Pressure transients to characterize cavities dissolved for natural gas storage

• Characteristics of pressure wave inside a submerged pipe significantly change responding to its boundary conditions.
• They include diameter of a conduit containing the pipe and an interface between contrasting fluids in the conduit.
• Pressure transient can be a useful tool for characterization of subsurface structures.

This paper describes an analysis of pressure transients in conduits that includes effects of variable conduit diameter and contrasts in fluid material properties. The analysis considers pressure applied to the outside of a submerged conduit, as well as that transmitted along the submerged conduit's interior with potential applications such as analysis of submerged pipelines or tubing in oil wells or in wells used in solution mining.In this work, we present a method for monitoring the dissolution of carbonate rock with acid to create a cavern for storing natural gas. This process includes an acid injection pipe and a brine production pipe submerged in the fluid-filled cavern. The motivation is to characterize the geometry of the cavern by analyzing pressure transients traveling down the acid injection pipe, interacting with the walls of the cavern and compressing the brine pipe, and traveling to a pressure transducer in the brine pipe. The conventional water-hammer equations were modified to include the pressure applied to the outside of a submerged conduit, and the resulting boundary value problems were solved with perturbation analysis. The results show that peak pressure at an observation point decreases more than one order of magnitude when the cavern diameter increases one order of magnitude. This suggests that the proposed approach is sensitive to cavern diameter. Consideration of the external loading provides a potential mechanism for locating an interface between fluids of different properties, such as the interface between acid/CO2 mixture and underlying brine.