Abstract

In Te Giac Trang oil field, wells are completed with monobore design and with thru-tubing perforation carried out offline following the well completion phase. Producers in the field are perforated with underbalanced conditions through gas-lift application, which has resulted in relatively low-skin completions. In 2014 the first water injector was drilled in the field, with the same completion design applied as for the producers, however with no gas lift mandrels and surface gas lift line installed for cost saving. As a result, perforation of the injector was carried out without underbalanced conditions. The well started injection in November 2014 at 4k bwpd, with the bottom-hole injection pressure (BHIP) kept below the formation fracture pressure (FFP). Injection performance was below expectation, therefore in November 2015 re-/extended-perforation of key sand layers was carried out in order to increase the injection rate. Well injectivity however significantly reduced immediately following the re-/extended-perforation.

Pressure fall-off (PFO) tests were carried out on the injector, indicating significant near-wellbore damage following the re-perforation. Skin factor increased from +14 before the re-/extended-perforation to +50 afterwards. Impairment of the well injectivity was expected to be due to debris plugging the perforation tunnels. After thorough review of the options for remedial treatment, it was proposed to hydraulically fracture the formation using the existing water injection system. The objective of this treatment was to create and keep fractures open over a period of time, so that debris in the perforation tunnels could be pushed far away into the formation.

The treatment was carried out in February 2016, during which the water injection rate was increased to 12k bwpd, with BHIP significantly higher than FFP, and maintained at this over 3 days. PFO testing following the treatment indicated successful damage removal, with skin factor reduced to + 10. Subsequent well injection rate has been stable at 8k bwpd, with BHIP lower than FFP.

This paper describes the process of injection well performance evaluation, including PFO testing, acquiring reliable FFP, and application of remedial treatment through proppant-free hydraulic fracturing. The paper also covers the lessons learned on the design phase of injection well completion, on perforating and on the capacity of the water injection facility.

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