Innovative and Tailored Refracturing to Rejuvenate the Largest Shale Gas Field of China: Case Study

Abstract

The Fuling shale gas field in China is one of the largest shale gas fields outside of North America. After a long period of production, some gas wells showed significant production decline and an effective refracturing treatment is designed to rejuvenate production. This Casing-in-Casing (CiC) refracturing method with customized design was successfully implemented with production increase beyond expectation. It was the first successful trial of a CiC refracturing treatment on a horizontal shale gas well in China. Bullheading (BH) refracturing with diverting balls was attempted in this field in past years, with high initial production observed. However, the production was inconsistent and declined quickly. The operator investigated and decided to attempt a CiC refracturing method in an under-stimulated candidate well. The CiC refracturing method is to cement a 3.5-in. liner in the legacy 5.5-in. casing to isolate the perforations, and new plugging and perforating (P-n-P) operations can be performed in the reconstructed wellbore. A refracturing design was customized integrating the production profile, residual recoverable reserves, and the specific 5.5- × 3.5-in. reconstructed wellbore limitation. During the BH refracturing, the treating pressure gradually increased with the drops of diverting balls, but the proppant placement became more and more difficult accordingly. The BH refracturing was finally completed with five cycles which was less than the design. High initial production after the BH treatment was observed but it declined quickly. In the CiC refracturing, a new 5.5-in × 3.5-in wellbore was constructed successfully, and it passed pressure test for stage frac. New clusters were added between the original clusters and stimulated with more intensive fracturing treatment in the new wellbore. Dissolvable particulate diverting agent was used for even cluster initiation and fracture geometry. The pressure response of the diverting agent was obvious indicating good diversion result and new reservoir contact. The initial production recovery rate was 88.1% compared to that of the original fracturing and the production increased 819% compared to the production before the CiC refracturing, which was much higher than BH refracturing. This case study illustrates that the CiC refracturing method is an effective method for refracturing. It overcomes the uncertainty and difficulty of proppant placement, achieves a higher intensity of fracturing treatment, and enables stimulation of the previously bypassed reservoir to improve the recovery of the well. The method has provided the industry with a new and reliable option to rejuvenate the aging wells.