2018 年 83 巻 6 号 p. 483-490
We have made continuous efforts to develop a middle mioccene siliceous shale distributed in northern Japan since 2012 with governmental supports. The shale formation is called as Onnagawa formation and is roughly estimated to have an order of hundred million barrels of oil as an original in-place. Although we have successfully recovered a certain amount of oil from Onnagawa shale, cost-effective and efficient production methods to exploit oil from Onnagawa shale are still unclear. In our project, two different stimulation approaches were tried: multi-stage hydraulic fracturing and acidizing.
In the multi-stage hydraulic fracturing job along a newly drilled horizontal well, we met serious issues to occur screenouts at some frac stages. Although the high viscous fluid slug and pre-injection of fine particles were adopted to prevent screenouts, it did not work. From the detail investigation of microseismic event rates as well as pumping pressure responses, the cause of screenouts is interpreted to be in excessive leak-off of fracturing fluid to the natural fracture system.
On the other hand, the acidizing job was a great success using mud acid system applied at an existing deviated-well.
After the job, the oil production increased dramatically from 10 bbl/D to 315 bbl/D and shows 100,000 bbl cumulative production for a year. The production improvement is inferred to come from contributions by stimulating natural fracture system filled with acid soluble minerals.
Based on the interpretation of the two field trials and a preliminary acidizing simulation, a hybrid fracturing concept has come up as a cost-effective approach for Onnagawa shale. In the hybrid fracturing process considered, the acid fracturing stimulate natural fractures developed in the deep field away from wellbore and the proppant fracturing injects the proppant placed in the near-wellbore region to prevent closing of fractures from high drawdown pressure at oil production.