Summary
In water injectors, rapid shut-in creates a water hammer. Water-hammer
effects resulting from the shutting in of water-injection wells have
considerable impact on injection-well performance and longevity. Over time,
injectors that undergo repeated rapid shut-ins often have significantly reduced
injectivity and show evidence of sanding and even failure of the downhole
completion. This study seeks to provide an operational reference for the
well-injection operation and valve installation to mitigate backflow and
maintain the downhole-sand-control-device integrity and thus good water
injectivity.
Two different shut-in scenarios for an offshore injection system have been
investigated. One of the scenarios is that two wells are shut in and the third
well is kept open. The other scenario is that the topside pump is stopped while
the injection wells are still kept open. Water-hammer sensitivity on different
parameters, such as valve-installation position, stroke time, water-backflow
conditions, and the hydraulic characteristics, has been performed.
For Scenario I, the pressure change because of the wellhead shut-in is
approximately 200 psi at bottomhole, which is much lower than the amplitude
seen at the wellhead (3,400 psi). The third well, that stays open for
injection, experiences an even larger pressure surge (approximately 450 psi at
bottomhole). Backflow for the opening well could be close to 10,000 STB/D. With
increasing skin because of cumulative injectivity damage by water-particle
plugging and thermal-induced fracture closure at shut-in, the
water-hammer-pressure fluctuation can be as high as 1,200 psi. For Scenario II,
pressure fluctuation because of topside shut-in is 300 psi. With the
surface-controlled subsurface safety valve (SCSSV) closing when backflow is
felt, the water-hammer fluctuation can be reduced to 200 psi.
Unlike the classic water hammer in pipelines, water hammer in injection
wells is much less in surge amplitude because the high-injectivity reservoir
behaves like a cushion to absorb the water-hammer impact on the downhole
completion and sand-control infrastructure. Water hammer in the wells and
pipeline system experiences (1) after-flow with reduced bottomhole pressure
(BHP) and flow rate into formation, (2) backflow when BHP becomes less than
reservoir pressure, and (3) resumption of water flow into reservoir when BHP
starts to increase. This cyclic process continues with reduced amplitude in
each cycle because of friction. A check valve set at the bottomhole could stop
the backflow in less than 1 second. This study provides useful reference and
operation guidelines on offshore water-injection and completion design
consideration.
© 2010. Society of Petroleum Engineers
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History
- Original manuscript received:
2 March 2009
- Meeting paper published:
9 June 2010
- Manuscript approved:
23 April 2010
- Published online:
28 October 2010
- Version of record:
17 November 2010
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