HCl
CO2H HO HO2C N N CO2H
> Carbonate core tests. A coreflood test was performed on Indiana limestone with 15% HCl at 150°F [65°C]. A photograph of the core face shows dissolution ending in a single dominant wormhole (top left). A longitudinal CT scan of this core indicates that this single wormhole extended the entire length of the sample (top right). Similar testing was carried out on a limestone sample with HEDTA at 350°F and the same flow rate (bottom left). Use of a chelant resulted in a complex network of wormholes at the higher temperature level (bottom right).
This layered reservoir consists of varying thicknesses of sandstone, limestone and shales. Although some high-permeability streaks exist due to fissures and fractures, permeability elsewhere is low and temperature is high— 149°C. The Nemba formation contains high levels of native calcium carbonate, making the formation particularly difficult to acidize at elevated temperatures without causing decon soli - dation. Prior treatment and workovers in the Nemba formation had caused significant damage related to carbonate scale. Nemba sandstone samples represent good candidates for evaluating the use of chelants in high-temperature acidizing. Ten core samples were taken from the Nemba field over a narrow depth interval at about
The Nemba reservoir is one of a group of production zones lying offshore Cabinda, Angola.13
9. Frenier WW, Wilson D, Crump D and Jones L: “Use of Highly Acid-Soluble Agents in Well Stimulation Services,” paper SPE 63242, presented at the SPE Annual Technical Conference and Exhibition, Dallas, October 1−4, 2000.
10. Frenier W, Brady M, Al-Harthy S, Aranagath R, Chan KS, Flamant N and Samuel M: “Hot Oil and Gas Wells Can Be Stimulated Without Acids,” paper SPE 86522,
3,534 m [11,595 ft] and subjected to a variety of experiments with an HEDTA chelant. These experiments measured composition, examined metals evolution during reaction and determined permeability. The composition of the Nemba core samples ranged from 5% to 44% calcium carbonate with significant amounts of feldspar and chlorites. Two different procedures were performed in the laboratory to determine the results of HEDTA treatment—slurry reactor tests and coreflood permeability tests. The slurry reactor tests on the Nemba sandstone samples used an isothermal, stirred reactor to measure product composition as a function of time. Powdered sandstone samples containing 24% and 44% carbonate levels were treated in the reactor with HEDTA at 149°C. Samples of the reaction mix were withdrawn over
presented at the SPE International Symposium and Exhibition on Formation Damage Control, Lafayette, Louisiana, February 18−20, 2004.
11. Frenier et al, reference 9. 12. Frenier et al, reference 10.
13. Ali S, Ermel E, Clarke J, Fuller MJ, Xiao Z and Malone B: “Stimulation of High-Temperature Sandstone Formations from West Africa with Chelant Agent-Based Fluids,”
time and analyzed by inductively coupled plasma emission spectrometry. For both carbonate levels, the concentrations of calcium, silicon, aluminum and magnesium rose smoothly over time with no decreases that would indicate precipitation. The same slurry reactor test was repeated for a 30% carbonate-containing sample using a conventional 9:1 mud acid.14
In this experiment,
concentrations of calcium and other components showed an initial rise followed by a decrease— indicating precipitation—a common cause of sandstone treatment failure. The slurry reactor data on HEDTA suggest that this chelant dissolves the pore-filling and blocking minerals at high temperature without causing precipi - tation. These positive results for HEDTA were followed by coreflood tests at two carbonate levels. Results from these tests show that the
paper SPE 93805, presented at the SPE European Formation Damage Conference, Scheveningen, The Netherlands, May 25−27, 2005.
14. A conventional 9:1 mud acid is 9% by weight HCl combined with 1% by weight HF.
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