Silurian reservoir in Ta-zhong Oilfield, western china, is a thin-interbed, low-permeability reservoir with buried depth from 4000 to 4500m. The sand distribution in this reservoir is heterogeneous both vertically and horizontally. Horizontal well with segregated completion and multi-stage fracturing is taken as an efficient technology to exploit this kind of reservoir. However, the existence of many mudstone interlayers makes the design of completion and fracturing difficult.

This paper will provide a case study on design and operation of multi-stage fracturing of a horizontal well in Ta-zhong Silurian reservoir. This well was fractured twice using two different technologies, firstly hydraulic jetting fracturing and then packer-segregated fracturing. Both the two operations encountered abnormal pumping pressure and sand out. The design and implementation process of both technologies will be introduced in this paper, and the challenges facing the completion and fracturing design will be highlighted. The causes to the abnormal pumping pressure and sand out will be analyzed comprehensively through numerical modeling and comparative calculation.

The results show that the defective completion design is the main cause of the failures of the two operations. For hydraulic jetting fracturing, the ill-considered jetting points and improper jetting space cause the difficulties for fracture initiation and propagation and therefore the sand out. For packer-segregated fracturing, the inappropriate perforating interval leads to simultaneous initiation of multiple fractures in each stage. These fractures interfere with each other during propagation, leading to great near-wellbore pressure loss and width reduction for each fracture, therefore the abnormal high pumping pressure and sand out were encountered. The acting mechanism of induced stress and fracture interaction and reorientation are illuminated in this paper. A bunch of ways to avoid these problems during multi-stage fracturing of a thin-interbed sandstone reservoir is also proposed from both designing aspect and operating aspect.

Nowadays, horizontal well with multi-stage fracturing is a common-used technique in tight thin-interbed formations. However, the design of a segregated completion and multi-stage fracturing is still a nodus for such formations. This paper gives a detailed introduction of a failure case and comprehensively analyzes the causes of these failures from geology to engineering, therefore gives a lesson for avoiding this kind of failures and optimizing a staged fracturing design in tight thin-interbed reservoirs.

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