In China, about 70% of new discovered oil reserves and 80% of new discovered natural gas reserves are in the low permeability reservoirs, and the lower limit of reservoir permeability is becoming lower and lower in new oilfields. The blocking and tip-screen-out (TSO) fracturing are introduced to create more complex fractures and increase the stimulating reservoir volume, comparing to the 2-wing symmetrical planar fractures, and so develop these ultra low permeability oilfields.

A model was set up to describe when the complex fractures form, and the key point is when fracturing elevating fracture net pressure to exceed the sum of tensile strength and the difference of maximum and minimum principal stress. And then two cases are detailed to illustrate how the blocking and TSO fracturing are designed and executed. The blocking fracturing is pumping some wax balls or larger size proppant to temporarily block the fracture and so increase the net pressure. And the TSO fracturing is similar to that used in the high permeability reservoir, but the aim is also increasing the net pressure not creating a wide and short fracture. The net pressure and the oil production are only matched when considering multiple fractures, so the blocking and TSO fracturing actually make more fractures spread, and increase the stimulating reservoir volume.

The two fracturing methods have been applied in several ultra low permeability oilfields in China, and average oil production is much more than that of neighbor wells. So the fracturing makes these ultra low permeability oilfields can be developed economically.

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