John E. Oddo

The inhibition of gypsum and barite nucleation in NaCl brines at temperatures from 25 to 90°C

Abstract The inhibition of gypsum and barite nucleation in NaCl brines by phosphonates and polycarboxylates was studied at 25, 50, 70 and 90°C in terms of the prolonged induction period. Hexamethylene-diaminetetra (methylene phosphonic) acid (HDTMP) was found to be an effective inhibitor for the scaling of calcium sulfate dihydrate. Hydroxyethylene-1, 1-diphosphonic acid (HEDP) and phosphinopolycarboxylic acid (PPPC) were found to be effective inhibitors for barium sulfate scaling. The inhibition of nucleation through prolongation of the nucleation induction period likely resulted from an increase in the interfacial tension between the crystal and aqueous solution due to the presence of the inhibitors. It was observed that the inhibition of nucleation depends highly on the lattice cation/anion molar ratio and the pH of the solution as well as the degree of supersaturation and temperature for a given inhibitor.

Simplified Calculation of CACO3 Saturation at High Temperatures and Pressures in Brine Solutions

A simplified method to calculate CaCO/sub 3/ saturation is developed using only commonly measured field parameters. The calculated saturation index, I/sup S/, and pH values are accurate at high temperatures and pressure in brines and are compared with less sophisticated and more complex calculations. The final forms of I/sub S/ and pH calculations are derived using conditional equilibrium constants dependent on temperature, pressure, and ionic strength, which eliminate the need for activity coefficients. The I/sub S/ equation is presented in forms for calculation with known or derived pH and where the pH of the solution is known. Practical application of I/sub S/ is shown by calculating the scaling tendency of several geopressure energy wells of the U.S. Gulf Coast region. 18 refs.

The solubility and stoichiometry of calcium-diethylenetriaminepenta(methylene phosphonate) at 70° in brine solutions at 4.7 and 5.0 pH

Abstract Phosphonic acids are commonly used as scale and corrosion inhibitors, but little is known about their solubility and stoichiometry. Hence, questions concerning the retention mechanisms of phosphonates in reservoir systems remain unanswered. A method is described in this paper to measure the solubility and stoichiometry of the phosphonates and is applied to diethylenetriaminepenta (methylene phosphonic acid) (DTPMP). The experiments reported herein were performed in 2.0 M brine solutions at 70°C for varying Ca and DTPMP concentrations at 4.7 and 5.0 pH. The conditional equilibrium constants of the Ca-DTPMP salts formed under the conditions of the experiment are 4.34 × 10−4 and 3.04 × 10−4 at pH values of 4.7 and 5.0, respectively. The stoichiometry of the solid phase precipitated in the experiments and the dominant solution complex are Ca3.5H3PHN·3H2O and Ca3H3PHN1− where PHN refers to the deprotonated DTPMP ligand. These results suggest that retention mechanisms involved in an inhibitor squeeze are either adsorption or precipitation/coprecipitation with cations other than Ca and/or other ligands.

Inhibition of CaCO3 Precipitation From Brine Solutions: A New Flow System for High-Temperature and -Pressure Studies

The study of calcium carbonate (CaCO/sub 3/) scale inhibition in geopressured energy systems has led to the development of a flow system with high temperature and pressure capability. A high-pressure (performance) liquid chromatograph (HPLC) was modified by the addition of a backpressure valve and by bypassing the HPLC packed column. Solution mixing and temperature are computer-controlled, and CaCO13B precipitation is monitored by in-line pH measurement. Simulated and real brine samples were used to evaluate scale inhibitors, and CaCO/sub 3/ precipitation was related to the saturation index, I/sub s/.

Scale Control Aids Gas Recovery

The East Hitchcock field has produced natural gas since 1958. Toward the end of the primary production phase, the wells watered out and formed excessive amounts of calcite scale. Acidizing was required frequently. In recent years, some of these wells were recompleted for coproduction. Because large volumes of brine are produced, scale control is necessary. Prets Unit No. 1 was successfully treated by an inhibitor squeeze, while Thompson Trustee Unit No. 1 was completed with two treat strings installed at calculated depths to administer scale inhibitor downhole. An inhibitor squeeze protects the formation and the perforations as well as the production tubing, surface equipment, and disposal system. Periodic interruption of production, however, is required to repeat the squeeze, and the concentration of inhibitor in the produced brine is controlled by the chemistry of the inhibitor salt. Installation of treat strings requires that the tubing be pulled and reinstalled with the treat strings clamped to the tubing OD. The concentration of inhibitor in the effluent brine is controlled by a surface injection pump connected at the top of the treat string.

Novel Approach to Eh-pH Diagrams and Their Relation to Uranium In-Situ Leaching: ABSTRACT

When constructing Eh-pH diagrams, workers have previously assumed a fixed total concentration of a substance in solution, and calculations (assuming equilibrium) were made to predict mineral phases and aqueous species at various Eh-pH conditions. When dissolving a solid compound (as in in-situ leaching), it seems more plausible to assume a solid phase in solution at various Eh-pH conditions and allow the aqueous species the freedom to assume thier equilibrium concentrations. The computer program EHPHUR was written to thermodynamically simulate the dissolution of a solid phase leading to diagrams which differ markedly from conventional Eh-pH diagrams. The name proposed for these new diagrams is dissolution diagrams. The assumptions include unit activity of uraninite, and fixed amounts of O2, CO2, and H2S at 25°C and 1 atm. The parameters were chosen to simulate in-situ leaching systems currently in use. Using these diagrams, thermodynamic arguments can be presented which are consistent with the results of kinetic experiments, for example: the first and zero order dependence of the uraninite dissolution rate on the carbonate concentration; the decrease in the dissolution rate as the pH approaches 6.0; and the dependence of the rate on the oxygen and hydrogen ion concentrations. Apparent optimum pH values for in-situ leaching are discussed. End_of_Article - Last_Page 966------------