Sandstone acidizing has been carried out with mud acid which combines hydrochloric acid and hydrofluoric acid at various ratios. The application of mud acid in sandstone formations has presented quite a large number of difficulties like corrosion, precipitation of reaction products, matrix deconsolidation, decomposition of clays by HCl, and fast spending of the acids. There has been a recent trend to use chelating agents for stimulation in place of mud acid which are used in oil industry widely for iron control operations. In this study, two chelates, L-glutamic-N, N-diacetic acid (GLDA) and hydroxyethylethylene-diaminetriacetic acid (HEDTA) have been studied as an alternative to mud acid for acidizing. In order to analyze their performance in the application of acidizing, coreflood tests were performed on Berea and Bandera sandstone cores. Another disadvantage of mud acid has been the fast spending at clay mineral surfaces leading to depletion of acid strength, migration of fines, and formation of colloidal silica gel residue. Hence, compatibility of chelates with clay minerals was investigated through the static solubility tests. GLDA and HEDTA were analyzed for their permeability enhancement properties in Berea and Bandera cores. In the coreflood experiments conducted, it was found out that chelating agents can successfully stimulate sandstone formations. The final permeability of the Berea and Bandera cores were enhanced significantly. GLDA performed better than HEDTA in all applications. The substitution of seawater in place of deionized water for mixing purposes also led to an increased conductivity of the core implying GLDA is compatible with seawater. In the static solubility tests, chelates were mixed with HF acid at various concentrations. GLDA fluids kept more amounts of minerals in the solution when compared with HEDTA fluids. Sodium-based chelates when mixed with HF acid showed inhibited performance due to the formation of sodium fluorosilicates precipitates which are insoluble damage creating compounds. The application of ammonium-based chelate with HF acid was able to bring a large amount of aluminosilciates into the solution. The study recommends the use of ammonium-based GLDA in acidizing operations involving HF acid and sodium-based GLDA in the absence of the acid.

Iron control during acidizing plays a key role in the success of matrix treatment. Ferric ion precipitates in the formation once the acid is spent and the pH exceeds 1-2. Precipitation of iron (III) within the formation can cause formation damage. Chelating agents such as EDTA and NTA are usually added to acids to minimize iron precipitation. Drawbacks of these chelating agents include limited solubility in strong acids and poor environmental profile. Hydroxy EDTA was introduced because of its higher solubility in 15 wt% HCl. However, its solubility in 28 wt% HCl is low and it is not readily biodegradable. In this study we studied the formation damage caused by iron precipitation in acidizing operations and tested the chelate L-glutamic acid, N,N-diacetic acid (GLDA). This chelant is soluble in higher concentrations of HCl. It is readily biodegradable, and is an effective iron control agent. A study was conducted to study the concentration of iron at different pHs ranging from 1-4 without the presence of any chelating agent at room temperature. A similar study was conducted in the presence of a chelating agent. To simulate field conditions, coreflood tests were conducted on Indiana Limestone, Austin Chalk and Pink Desert. Tests were conducted with and without the chelant. Samples of core effluent were collected and iron and calcium concentrations were measured using atomic absorption spectroscopy (AA). The cores were scanned using X-ray before and after acid injection. Results indicated that precipitation of iron can cause serious reduction in core permeability. The chelate was found to be very effective in chelating iron upto 300 degrees F. No permeability reduction was noted when GLDA was added to the acid. Material balance calculations show that significant amount of the iron that was added to the injected acid was produced when GLDA was used. This chelant is effective, environmentally friendly and can used up to 300 degrees F.

Well stimulation is the treatment remedy when oil/gas productivity decreases to unacceptable economical limits. Well stimulation can be carried out through either "Matrix Acidizing" or fracturing with both "Hydraulic Fracturing" and "Acid Fracturing" techniques. "Matrix Acidizing" and "Acid Fracturing" applications involve injecting an acid to react with the formation and dissolve some of the minerals present and recover or increase the permeability. The permeability enhancement is achieved by creating conductive channels "wormholes" in case of "Matrix Acidizing" or creating uneven etching pattern in case of "Acid Fracturing" treatments. In both cases, and to design a treatment successfully, it is necessary to determine the distance that the live acid will be able to penetrate inside the formation, which in turn, determines the volume of the acid needed to carry out the treatment. This distance can be obtained through lab experiments, if formation cores are available, or estimated by modeling the treatment. The successful model will depend on several chemical and physical processes that take place including: the acid transport to the surface of the rock, the speed of the reaction of the acid with the rock, which is often referred to as "Reaction Rate", and the acid leak-off. The parameters describing these processes such as acid diffusion coefficient and reaction kinetics have to be determined experimentally to ensure accurate and reliable modeling. Hydrochloric acid and simple organic acids such as acetic and citric acids have been used extensively for stimulation treatments. The diffusion and reaction kinetics of these acids, in a straight form, were investigated thoroughly in literature. However, solely these acids are used in a simple form in the field. Acid systems such as gelled, crosslinked gelled, surfactant-based, foam-based, or emulsified acids are used to either retard the reaction rate or to enhance acid diversion. Literature review shows that additional work is needed to understand the reaction and report the diffusion and kinetics of these systems with carbonate. In addition, a new chelating agent (GLDA) was recently introduced as a stand-alone stimulating fluid. The kinetics and the mass transfer properties of this acid were not studied before. Therefore, the objective of this work is to study the reaction of different acid systems with calcite and dolomite and report the mass transport and kinetic data experimentally. Lactic acid, a chelating agent (GLDA), and in-situ gelled HCl-formic acids were investigated in this study. In some cases, rheology measurements and core flood experiments were conducted. The data were combined with the reaction study to understand the behavior of these acids and examine their efficiency if injected in the formation.