Alternative Stimulation Fluids and Their Impact on Carbonate Acidizing

Acidizing treatments are commonly used to remove near-wellbore damage and create artificial flow channels in carbonate formations. Matrix acidizing treatments are most useful when fracture acidizing is undesirable, such as when a shale break or other natural boundary must be maintained to prevent water or gas production or where fracture acidizing is ineffective, such as in soft chalk formations. Unfortunately, matrix treatments often require low injection rates to prevent fracturing the formation rock. These treatments may also be required in heterogeneous formations with zones of low conductivity (which need stimulation the most) that accept acid at low rates. It is at these low injection rates that the problem of rapid acid spending severely limits the acid penetration distance. The injection of hydrochloric acid (HCl) into carbonate formations at low rates results in face dissolution, or complete dissolution of the carbonate matrix near the wellbore. This face dissolution consumes large volumes of acid and provides negligible increases in the conductivity of the formation. Various acid systems have been used to reduce the limitations of rapid acid spending at low injection rates. A few of the acids include: weak acids, such as acetic and formic acid, which have relatively low H concentrations and therefore react with carbonates at a slower rate than HCl; chemically retarded acids, such as oil external microemulsion systems containing HCl, that retard acid diffusion to the carbonate surface and thus allow deeper penetration of live acid; and foamed acids (nitrogen gas and aqueous HCl) that prevent acid from spending outside the primary dissolution channel, thereby promoting the growth of wormholes. Although retarded and foamed acid systems can stimulate carbonate formations at lower injection rates, strong acids such as HCl induce the precipitation of asphaltic sludge from crude oil. This sludge can plug the formation and restrict production after an acidizing treatment. When ferric ions are present, this problem is even more severe. Thus, adequate corrosion protection becomes more essential. Acetic acid (HAc), an iron chelating agent, does not reduce sludging tendencies in the presence of ferric and ferrous iron. A variety of acid additives (antisludging agents, corrosion inhibitors, and iron-reducing agents) have been used to prevent the sludging problem. However, their effectiveness is limited by the need to obtain a compatible combination of additives and a lack of understanding of the complex chemistries involved in the precipitation reactions. These limitations demonstrate the need for an alternative stimulation fluid that combines the ability to stimulate at low injection rates with fluid properties that are not conducive to asphaltic sludge precipitation or corrosion problems. Ethylenediaminetetraacetic acid (EDTA) is an alternative fluid that is capable of stimulating carbonate porous media. EDTA is a chelating agent that stimulates by means of sequestering the metal components of the carbonate matrix. The dissolution mechanism is different from that of HCl in that hydrogen ions are not required. The dissolution, however, is enhanced at low pH through a combination of hydrogen ion attack and chelation. Although EDTA has not been used for carbonate acidization, it has been used successfully for the removal of calcium carbonate scale from the sandstone formations of the Prudhoe Bay field. Conventional acid treatments could dissolve the scale, but scale reprecipitation from the spent acid caused rapid productivity decline. EDTA effectively chelated or sequestered the metal ions of the dissolved scale, thus preventing their reprecipitation. Other related applications of EDTA include the removal of calcium sulfate anhydrite scale from brine heater tubes and boilers and the removal of sulfate and carbonate minerals from clay assemblages. This paper describes applications of EDTA in carbonate acidizing and compares results from coreflood experiments and from sludge tests for EDTA, HAc, and HCl.