More than 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. 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.
Casing damage has become a serious problem for mature oilfields. It is estimated that more than 9623 wells, about 18.51% of total wells, have casing damage problem till the end of 2003 in Daqing Oilfield. Casing damage has caused these wells difficult to be hydraulic fractured.
There are five main casing damage types in Daqing Oilfiled: deformation, break, cracking and corrosion. The main casing damage types are casing deformation and broken, which occupies 96.2% of the total casing damage wells. About 59.6% of total active casing damaged wells, 961 wells, the drifting diameter are larger than 105mm and these wells can be hydraulic fractured without any repairment. The drifting diameters of 651 active wells are smaller than 105mm and about 80% of them can be repaired. These wells can be hydraulic fracturing only when they are repaired. So small diameter hydraulic fracturing techniques have been studied.
Operation safety is the primary condition for hydraulic fracturing of casing damaged wells. A series studies were carried out to secure operation, such as stress status analysis on casing and cement annulus, damaged status of cement annulus, the critical distance between fracturing interlayer and casing damage point, and so on. These researches have been applied to guide the establishment of fracturing treatment criterions.
