Chelating agents are used to remove various inorganic scales, including sulfates and carbonates. They are also used as stand-alone stimulation fluids and as iron control agents during acidizing treatments. The main chelating agents used in the field, include: ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), N-(hydroxyethyl)-ethylenediamineteriaacetic acid (HEDTA), and recently, L- glutamic acid-N, N diacetic acid (GLDA). One of the concerns with these chelates is their thermal stability at elevated temperatures. A few studies examined the thermal stability of these chelating agents, and the impact of thermal degradation products on permeability. The objectives of the present study are to: 1) Examine the thermal stability of several chelating agents and their salts up to 450°F, and 2) assess the effect of thermal decomposition products on the permeability of carbonate and sandstone cores with different initial permeabilities. We prepared solutions (0.4 to 0.6 M) of HEDTA, GLDA, NTA, EDTA and their salts. The solutions of these chelates were heated at various temperatures and times (2 to 12 hrs.). The concentration of chelate was determined using a new analytical technique that was based on titration with FeCl3. The products of thermal decomposition of chelates were determined using MS technique. Core flood tests were conducted on Berea sandstone and Indiana limestone to determine the effect of thermal degradation products on the permeability of these cores. Coreflood tests were conducted at 325°F and 3 cm3/min. Most chelates decomposed at temperatures greater than 350°F. Among monovalent salts, potassium salt was found to be the most stable one. Chelates with two nitrogen atoms were more stable than those with one nitrogen atom. For example, diammonium salt of EDTA is more stable than diammonium salt of GLDA. Analyses of chelate solutions after heating using MS technique highlighted that the decomposition products included: iminodiacetic acid, formic acid, and α-hydroxy acids. Results of the coreflood indicated that some of the thermal decomposition products can cause formation damage. This paper will summarize the results obtained, and explain how chelates can be used to improve field treatments, especially at high temperatures.

Summary Hydraulic fracturing has become a common practice in the petroleum industry, and several systems have been developed to obtain a suitable crosslinked polymer for the treatment. However, each system has its strengths and weaknesses. This study aims to investigate the effect of three different ligand types attached to zirconium (Zr) on the performance of carboxymethylhydroxypropylguar (CMHPG) crosslinked with Zr-based crosslinkers with the different ligands. The shear recoverability and rapid viscosity buildup at the high pH of Zr-based crosslinkers were overcome by a new aluminum-zirconium (Al-Zr) dual crosslinker in this research. The polymer used was CMHPG, and the tests were conducted at pH of 3.8 and 10.8. One of the factors that affects the gel performance is the type of ligand attached to the Zr. Because the Zr-based crosslinkers are shear-sensitive, ligands were introduced to delay the crosslinking until the fluid passes the high-shear environments (perforations). Therefore, in this study, lactate, propylene glycol, and triethanolamine (TEA) were studied as ligands attached to the Zr. Two Zr crosslinkers with almost the same concentration of Zr can display different performances if the ligand attached to the Zr is not the same. The rapid viscosity buildup at high pH had always been a limitation of Zr crosslinkers; however, a new Al-Zr dual crosslinker was introduced in the present study to address this limitation. The Al-Zr crosslinker outperformed all the other crosslinkers examined in the present study. Immediate viscosity buildup at a high pH and a lack of shear recoverability of Zr-based crosslinkers was addressed through the Al-Zr crosslinker. The Al-Zr crosslinker introduced in this study is one compound that is easy to use in the field. The Al-Zr crosslinker performance was compared with the boron-zirconium (B-Zr) crosslinker: Both had lactate as a ligand attached to them. Among all the Zr-based crosslinkers in this study, the Zr crosslinker with lactate and propylene glycol as a ligand performed the best. The CMHPG crosslinked with each of the crosslinkers was tested for proppant-carrying purposes along with static leakoff rates. The results revealed gel-proppant-suspending capabilities and acceptable leakoff rates. Extensive laboratory research is a key to a successful field treatment. These results indicate that fracturing fluids are complex, and the ligand type is one of the important factors in determining the final properties of fracturing fluids. Therefore, the results of this study will assist in developing Zr-based crosslinkers that address their current shortcomings.

A new in-situ generated HCl acid was developed to overcome the fast reaction rate and high corrosion rates of 15 wt% regular HCl acidizing system. The objectives of this work are to: (1) examine the reaction rate of the new in-situ generated HCl with calcite at 100, 150, and 200℉, and (2) compare the reaction rate of 15 wt% regular HCl with the new in-situ generated HCl. The rate of the reaction of 15 wt% HCl and the new in-situ generated HCl was measured using the rotating disk apparatus (RDA). Calcite disks were used with the specifications of 1.5 in. the diameter and 0.65 in. thickness. The effects of disk-rotational speed (200-1,200 rpm) and temperature (100-200℉) were investigated. Calcium concentrations were measured in the samples collected from the RDA, which were used to calculate the rate of dissolution. The disk surface after the tests was analyzed using Scanning-Electron-Microscope–Energy-Dispersive Spectroscopy (SEM-EDS). Experimental results showed that the rate of dissolution at 100 and 150°F was controlled mainly by the rate of mass transfer of the acid to the surface. By increasing the temperature to 200℉, the overall rate of reaction for the in-situ generated HCl was mass transfer limited up to 800 rpm and surface limited above 800 rpm. Based on the dissolution rate results, the diffusion coefficient, the activation energy, and the reaction rate constant at 100, 150, and 200°F were determined for the new developed in-situ generated HCl and were compared to 15 wt% regular HCl. This study will assist in developing a more cost-effective and efficient design of acid treatments through a slower reaction rate of the in-situ generated HCl. This new in-situ generated acid system reacts slower and more efficient compared to regular HCl in carbonate and sandstone reservoirs.