ABSTRACT:

Hydraulic fracturing and gravel packing (frac&pack) has been routinely used in completing oil wells drilled in poorly consolidated sandstone reservoirs of Bohai oil field of China. However, due to the high porosity, high permeability and complex mechanical behavior of poorly consolidated sandstones, initiation and propagation of hydraulic fractures in poorly consolidated sandstone are still less understood compared to those in competent rocks, leading to difficulties in developing adequate modeling techniques and optimizing the hydraulic fracturing operations in poorly consolidated sandstones. In this paper, we report the results of laboratory experiments on hydraulic fracturing in poorly consolidated sandstones. Cylindrical samples of poorly consolidated sandstones were prepared by mixing silica sand and clay together and compacting the mixtures with confining pressures of 10 MPa and durations of 2 hours. The particle size distributions of the sand-clay mixture are the same as those of typical poorly consolidated sandstone found in Bohai oil field. The size of the samples is 10 mm in diameter and 12 mm in height, while the permeability of the samples is either about 900 mD or about 60 mD. Before the experiments, the samples were saturated with oil. During the experiments, borate cross-linked guar fracturing fluids with different viscosities were injected into the specimens with different injection rates under various confining pressures. X-ray CT scanner was used to inspect the fracture patterns after the experiments and then the samples were sectioned for visual examination of the fractures. The experimental results span from fully infiltration, branching fractures to quasi-planar fractures, depending on the permeability, the viscosity and the injection rate. Clear quasi-planar fractures were formed when high viscosity fracturing fluid was injected into relatively low permeability samples with high injection rates, while fully infiltration or branching fractures were likely to occur if low viscosity fracturing fluid was injected into relatively high permeability samples with low injection rates. Insights obtained from the experiments in this paper are expected to be helpful in developing reasonable modeling techniques for hydraulic fracturing in poorly consolidated sandstones.

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