Hydraulic fracturing using heavy brine was conducted to stimulate the deeper part of the completion interval in the naturally fractured reservoir, Yufutsu Japan. The microseismic monitoring and the temperature surveys showed that the deeper part was stimulated effectively as expected.

The Yufutsu oil and gas reservoir is seated at around 4 km depth in the Southern Ishikari Plain, central Hokkaido, in northern Japan is known as a basement fractured reservoir. The water frac inducing shear dilation is one of the effective stimulation methods for the fractured reservoir. However it is difficult to stimulate the bottom part of the vertically long completion interval with seawater or slick water because the gradient of the fracture closure stress (that is typically 0.6-0.7 psi/ft, equivalent mud weight 1.4-1.6 sg) is commonly larger than that of the injection fluid. The heavy brine (CaCl2/CaBr2 brine 1.8 sg) realizes a pressure gradient larger than the fracture closure stress gradient that can stimulate deeper zone effectively. The numerical prediction using "SHIFT" simulator shows consistent results with the above-mentioned considerations. SHIFT simulates shearing of fractures and related permeability changes in a dynamic process by coupling analyses of the fluid flow and the shear dilation along fractures.

The target well has a long completed interval of 700 m. The deeper part of this well showed poor productivity due to a damage of lost circulation materials. The fracturing operation using the 1.8 sg CaCl2/CaBr2 brine was conducted with the objective to stimulate the damaged zones. About 200kL of heavy brine was injected. The microseismic array sensor was installed in the adjacent well to monitor the stimulation during the injection.

Approximately a thousand microseismic events were observed during the injection. The location of the microseismic events showed that the deeper part was stimulated effectively. A temperature survey conducted after the stimulation revealed the new production zones appeared on the temperature profiles as anomalies which had not been seen before. The AOFC of the well increased by a factor of 1.5.

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