Technical and Risk Assessment of Underground Gas Storage Construction and Operation in China and Caspian Region

Abstract

Since China proposed "Belt and Road" initiative in 2013, Caspian countries have become a crucial region for oil and gas cooperation. However, it has been a common technical issue to develop presalt oil and gas resevoirs in the world. Caspian region is known for the wide-spread thick salt-gypsum formations. In China some old wells that were depleted due to engineering difficulties arising from presalt formations started to be developed as underground gas storages. The crude oil price has been keeping at a low level in recent years, which made China's government put more efforts in the development of underground oil storages in depleted salt caverns. As the largest project of underground gas storages, Jintan gas storage construction project successfully made best use of depleted salt caverns. A Sonar test was applied on 4 Jintan underground salt caverns that are 900m deep and 1000m away from the surface in order to reveal their shapes. Salt and muddy salt rocks were collected in Jintan as samples to be analyzed in 3-axis pressure tests. Numerical results with finite element method (FEM) code ABAQUS were studied to approach the creep behaviors of caverns and rock pillars. The pressure arch model and the most dangerous sliding surface model were both utilized to optimize geometric parameters of the upper and lower walls of dissolved cavities. Creep happens on salt rocks under deviator stress, which has non-linear relationship with time. In the simulated gas storage process the internal pressure went up to 14.0 MPa within 3 months but dropped to 7.0MPa in the following 3 months. The above condition may last for 9.5 years in real scenarios. Surrounding rocks of caverns deformed during construction but the most obvious creep was found at their tops and bottoms. Within 10 years salt pillars between caverns are under high deviator stress, thus their creep is the most serious. Surrounding rocks become unstable as the internal pressure of caverns decreases. The creep scope of salt caverns' tops was determined, because casing shoes shall be away from it in order to enhance the sealing quality. Shapes of dissolved cavities' tops are determined by minimum internal gas pressure which is inversely proportional to arches' heights if the span is given, while shapes of dissolved cavities' bottoms are determined by maximum internal gas pressure which is inversely proportional to stability coefficients of the most dangerous sliding surface. If the dissolved cavity's shape is given, the cavity is more stable with a higher cohesion. The maximum span of a cavity determines its stability. It is the first time to use the pressure arch model and the most dangerous sliding surface model to optimize engineering parameters of underground gas storages in depleted salt caverns, which improves cavities' stability and optimized their shapes. Casing shoes used to be positioned directly above salt segments but now they are set away from the creep scope of salt rocks, which greatly contributes to the long-term stable operation of underground gas storages.