Development and Research on Fracturing Technology in Low-permeability Ultra-deep Wells in China
Authors: Wang Zhenduo , Wang Xiaoquan , Shan Wenwen , Lu Yongjun 
Published: December 2007

Abstract
Fracturing in ultra-deep wells is one of the important means of improving ultra deep reservoir conditions and making it reach commercial development value. There are more difficulties in the optimization design and the safety of operation due to factors such as higher formation temperature, higher pumping pressure, and higher investment risk. This paper discusses the recent development of fracturing technology in ultra deep wells in China:history of fracturing in ultra-deep wells;successful fracturing experience in ultra-deep well in DH Oil Field, Tarim, China.
Drawing the lesson and experience of fracturing in ultra-deep well in early years, combining with the research and practice of stimulation in ultra deep low-permeability reservoir in Tarim Basin(5800 to 5900 m in depth,140°C), aiming at the properties of fracturing for ultra deep formation, applying fully 3D fracturing design software, a methodology of predicating the accurate pumping pressure, understanding the state of fracture extension, optimizing the fracturing fluid characteristics has been used to optimize the fracturing designs. In addition, advanced operational equipment and matching techniques have been applied to DH Oil Field.
Significant stimulation results in production and injection wells were obtained with the applications of the fracturing technology in DH ultra deep oil field. The water injection profile testing results showed that the post-fracture water injection rate of the 4 fractured wells was at the range of 140 to 170 m3/d, the operational success rate of the four fractured wells was 100%, and the effective rate of water injectivity was 100 %.
Field practice shows that China not only has the capability of fracturing in ultra deep wells but also obtains good post-fracture response at about the depth of 6000m at present.

Introduction
With the extension of exploration and development of hydrocarbon into deeper formations, hydraulic fracturing in these formations has become more and more important. The properties of fracturing for ultra deep wells include that:deep formation(&gt;4200m);high temperature (&gt;120°C);high fracture pressure&gt;100MPa); andhigh operational pumping pressure(&gt;60MPa).
Therefore, as compared with conventional fracturing, fracturing in ultra-deep wells add more difficulties to the optimization design and the safety of operation due to the above factors. In addition, the investment risk is raised.
From the first fracturing treatment in Yumen Oil Field, China in 1955, not only the quantity and scale of fracturing operation, but also the depth of fractured wells have been deepen with the extension of exploration and development of hydrocarbon. The deepest ultra deep well in China's fracturing history at that time—Wugu 1 well (4827.41 to 4911.0m in depth), Huabei Oil Field. Had been fractured successfully on Feb.1,1989. The main area of fracturing for ultra deep well in China had been switched from east to west with exploration and development of the west Tarim Basin at the end of 1980's, the depth of ultra deep fractured well was beyond 5000m, too. However, because of the limitation of fracturing equipment and technology for ultra deep well, the operational success rate was only 50% from 1989 to 1993, as shown in Table 1. Since 1995, considering the under-injected conditions of water injection well, the following aspects have been deeply performed to improve the ultra deep well fracturing technology. That is the understand of formation parameters, the optimization of 3D design, the selection of operational equipment and down hole tools, the optimization and selection of fracturing material and fluid formula, and the control of fracturing fluid quality on the spot. The operational success rate was 100%, as shown in Table 4. It has eventually changed the passive situation of the formerly ultra deep well fracturing—"lower operational success rate and easy sand screen-out".