A new water injection technology of concentric dual string in high-angle deviated wells of offshore oilfields

At present, the water injection technology of concentric dual string used in offshore oilfields is faced with a series of difficulties. For example, the safety control is unreliable, the stratification of inner and outer string tends to fail easily, and well flushing is difficult due to incomplete unsetting of the packer. To solve these problems, a new water injection technology of concentric dual string was studied in this paper. In this technology, water injection strings are independent from each other, so the construction is relatively simple, the additional load on the outer string is reduced greatly and three water injection channels can all realize independent safety control. The annulus safety valve adopts the plunger setting mode and has a standby water injection channel, so its reliability and service life are improved. The pattern of combined packing is adopted for the stratification of inner and outer string, and two-stage sealing mode of combined packing and inner packer rubber is adopted at the easy-failure positions, which makes the sealing effect more reliable. For well flushing, the flushing sliding sleeve is controlled independently on the ground so as to prevent the formation pressure difference from leading to the self-switching of flushing piston and satisfy the needs of large-displacement inverse well flushing in offshore oilfields. This technology has been successfully applied in over 10 wells in Shengli offshore oilfield, and its suitable maximum well deviation and maximum interwell pressure difference are 74.18° and 10.5 MPa, respectively. In conclusion, this technology effectively realizes the separate layer water injection in the oil layers with large pressure difference and the high-angle deviated wells.