Guidelines Regarding the Use of Alcohols in Acidic Stimulation Fluids

Abstract

Using alcohols in acidic stimulation fluids for fast cleanup of spent acid can lead to the formation of high concentrations of organic reaction products and an attendant loss of available acid. The problem can be minimized by limiting the alcohol concentration to a maximum of 32 percent (by volume) and by limiting treatment temperatures to 175 degrees F for sandstone and 200 degrees F for limestone formations. Introduction One of the problems associated with treating gas wells in sandstone formations is the slow recovery of the spent acid solution. This problem often results from high water saturation in the critical matrix surrounding the wellbore. Water blocks can be caused by the capillary forces present in porous rock and by the high mobility ratio of gas and water. When a well is put on production following a workover or a stimulation treatment, the gas forces some of the water out of the rock and then may break through, leaving a high water saturation around the wellbore. This residual high water saturation reduces the effective permeability of the formation to gas flow. permeability of the formation to gas flow. The lower alcohols have been used for a number of years in acidic stimulation fluids to aid in rapid recovery of spent acid. The reasons for this use are well founded. Alcohols aid in cleanup of spent acid by lowering the surface tension and increasing the vapor pressure of the solution. In recent years, this technique has gained popularity to the extent that it is used routinely in gas-well popularity to the extent that it is used routinely in gas-well stimulation treatments and, to some degree, in oilwell acidizing. This paper deals with an undesirable side reaction attendant when alcohols are used in acid. Even under moderate-temperature conditions, alcohols will react readily with acid. In the cases of organic acids (acetic or formic acid, for example), these reactions will result in ester formation, with the only resulting problem being possibly a loss of available acid for the stimulation possibly a loss of available acid for the stimulation reaction. Even this may be of small consequence since the esterification reaction is reversible and regeneration of the organic acid for the desired stimulation reaction is possible. possible. CH3COOH + CH3OH <- ->CH3COOCH3 + H2O acetic acid methanol methyl acetate.........................(1) However, in an alcoholic solution of hydrochloric acid, a reaction takes place that results in the formation of organic chlorides. The following are two examples. CH3OH + HCl -> CH3Cl methanol hydrochloric acid methyl chloride+H2O............(2) CH3CHOHCH3 + HCl -> isopropanol hydrochloric acid CH3CHClCH3 + H2O.........(3) isopropyl chloride This reaction poses a genuine problem in acidic gaswell stimulation. A reaction such as in Eq. 2 or Eq. 3 is irreversible, and any HCl lost to this reaction can not be regenenued, as was possible with organic acids (Eq. 1). Thus, this reaction competes with the acidic-stimulation reaction for the HCl content. A second problem concerns the reaction products themselves. Although the presence of organic chlorides in natural gas might not present any special problems, their presence in produced crudes or distillates is considered a serious produced crudes or distillates is considered a serious contamination. For these reasons, this paper is limited to HCl-alcohol reactions. JPT