MOLECULAR MODELING AIDS DESIGN OF DOWNHOLE CHEMICALS

Khamis S. Siam
Pittsburgh State University
Pittsburgh, Kan.

Rick D. Gdanski, Bruce E. Landrum, David Simon
Halliburton Services
Duncan, Okla.

Molecular modeling can aid the petroleum industry to design better, more efficient, and environmentally safer chemicals for downhole use.

Two prime areas of investigation are the interaction of clay stabilizers on clay surfaces and the mechanism of corrosion inhibitors.

Molecular modeling by computational methods can assist scientists in understanding oil field chemical interactions at the molecular level. This technique was first used in the petroleum services industry in the summer of 1990.

Modeling of chemical structures can help reduce the amount of laboratory tests required and guide the experimentalists in designing chemicals based on an understanding of chemical interactions at the molecular level.

Preliminary work has been done to model several processes. These include clay stabilizers and their interactions with clay surfaces and corrosion processes that do extensive damage to oil field tubular goods and other equipment.

EARLY MOLECULAR MODELING

In the late 1600s, Isaac Newton discovered classical mechanics through which the laws of motion of macroscopic objects were laid down. However, the behavior of very small particles such as electrons and nuclei of atoms and molecules could not be described by classical mechanics at that time.

In the early 1900s, Schrodinger discovered quantum mechanics which could describe the behavior of submicroscopic objects. Quantum mechanics is a science that relates molecular properties to the motion and interactions of electrons and nuclei.

The Schrodinger equation can lead to direct quantitative predictions of most chemical phenomena; however, the Schrodinger equation is a very complicated mathematical problem and a powerful computer is needed to solve it.

The numerous differential equations can generate millions of integrals with each calculation performed. The solution is iterative. Many computational cycles have to be performed before convergence is reached. Such calculations are referred to as 'lab initio" calculations.'

The information obtained from the ab initio calculations has been utilized in additional methods. One of these is called a molecular mechanical method utilizing a force field .2 This method uses the bonding and nonbonding parameters (referred to as a "force field") developed from ab initio calculations or experiments, and is less calculation intensive.

Where the assumptions to obtain the parameters are minimal in ab initio calculations, the parameters used in molecular mechanics are usually more generic for the particular atom being studied. This leads to a trade-off since the calculations require less time to complete, but they are not extremely accurate.

Additionally, the size of the systems can be increased due to lower computer resource requirements. Because the system sizes are larger, dynamics calculations of the interactions between large molecules are possible. Several appropriate force fields are described in the 2-9 literature.

The discovery of quantum mechanics and its calculations has enabled us to understand the interactions that take place at the molecular level.

Computerized molecular modeling studies have been developed only recently for large systems modeling. The need to understand these large systems has driven the pharmaceutical and biochemical industries to develop these types of calculation techniques.

With the improvements in computer graphics, the user can observe chemical structures in three dimensions. The chemist can draw the structures of interest and manipulate them in a real-time environment.

CLAY STABILIZATION

Productivity losses during well drilling, completion, and stimulation operations caused by the adverse interactions of treating fluids on clay minerals contained within the formation are well documented.10 Early works identified aqueous fluids as the major problem causing productivity losses. It has been suggested that by carefully controlling fluid salinity, productivity losses could be minimized.

In the early 1970s, inorganic and organic polymers were introduced as successful agents to "glue" clay minerals in place making them less subject to swelling and migration." This technology had been used until the mid1980s when sophisticated experimental procedures demonstrated that in some cases, the clay stabilizing chemicals were either ineffective or counterproductive.

Soon after, new classes of clay-treating chemicals were discovered and functioned effectively even in low-permeability formations. 12

Molecular modeling was used to study the interactions of various clay mineral treating agents with the clay surface.

By molecular modeling, it was possible to reproduce the features measured by X ray experiments. Clay models were developed using X ray crystal data of clays, and a clay stabilizer was inserted into the interlayer region.

The interatomic distances and the d-spacings found by X ray were also reproduced.

Studies have shown that the chemicals tend to associate with the edge of the clay sheet first and then, when the stabilizer invades the interlayer region, associate with the hexagonal formations of silicon atoms at the surface of the sheet. Molecular modeling has duplicated these interactions.

Fig. 1 shows the structure of the smectite clay. It is in the interlayer region that all the chemical interactions take place. Damaging chemicals will invade this region in such a manner that they require more space than is available.

The clay sheets must move apart, or swell, to accommodate these chemicals. An efficient clay stabilizer would fit in this interlayer region without swelling the clay.

With an inserted organic clay stabilizer in the clay interlayer, Fig. 2 shows that the chemical adheres to the clay formation and prevents it from sloughing off and blocking the channels that make the formation permeable.

