Water hammer effects resulting from the shutting in of water injection wells are an often ignored issue in petroleum production operations but they have considerable impact on injection well performance and longevity. Mismanaged, they can result in substantial and perhaps irreparable damage. This paper presents a study on the creation and propagation of water hammer due to rapid shut-in of water injectors.
Water hammer1-4 or pressure surge, is a pressure transient phenomenon which has long been known to occur as a result of a sudden change in fluid flow velocity. In water injectors, rapid shut-in creates a water hammer. 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 completion5 . It is therefore critical to understand the nature of water hammer including the magnitude, frequency, and energy dissipation.
To study the water hammer in water injectors, a field trial was conducted to record pressure pulses generated from rapid shut-ins, at different well depths, in a soft formation, cased and perforated (C&P) water injector. Modeling work was conducted to understand the data.
The results of the field trial and model work demonstrated that:
The magnitude of the first pressure pulse due to abrupt shut-in can be estimated by using the equation: Δp = V×ρ×c, where V is the flow velocity, ρ is the fluid density, and c is the speed of propagation of a pressure signal along the well.
High rate sampling (up to 100 samples/second) is required to capture subtle details of the water hammer signal.
Water hammer can be modeled as a low-frequency pressure wave similar to the higher frequency Stoneley waves produced during VSP and by acoustic logging tools.
Models of water hammer propagation in a synthetic analog of the test well reproduced most of the details of the signals recorded during the tests.