String Corrosion and String Protection During Constructing and Operating Gas Storage Facility in Bedded Salt Deposit
- Yuan Guangjie (PetroChina Co. Ltd.) | Wang Qinghua (PetroChina Co. Ltd.) | Shen Ruichen (PetroChina Co. Ltd.) | Yuan Jinping (PetroChina Co. Ltd.) | Lu Lijun (Drilling Research Institute, CNPC) | Wang Chunmao (Liaohe Petroleum Exploration Bureau, CNPC)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Operations
- Publication Date
- February 2008
- Document Type
- Journal Paper
- 63 - 67
- 2008. Society of Petroleum Engineers
- 1.6 Drilling Operations, 4.6 Natural Gas, 2.2.2 Perforating, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 4.3.4 Scale, 5.4.2 Gas Injection Methods, 4.2.3 Materials and Corrosion, 5.10.2 Natural Gas Storage, 3 Production and Well Operations, 4.3.1 Hydrates, 2.4.3 Sand/Solids Control, 2 Well Completion, 4.2 Pipelines, Flowlines and Risers
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With more and more salt cavern gas storage under construction, the corrosion problems of leaching string, production string, and production casing require more attention by experts on gas storage. This paper introduces several corrosion phenomena that are encountered during the operation of leaching salt caverns and during natural-gas injection and withdrawal. The main corrosion factors are discussed such as brine, air, microbes, components of natural gas, gas injection velocity, operating status, and others. The corrosion mechanisms are also analyzed. To prevent the corrosion effects, a series of measures are successfully applied during different construction phases of the gas storage facility in the Yangtze River Delta, such as oxygen scavenger and biocide, coating protection, annulus protection liquid, and cathodic protection.
The two principal types of underground storage sites used in China today are depleted reservoirs in oil and/or gas fields and salt formations. Each type has its own physical characteristics (porosity, permeability, retention capability) and economics (site preparation costs, deliverability rates, cycling capability), which govern its suitability to particular applications. Salt-formation storage facilities provide very high withdrawal and injection rates compared with their working gas capacity. Base gas requirements are relatively low. To date, a large amount of natural-gas storages in the bedded salt deposits are being constructed by PetroChina as a component of the infrastructure necessary to accommodate the gas flow from the west-to-east pipeline. It is critical to ensure safe gas consumption in the Yangtze River Delta, which is the main gas-consumption area.
Solution mining is the process by which a void or cavity is created in an underground salt formation for storage purposes (RP 1114 1994). This cavity is created by the dissolution of salt when fresh water is injected into the formation under controlled conditions. The resulting brine is displaced out of the developing caverns for further processing or disposal. Solution mining of the cavity is usually achieved by controlled circulation of water through two concentric leaching strings down the wellbore and up again to the surface. The leaching strings include an intermedium string and a central string, and it can be seen in Fig. 1. Solution mining of the caverns represents about 25 to 35% of the investment in gas storage (Favret 2004). Taking to several years to complete, it is a long process that requires large water resources and that produces just as much brine with a salt concentration of 200 to 310 kg/m3, which is used by the chlorine and sodium chemical industries, or reinjected into the subsoil, or even pumped into the sea.
During the process of solution mining, the leaching strings and wellhead are often corroded and eroded by the brine, which contains plentiful salt, dissolved oxygen, and particulates. Therefore, the circulating pump pressure is increased, and the service life of leaching strings is reduced. Furthermore, the sealing property of the wellhead is also affected. These can all be seen in Fig. 2. According to construction unit surveys, the corrosion of leaching string and wellhead contributes to a solution mining cost increase of at least 15%, and at worst it can even cause a salt cavern to be discarded as useless.
Once the cavern is completed, the well is recompleted for dewatering and first gas filling. Gas is then injected through the annulus between the debrining string and the production string, and brine is produced through the debrining tubing. This can be seen in Fig. 3. Natural-gas injection and debrining are a very important part of the construction process. This process generally takes 6 to 8 months, and then, after the debrining string is pulled out, the gas-storage facility will be used for daily peaking, seasonal gas deliveries, and emergency gas supply.
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