In the deep wells of more than 7,000 meters in the Tarim Oilfield in China, the opening of natural fractures in the reservoir directly affects the productivity of single wells, and it is necessary to start research on the mechanism of natural fractures.

Through the core mechanics experiment and the block geostress modeling, theoretical analysis, numerical simulation and case analysis are used to determine the shear failure of natural fractures during fracturing in ultra deep fractured reservoir. It is suggested that natural fractures become shear failure when pore pressure reaches the natural fracture opening threshold, and natural fractures with weak surface cementation and the dip angle greater than 60° are more likely to happen for such a shear failure.

The pumping pressure during fracturing increases linearly with the pore pressure in natural fractures, and it is difficult to achieve full opening of natural fractures in ultra-deep reservoir fracturing. Under the current working conditions, hydraulic fractures are opened along the natural fracture development section, and it is difficult to achieve the fracture of the rock body, and some natural fractures in favorable orientation also cannot be opened.

After researching the failure mechanism of natural fractures, the fracturing process can improve the stimulated reservoir volume by optimizing the fracturing parameters such as liquid combination, displacement, sand addition and pumping procedure. The field application achieved good results.

1. Introduction

The reservoirs in the Dabei and Keshen gas fields of the Tarim Oilfield are buried at a depth of 6000m-8882m, the formation pressure coefficient is 1.53MPa/100m-1.82MPa/100m, and the thickness of reservoir is 100m-300m. It belongs to high-pressure, high-stress and ultra-deep fractured sandstone reservoirs and needs to be stimulated to achieve economic development. Sand fracturing and acid fracturing technology are needed to increase single well production.

At present, some progress has been made in the fracturing technology of ultra-deep reservoirs (Ye Dengsheng, et al., 2009; Bartko K., 2009; Raid B K., 2010; Martin R., 2011; Joe C., 2011; Zou Guoqing, et al., 2012); however, the yield increase of single wells is small after acidification or acid fracturing of some fractured sandstone reservoirs. For the ultra-high pressure fractured sandstone reservoir with a depth of more than 7000m, there are even fewer reports of sand fracturing in single wells.

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