occurred as soon as the acid entered the core. This permeability occurred as soon as the acid entered the core. This temporary damage was later removed, but 30 to 50 PV of acid were required for the removal. The initial damage was attributed to the migration of formation fines rather than to the secondary deposition of spending products since the damage occurred immediately upon acid entrance.Against this background, we reasoned that a treatment generating HF in situ over a period of a few hours would not only place reactive HF far from the wellbore, but would also provide sufficient reaction time for clay dissolution. In provide sufficient reaction time for clay dissolution. In addition, the use of a low injection rate was thought desirable to minimize the shifting of fines (and, hence, temporary damage) at the beginning of the treatment.Dilgren's approach to producing HC1 in situ over a given period of time was to hydrolyze or solvolyze HC1 from a period of time was to hydrolyze or solvolyze HC1 from a chlorine-containing organic compound; his patents mention the parallel process of reacting HF off some fluorine-containing parallel process of reacting HF off some fluorine-containing organic compound. However, producing HF in this manner is much more difficult than producing HC1. The use of an in-situ aqueous mud-acid formulation initially containing fluoride ion (from NH4F, for example) and a neutral agent (such as an alkyl carboxylic ester) capable of generating some acid at a controllable rate is a more effective approach.This paper describes laboratory work on the use of methyl formate to generate formic acid in the presence of ammonium fluoride, thus dissolving clay suspended in the medium as illustrated in the following reactions:
(1)
(2)
JPT
P. 1199