Analysis of Concentration Field in Underground-Storage Salt Cavern Using Two-Well-Vertical Solution Mining

Abstract

ABSTRACT: The well spacing and the brine concentration are crucial for the volume and morphology of the two-well salt cavern gas storage by affecting dissolution rate of the cavity. However, it is hard to directly monitor the brine concentration inside the cavity. This study uses the classical method of Navier-Stokes, based on which the three-dimensional mathematical model of Two-Well-Vertical solution mining is established. Using Semi-Implicit Method for Pressure Linked Equations (SIMPLE) algorithm to solve the concentration distribution, while using Volume of Fluent (VOF) algorithm to solve the movement of boundary, so as to research the variation of cavity shape. According to the simulation results, it can be concluded that the region with the brine concentration of 210 g/L – 270 g/L is the key to enlarge the cavity volume in the connectivity stage. Compared with single-well salt cavern gas storage, two-well salt cavern gas storage mainly expands along the connection direction of two wells. The well spacing of 50 m can increase the maximum cavity diameter by nearly 78% compared to that of 10 m. With the increase of well spacing, the final cavity shape will change from cone shape to hump shape, however, the large well spacing may affect the cavity forming efficiency. 1. INTRODUCTION There are more than 800 underground gas storage facilities in the world, and the total capacity is close to 450 billion cubic meters (Caineng Zou et al. 2020). Among them, salt cavern gas storage is the main technology for natural gas storage due to its lower permeability, higher gas injection efficiency and less gas consumption of cushion. However, Considering the number of salt cavern gas storage, the storage volume of salt cavern gas storage is much smaller than other underground gas storages. Therefore, accelerating the construction and increasing the volume of salt cavern gas storage are the most dominant approach to improve the competitiveness and economy. The construction of salt cavern gas storage mainly uses the salt layer of certain depth to dissolve. When the string reaches a predetermined depth, fresh water is injected to dissolve the salt layer, forming a storage space for natural gas. Up to now, a series of experiments and theoretical studies have been carried out about salt cavern underground gas storage. Menezes, J. D. et al. applied mixed boundary and finite element methods to numerical simulations of salt caverns, in order to study the state of natural gas in underground rock salt (Menezes, J. D. and Nguyenminh, D. 1996). The ground subsidence and cavity dissolution were analyzed by comparing the numerical simulation results with actual data.