In situ gelled acids that are based on polymers have been used in the field for several years as an acid diversion agent. These acids should not cause permanent formation damage, and should clean-up rapidly and completely when the well is put back on production. However, recent lab studies indicated that a significant amount of gel was retained inside the core even when the recommended breaker concentration was used. This gel significantly reduced the core permeability. Therefore, the objective of this study is to examine the working mechanisms of the solid breaker and the factors that affect the breaking time. The flowback of the spent acid and the core damage induced by it will also be assessed in detail. Viscosity and rheological experimental studies were conducted using polymer-based in situ gelled acids that used Zr as a crosslinker agent. To form the gel, this acid was neutralized using Pink Desert limestone to pH 3.5-4.The rheological properties of the gel were measured under different breaking conditions for 2 hr using HPHT rotational and oscillatory rheometers. Temperature (150 to 250 degrees F), shear rate (0.1 to 1000 s^-1), and breaker concentrations (0 to 32 lb/Mgal) were the main parameters that were investigated in this study. Experimental results showed that there is no need to add more than 16-ppg breaker concentration at 150 degrees F, while 4-ppg breaker concentration is enough at temperatures higher than 200 degrees F. A full breaking of these acids was obtained when the values of elastic modulus (G') were equalized with the viscose modules (G"). An HPHT filter press was used to simulate the flowback of the spent acid. Viscosity measurements of filtered fluids were used to determine the degree of gel degradation. Carbonate cores that had a 2.5 in. diameter and 0.25 in.thickness were used. Temperature (75 to 250 degrees F) and breaker concentrations (0 to 16 lb/Mgal) were the main parameters that were investigated in this study. Experimental results showed that there was always a gel left inside the rock, and such caused permeability reduction. Time needed to flowback the solution decreased by 50% when the temperature was increased from 150 to 200 degrees F, while it decreased by only 30% when the breaker concentration was doubled. At 150 degrees F, 16-ppg breaker concentration was recommended for breaking the gel. At higher temperatures (>150 degrees F), 8-ppg breaker concentration was enough to help degrade the gel.

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.