SEAWATER CAN DAMAGE SAUDI SANDSTONE OIL RESERVOIRS

A.S. Dahab
King Saud University
Riyadh, Saudi Arabia

Experiments have shown that formation damage from waterflooding of the Aramco and Alkhafji sandstones of Saudi Arabia will not occur if the salinity of the injected brines is higher than 20% NaCl.

Because the connate water in these reservoirs has a high salt content of up to 231,000 ppm,l Saudi oil fields are almost always susceptible to formation damage when flooded with seawater (about 38,500 ppm).

The productive behavior of a reservoir can be affected by clay crystals developed within rock pores. Several papers have described the importance of dispersed clays in sandstones .2-7 Since dispersed clays generally occur as a rock-pore filling component and have a variety of crystal sizes and shapes, they exhibit a broad spectrum of adverse affects on rock fluid-flow properties.

SAUDI SANDSTONES

The Wasia formation of lower Cretaceous age forms the major producing sands in Alkhafji and Safaniya reservoirs.

These sands are coarse-to-fine grained, moderately sorted quartz and graywake, characterized by the alteration of authigenic clays.

A total of 20 sandstone cores were obtained from producing areas in the northeastern part (Alkhafji) and eastern part (Aramco) of the Kingdom of Saudi Arabia.

Core plugs were cut from the center of the drilled cores using simulated formation water as the cutting fluid to reduce possible mud contamination and to preserve the original state of the cores.

CORE ANALYSIS

The analyses of the cores are given in Table 1. Examination of the sandstones with the scanning electron microscope (SEM) showed that they consist of sand-sized grains cemented by partial grain coatings of clays (Fig. 1). Kaolinite, which is the dominant clay mineral in the cores, develops as pseudohexagonal, platy crystals scattered throughout the pore system in a random arrangement. It affects rock petrophysical properties by reducing intergranular pore volume and by migrating in the pore system rather than changing crystal lattice structure by swelling.

Other SEM photomicrographs for the same studied cores show pore walls extensively coated with clay crystals, yet the intergranular pores remain relatively open (Fig. 2). Such cores have relatively good permeability.

The micrographs of lowpermeability sandstones show clay crystals attached to sand grain surfaces and bridging across intergranular pore space.

SEM examination of the same rocks after flow tests showed that the clay coating on the sand grains had become detached and had migrated into pore throats (Fig. 3 and 4). The formation damage represented by partial blocking of openings due to dispersed clays reduced permeability ratio, i.e., measured permeability/initial permeability (Figs. 5 and 6).

Permeability measurements were carried out at a temperature of 90 C. and 3,000 psi overburden pressure.

Aramco cores exhibited the maximum damage, This could be attributed to the fact that they had higher percent of dispersed clays and higher base exchange capacity-25 milliequivalents (meq)/100 g-compared to Alkhafji cores which had lower base exchange capacity (8 meq) 100 g.

The lower dispersed clay content in Alkhafji cores and their high porosity were the reasons for showing less permeability drop.

Dispersed clays readily contact the passing fluids because they commonly occur within pores. In contrast, laminae of clays are less affected by the passing fluids.

ACKNOWLEDGMENT

The author would like to thank Mr. M. Bakry, responsible for the SEM laboratory, King Faisal Specialized Hospital for his cooperation in making SEM micrographs.

REFERENCES

Dahab, A.S., Omar, A.E., El-Gassier, M.M., and Awed el Kariem, H., "Effect of clay mineralogy and exchangeable cations on permeability of Saudi sandstone reservoirs," Revue de l'institut Francais de Petrole, Vol. 44, No. 5, 1989.
Wilson, M.D., and Pittman, E.D., "Authigenic clays in sandstones: recognition and influence on reservoir properties and paleoenvironmental analysis," J. Sed. Patrol, Vol. 47. No. 1, 1977, pp. 3-31.
McHardy, W.J., Wilson, M.J., and Tair, J.M., "Electron microscope and X-ray diffraction studies of filamentous illitic clay from sandstones of the Magnus Field," Clay Minerals, Vol. 17, 1982, pp. 23-39.
Neasham, J.W., "The morphology of dispersed clay in sandstone reservoirs and its effect on sandstone shalines, pore space and fluid flow properties,"' paper SPE 6858, SPE Annual Technical Conference and Exhibition, Denver, Oct. 9-12, 1977.
Pittman, E.D., and Thomas J.B., "Some applications of scanning electron microscopy to the study of reservoir rock," J. Pet. Tech., Vol. 31, No. 11, 1979, pp. 1375-80.
Timur, A., Hempkins, W.B., and Weinbrandt, R.M., "Scanning electron microscope study of pore systems in rocks," J. Geophys., Vol. 76, 1971, pp. 4932-47.
Pallatt, N., Wilson, J., McHardy, B., "The relationship between permeability and the morphology of digenetic illite in reservoir rocks," J. Pet. Tech., Vol. 36, No. 13, 1984, pp. 2225-27.