About 9600 casing-damaged wells were identified till 2004, up to 18% of total wells in Daqing oilfield, China. The number of casing-damaged wells is increasing at a rate of 600 wells annually. So how to apply hydraulic fracturing technique for those wells is of major importance to recover and improve oil production in related mature reservoirs. A new technique called small-diameter separate-layer hydraulic fracturing is investigated for those damaged wells whose diameter is above 105 mm. Fracturing string and its matching tools are specially designed and tested, including packer and its packing element, sand jet and safety joint. Testing results show that stabilized pressure time can maintain 5~8 minutes at the testing pressure of 40-45 MPa for all tools. Mathematical model that can be used to predict stress of casing-damaged wells was developed based on the analysis of ground bearing mechanisms under casing damages. Calculation results from the model matched very well with practical testing results. Criteria of well and layer selection will be provided based on theoretical analysis. The technique can be used to hydraulic fracture multiple layer in one time.

The technique has been successfully applied to hydraulic fracture casing damage wells and is underway to deploy in Daqing oilfield. Until the end of 2004,, fourteen wells were hydraulic fractured using this new technique. The average fracturing layer is two for each well. All treatments were successful. The average number of fracturing layers was two per well, and the average fluid increase was 20 tons/day/well, and the average oil increase reached 5.0 tons/well/day.

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