Optimization and Evaluation of Chemical Shrinking Agent for Deposits in Salt Cavern Gas Storage

Salt cavern gas storage is an important strategic method to shave the fluctuation of supply-demand of natural gas in China. However, due to low grades of salt beds, there remains lots of insoluble sediments accounting for 1/3 up to 2/3 of the storage capacity at the bottom of cavity. The use of chemical agent with the function of swelled-clay-shrinking is an effective method for enlarging actual cavity volume. Clay swelling and physical deposits experiments were conducted to select the suitable chemical shrinking agent and study the relation between salt rock and agent. A device simulating the leaching process of insoluble sediments was developed to evaluate different factors on residue deposits and XRD (X-Ray Diffraction) was used to analyze mineral compositions of various salt caverns. The results showed that the main controlling factor for the volume swelling of the bottom insoluble sediments in the salt cavity is the electrostatic repulsion. These hydrated cuttings carry a negative charge leading to the electrostatic repulsion between each other, which promotes the loose accumulations of these physical deposits. The relation between rock and shrinking agent is clarified and the selected chemical agent has an excellent adaptation in salt cavern gas storages through the tests above. In addition, the result provides an experimental basis for minimizing the volume of the salt carven sediments to store more natural gas.

Numerical Investigation on Shape Optimization of Small-Spacing Twin-Well for Salt Cavern Gas Storage in Ultra-Deep Formation

Small-spacing twin-well (SSTW) salt caverns have an extensive application prospect in thin or bedded rock salt formations due to their good performance, while they are rarely used in ultra-deep formations. The target strata depth of Pingdingshan salt mine is over 1700 m, and it is planned to apply an SSTW cavern to construct the underground gas storage (UGS). A 3D geomechanical model considering the viscoelastic plasticity of the rock mass is introduced into Flac3D to numerically study the influence of internal gas pressure, cavern upper shape and well spacing on the stability of an SSTW salt cavern for Pingdingshan UGS. A set of assessment indices is summarized for the stability of gas storage. The results show that the minimum internal gas pressure is no less than 14 MPa, and the cavern should not be operated under constant low gas pressure for a long time. The cavern with an upper height of 70 m is recommended for Pingdingshan gas storage based on the safety evaluation and maximum volume. The well spacing has a limited influence on the stability of the salt cavern in view of the volume shrinkage and safety factor. Among the values of 10 m, 20 m and 30 m, the well spacing of 20 m is recommended for Pingdingshan gas storage. In addition, when the cavern groups are constructed, the pillar width on the short axis should be larger than that on the long axis due to its greater deformation in this direction. This study provides a design reference for the construction of salt cavern gas storage in ultra-deep formations with the technology of SSTW.

Construction Progress of Deep Underground Salt Cavern Gas Storage and Challenges of its Drilling and Completion Technology

With the rapid development of the national economy and the increasing demand for energy, and the acceleration of natural gas storage strategies, underground gas storage has become more and more important in China’s oil and gas consumption and energy safety. Underground salt cavern gas storage has many advantages and is very suitable for gas storage and peak shaving。However, most of the underground salt rocks in China are layered salt beds, and some of the salt beds are deeply buried. This article introduces the development history of deep underground salt cavern gas storage and the technical challenges faced in the process of drilling and completion of salt cavern construction.