Mathew M. Samuel

Development and Field Application of a New, Highly Stable Emulsified Acid

Acid-in-diesel emulsified acid has been used in the oil field for many years. It is known that the emulsified acid should be stable at ambient conditions for a long period of time. It should be stable also at downhole conditions for a period of time long enough to pump the acid without encountering operational problems. Application of emulsified acid at higher temperatures requires 20 gallons per thousand barrels (gpt) to 30 gpt of a cationic emulsifier. A thorough laboratory study was conducted to select an emulsifier that can be used at lower loadings, and meet the stability criteria that are needed to stimulate oil and gas wells. Laboratory work included measuring stability, apparent viscosity, and droplet size distribution as a function of emulsifier type and concentration, temperature and additive type and concentration. A new emulsified acid was developed and used to acid fracture over 10 wells in a deep gas reservoir in Saudi Arabia. The formation is predominantly limestone and dolomite with streaks of anhydrite. Acid fracturing practices consist of pumping 28 wt% emulsified acid (acid to diesel volume ratio = 70:30), a pad and in-situ gelled acid to create long conductive fractures. The new emulsified acid used a significantly lower amount (4 gpt to 6 gpt) of the emulsifier, albeit it produced a stable emulsion over a wide range of temperatures (from 75 oF to 275 oF). The droplet size of emulsions produced from the new emulsifier was much smaller and, as a result, the apparent viscosity of the acid-in-diesel emulsion was higher. Field data showed greater reduction in the time needed to prepare the new emulsion in the field. The performance of wells stimulated with the new emulsifier was significantly better than those stimulated with the old emulsifier.

The low pH stability of human coagulation factor XII (hageman factor) is due to reversible conformational transitions

Factor XII undergoes autoactivation when bound to negatively charged surfaces. To gain insight into the mechanism of factor XII autoactivation and stability at low pH, structural studies in the presence and absence of a soluble surface, dextran sulfate, at pH 5.3 and pH 8.3 were carried out. The circular dichroism data indicate that the secondary structure at pH 5.3 is only modestly different from that at pH 8.3. However, large changes in the secondary structure are found to occur when factor XII is exposed to pH 5.3 in the presence of surface. Changes in tertiary structure at low pH are also evident from the difference in tryptophan fluorescence and chemical reactivity of the histidine residues. Factor XII binds to the surface even at pH 5.3 though it is inactive at this pH. It is concluded that factor XII adopts a different conformation at pH 5.3 and causes it to interact differently with dextran sulfate. This results in an obstructed cleavage site that accounts for its stability at low pH.