Influence of water hammer effect on the well barrier integrity of shale gas well during SRV and the countermeasures

The failure of well barrier integrity is one of the key factors restricting the safe and efficient shale gas development. In the initial stage of shale gas development in the Changning-Weiyuan Block, the failure of well barrier integrity is obvious in the process of stimulated reservoir volume (SRV). In this paper, the well barrier system and its failure mode in the fracturing process of shale gas well were analyzed based on the technical characteristics of shale gas well SRV and the MIT24 arm caliper log data. According to the hydraulics principles, water hammer effect is generated by the sudden change of fluid flow rate inside the wellbore at the moment of SRV sand blocking and pump off, and water hammer pressure is propagated in the form of Stoneley wave along the solid-liquid contact inside the wellbore. Then, model for calculating the maximum surge pressure of wellbore fluid during the occurrence of water hammer effect was established by taking comprehensive consideration on the effects of water hammer pressure and fluid compression force. Finally, characteristics of the wellbore fluid pressure change caused by the water hammer effect at the moment of fracturing sand blocking and pump off and its effects on casing’s safety strength coefficient were analyzed. It is shown from the field construction parameters that the maximum water hammer pressure in the process of SRV sand blocking and pump off is as high as 31.88 MPa. Considering the influence of water hammer effect, casing’s collapsing strength coefficient is reduced to 0.95. And in view of the slippage and moving of shale gas formations and the reduction of casing’s collapsing strength caused by its shrinkage at low temperature, the non-uniform external loads are not high enough to damage the casing locally in the process of SRV. Based on these research results, the construction countermeasures to deal with the fluctuation of wellbore fluid pressure were proposed, which can be used as the reference for the well barrier integrity management of shale gas well with the completion mode of SRV.