Fresh Water Fracturing in Oil Reservoirs

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The use of fresh water as a fracturing fluid in oil reservoirs became widespread in the Permian Basin in 1956,following its earlier introduction in the San Juan Basin. Although water was first used because of its economy, it soon became apparent that water often gave better production increases than oil-base fracturing fluids. Investigations showed that these improved results were caused by the action of fresh water in dissolving and removing salt from treated formations. Salt occurs in reservoir rocks as matrix material, pore-filling material, fracture-filling material and as fall-out or precipitate from crude oils or brines. The salt content of a core sample can be determined by petrographic study or by chemical tests. Examinations of thin sections of cores show not only how much halite [salt] is present, but also whether the halite is in the form of cementing material, individual crystals, or secondary deposition in pores, channels, or natural fractures. Fig. 1 is a photomicrograph [X300] of a halitic sandstone thin section from an upper Permian formation. Halite is the cementing material binding together chert and quartz grains. At the upper left hand corner, marked "C", is a chertgrain. The grains marked "Q" are quartz, and the black grains at upper right are magnetite or ilmenite. The arrows point to characteristic cubic openings in the halite. Others are between the arrows and below them. The opening at the right arrow contains a gas bubble. Although many more such openings are in the sample, they cannot be brought into focus at such high magnification. Fig. 2 is a photomicrograph [x69] of loose quartz sand grains from the same core sample after the cementing halite was removed. This is the effect that can be expected when fresh water is used in treating such a formation. The presence of halite as individual crystals in the formation is shown in Fig. 3, a photomicrograph [X300] in which both halite and quartz grains are prominent. It is apparent that halite grains may be as large as or larger than quartz grains. Dissolving and removing many such halite grains during a fracturing treatment using fresh water will substantially increase formation permeability.