Study and Evaluation of Wellbore Integrity Strategy for Unconventional Resources Restimulation Purpose
Authors: Chen Qiang , Li Tao , Sun Qiang , Li Yiliang , Han Weiye , Huang Shouzhi 
Published: April 2019

Abstract
Fracturing activities had been a big success in the petroleum community, particularly when it came to the development of unconventional resources such as shale gas, however, as the production rate went down over time, secondary fracturing was necessary to restimulate the reservoir, how to deal with the original perforation holes and maintain the wellbore integrity during the refracturing process had been a major concern in the oil and gas industry.
In this paper, methods commonly used to plug the original perforation holes were discussed, namely chemical agents, coiled tubing plus saddle packers and mechanical patching, among them expandable system was highlighted and its technical details were disclosed, the system was also adapted and tested indoors, finally field operations were carried out and experiences were shared in detail.
The results indicated that right now there were three ways to initiate the start of refracturing operation, using chemical agent to plug the unwanted fractures in the reservoir which was proved to be uncertain in terms of amount and polluting; coiled tubing assisted with straddle packers could be employed to seal off the refracturing zone, but it was costly and risky since the packers had to be set and unset repeatedly, it was a threatening issue in the horizontal wellbore, mechanical device such as expandable tubular could be used to patch the perforation holes left during the last fracturing operation, it was solid and measurable. Solid expandable tubular was redesigned to overcome the challenge of high pressure during the restimulation operation, the material and sealing element were manufactured in response to the requirement of high-pressure refracturing demand. Field operations were performed in a number of wells, some were horizontal wellbores, the maximum expansion force was 30MPa, 70MPa broken pressure was recorded in the refracturing operation afterward, there was no report of any unsatisfactory issues, significant economic results and experience were secured, highly valuable to the operations in future.
This paper contributed an approach to maintain the integrity of wellbore for the purpose of refracturing operation, its contents helped us better understand the process, characteristics, operation process and experience summery in the wellbore integrity strategy.
Fracturing activities have been a big success in the petroleum community, particularly when it comes to the development of unconventional resources such as shale gas. In china, thousands of fracturing operations have been carried out over the past decade to stimulate the reservoir, almost 1/3 investment budget of petroleum industry comes to fracturing annually, reflecting its position in the development phase, as shown in the fig1 and fig2. However, due to multiple reasons, some fracturing operations may fail our expectations; meanwhile the production rate usually goes down over time, therefore secondary fracturing (refracturing) is demanded to restimulate the reservoir, the first step of refracturing was to plug the original perforation holes, how to finish the job and maintain the wellbore integrity have been a major concern worldwide. Figure 1horizontal wellbore fracturingFigure 2field facilities