In water injectors, rapid shut-in creates a water hammer effect. Over time, injectors that undergo repeated rapid shut-ins often have significantly reduced injectivity and show evidence of sanding and even failure of the down-hole completion. This study is to provide an operational reference for the well injection for an offshore deepwater field to mitigate back-flow and maintain the downhole sand-control device integrity and thus satisfactory water-injectivity.

Two different shut-in scenarios have been investigated. One of the scenarios is that among the three water injection wells, two are shut-in and the third one is kept open. Another scenario is that the top side pump is stopped while the three injection wells are still kept open. Water hammer sensitivity on different parameters, such as valve installation position, stroke time, water back flow conditions and the hydraulic characteristics, has been performed.

For Scenario I, the pressure change due to wellhead shutting-in is around 200 psi at bottomhole (BH), which is much lower than the amplitude seen at the wellhead (3400 psi). The third well which stays open for injection experiences even larger pressure surge (around 450 psi at bottomhole). Back flow for the opening well could be close to 10,000 stb/d. With increasing skin due to cumulative injectivity damage by water particles plugging and/or thermal induced fracture closure at shut-in, the water hammer pressure fluctuation can be as high as 1200 psi.

For Scenario II, pressure fluctuation due to top side shut-in is 300 psi. With SCSSV closing when back flow is felt, the water hammer fluctuation can be reduced to around 200 psi.

Unlike the classic water hammer in the pipeline, water hammer in the water injection wells is much less in terms of surge amplitude, as the high injectivity reservoir behaves like a cushion to absorb the water hammer impact on downhole completion and sand-control infrastructure. Water hammer in the wells and pipeline system experiences: (1) after-flow with reduced BHP and flow rate into formation; (2) back flow when BHP becomes less than reservoir pressure; (3) water flows into reservoir again when BHP starts to increase. This cyclic process continues with reduced amplitude in each cycle due to friction.

Results also show that check valve set at the bottomhole could stop the back flow in less than 1 sec.

This study provides useful reference and operation guidelines on offshore water injection and completion design consideration.

You can access this article if you purchase or spend a download.