Fig. 3 shows how another organic clay stabilizer deforms the clay formation; it would not be a good clay stabilizer. This model shows that the ineffective stabilizer leads to clay swelling because it is too large to fit in the interlayer region.

These results were compared to X-ray studies of clay mineral structures. The first generation organic polymer stabilizer was found to invade the interlayer of smectite, producing a saline-stable product. However, the resulting clay structure increased in volume as the interlayer spacing changed from about 14 to 21A. The later generation treating agents also appeared to invade the interlayer zone, but may actually reduce the interlayer spacing. With the use of the computerized model, rapid screening of chemical agents can determine their relative effectiveness.

CORROSION DAMAGE

Another area of interest is the kinetics of chemical reactions. Molecular modeling gives insight into possible reaction pathways. Depending on the reaction mechanism, these pathways may govern how products provide problem-solving performance such as preventing costly corrosion damage on oil well tubular goods.

For many years, research has led to the discovery of numerous chemicals to reduce the rate of corrosion .13 However, no one type of chemical or group of chemicals can solve all corrosion problems because of the variety of conditions in producing areas.

An understanding of how a corrosion inhibitor works can lead to optimization of the product or to new products. Because the reaction pathways are described by molecular dynamics, it has become apparent that the use of molecular modeling can help identify corrosion and inhibitor problems and help find ways to solve them.

Molecular modeling was applied to kinetic studies in which the reaction pathways and the stability of intermediates were investigated by ab initio methods. A pathway leading through an intermediate or transition state of a lower energy is more favorable than that of an intermediate with a higher energy. This is important in determining whether a particular reaction is governed by kinetic pathways or thermodynamic stabilities of reaction products.

The factors governing the direction and rate of the reaction determine what type of modifications to the molecules should be made to improve performance in helping solve corrosion problems.

Electron density maps provide a graphical illustration of the electron-rich sites on any chemical structure. The electron density map of propargyl alcohol (Fig. 4) presents a typical acid corrosion inhibitor. It illustrates where the electron-rich sites are located in the molecule. Such sites are prone to attack by electrophiles, which may explain the mechanism of corrosion inhibition occurrence.

The highest occupied and lowest unoccupied molecular orbitals can be determined as well. This information can help determine probable reaction products through some types of mechanisms.

REFERENCES

1. Simons, J., "An Experimental Chemist's Guide to ab Initio Quantum Chemistry," Journal Physical Chemistry, Vol. 95, 1991, pp. 1017-29.

2. Allinger, N.L., J. Am. Chem. Soc. Vol. 99, 1977, p. 8127.

3. Sprague, J.T., Tai, J,C., Yuh, Y., Allinger, N.J. J. Comput. Chem. Vol. 8, 1987, p. 581.

4. Brooks, B.R., Bruccoleri, R.E., Olafson, B.D., States, D.J., Swaminathan, S., and Karplus, M., J. Comput. Chem. Vol. 4, 1982, p. 187.

5. Nillson, L., and Karplus, M. J. Comput. Chem. Vol. 7, 1986, Ibid., p. 591.

6. Weiner, S.J., Kollman, P.A., Case, D.A., Singh, U.C., Ghio, C., Algona, G., Profeta Jr., S. and Weiner, P. J., Am. Chem. Soc. Vol. 106, 1984.

7. Weiner, F.J., Kollman, P.A., Nguyen, D.T., and Case, D.A., J. Comput. Chem., Vol. 7, 1986, p. 230.

8. Clark, M., Cramer III, R.D., and Van Opdenbosch, N. J., Comput. Chem., Vol. 10, 1989, p. 982.

9. Mayo, S.L., Olafson, B.D., and Goddard III, W.A., J. Phys. Chem., Vol. 94, 1990, p. 8897.

10. Hower, W.F., "Influence of Clays on the Production of Hydrocarbons," Paper SPE 4785, presented at the Symposium for Formation Damage Control, New Orleans, February 1974.

11. McLaughlin, H.C., Elphingstone, E.A., and Hall, B.E., "Aqueous Polymers for Treating Clays in Oil and Gas Production Formations," Paper SPE 6008, SPE 51 st Annual Technical Conference and Exhibition, New Orleans, October 1976.

12. Himes, R.E., Vinson, E.F., and Simon, D.E., "Clay Stabilization in Low-Permeability Formations," Paper SPE 18881, SPE Production Operations Symposium, Oklahoma City, March 1989.

13. Housler, R.H., "Corrosion Inhibition and Inhibitors" Corrosion Chemistry, ACS Symposia Series 89. American Chemical Society, Washington, D.C., 1979, pp. 262-320.

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