Thousands of salt caverns (100 in France alone) are being used to store hydrocarbons. This is the safest way to store large quantities of hydrocarbons: salt formations are almost perfectly impermeable, and fire or explosion is impossible underground. However, a small number of accidents (blow-out, product seepage, cavern instability) have occurred in the past. Cavern abandonment is also a concern in some cases. This paper describes several accidents and the lessons that have been drawn from them, leading to considerable improvements in storage design and operation.

/pdf/safety-of-salt-caverns-used-for-underground-storage-blow-out-7z5edbqru3.pdf

Safety of salt caverns used for underground storage: Blow out; mechanical instability; seepage; cavern abandonment

A small-spacing two-well salt (SSTW) cavern is a relatively new type of cavern. As this type of cavern is less frequently used in underground natural gas storages, study on its stability is quite limited. To promote the application of this type of cavern and accelerate the construction of underground salt cavern gas storages, stability analysis and optimization design of this type of cavern are indispensable to the security of underground natural gas storages. In this paper, numerical calculation and parameters analysis were performed to analyze the effects of internal gas pressure and ratio of long axis to short axis (RLS) on the stability of SSTW caverns. The results show that internal gas pressure has an influence on the stability of SSTW caverns and low pressure should be limited in terms of duration and magnitude. And the ratio of long axis to short axis (RLS) has limited influence on the stability of a SSTW cavern by ways of volume loss rate and roof settlement. Overall, the SSTW cavern behaves similar stability state with that of a SW cavern. In addition, a flatter roof shape appears under greater Ratio value (RLS) and thus a thicker roof salt rock thickness is suggested for engineering design. The most different between them are the waist displacement along short axis, therefore the pillar width close to the adjacent cavern should be a certain lengthened at this direction.

Stability study and optimization design of small-spacing two-well (SSTW) salt caverns for natural gas storages

Tightness and suitability evaluation of abandoned salt caverns served as hydrocarbon energies storage under adverse geological conditions (AGC)

Permeability characteristics of mudstone cap rock and interlayers in bedded salt formations and tightness assessment for underground gas storage caverns

In the thinly bedded salt rocks, the roofs of salt cavern gas storage may have different lithology, some cavern roofs of these caverns may be damaged and lose tightness. Thus it is significant to study the characteristic of the gas leakage through the cavern roof of the gas storage cavern in bedded salt rocks. To approach such a goal, the gas leakage through a cavern roof and the induced collapse are investigated in this study. At first, the salt cavern of ZJ-block of Huai'an salt mine is selected as the potential gas storage site, which has a roof consisting of salt rock, a thin gypsum layer, and thick argillaceous siltstone. And then, the porosity and permeability of rock samples of the roof strata are measured in the laboratory. Thirdly, a numerical simulation model is established based on the geo-conditions of the ZJ-block. The gas seepage and cavern tightness under three different cavern roof conditions are simulated and analyzed. The results show that, if the cavern roof is integral, the main gas seepage channels of the cavern are the interlayers, and the tightness of gas storage salt cavern is satisfactory once the permeability of interlayers is around 10−17 m2 or lower. After the salt rock layer of the roof is damaged and a partial gypsum layer is exposed, the gas leakage amount can increase about one order of magnitude. But the seepage rate and pore pressure both change slowly in the roof. Thus the operators have sufficient time to survey the accidents and transfer the gas away. However, after the thin gypsum layer is damaged and results in the exposure of argillaceous siltstone, the gas seepage sharply increases. Including the leakage amount, the seepage range, and the pore pressure are all increased much than that of the above two conditions. Due to the serious consequence of storage safety, this condition should be absolutely avoided. To ensure the roof safety and tightness of gas storage, it is suggested that the reasonable construction of a cavern roof is the prerequisite, and the internal pressure monitoring, as well as sonar measuring, should be engaged regularly during the operation period.

Research on gas leakage and collapse in the cavern roof of underground natural gas storage in thinly bedded salt rocks

A salt cavern abandonment test

The method comprising measuring the rate at which working fluid is expelled from a cavity through a central tubing to a well head under atmospheric pressure at the well head; determining the shape of a zone of the well selected for positioning an interface between a test fluid and the working fluid; performing temperature measurements and pressure measurements over the full height of a cavity under atmospheric pressure at the well head; injecting additional working fluid into the central tubing or into the annular space until a predetermined test pressure is reached; measuring the compressibility of a cavity during the preceding step; injecting test fluid into the annular; measuring the depth of the interface between the test fluid and the working fluid for a first time at the end of injecting test fluid; measuring the test fluid and the working fluid pressures at the well head; measuring the level of the interface between the test fluid and the working fluid for a second time; and evaluating the real leakage rate of test fluid that took place during said test period.

Method of leak testing an underground fluid-storage cavity

Rock mass properties differ significantly from those measured on samples in the laboratory. A test has been performed on a deep brine-filled cavern, with the objective of measuring the equilibrium pressure reached when the cavern was closed. Such an equilibrium is reached when salt mass creep, which leads to cavern shrinkage, balances brine permeation through the cavern wall. A K= 2.10 -19 m 2 value of the average in situ intrinsic permeability has been deduced from the test; it is significantly higher than the intrinsic permeability measured in a well or in the laboratory. This result supports cavern abandonment scenarios in which the risk of natural fracturation due to high brine pressures is alleviated.

Une mesure de la perméabilité et du fluage d'une caverne dans le sel : A measurement of creep and permeability of a salt cavern

Process is claimed for testing the seal of an underground salt cavity for storing hydrocarbons, where the cavity (10) is linked to the surface by a shaft comprising firstly a central tube (20) from the top of the shaft (18) to the cavity allowing the injection and extraction of a first working fluid, and secondly an annular space (22) between the central tube and a cement lining (16) which allows the injection and extraction of a test fluid. Process comprises (a) measuring the flow of working fluid expelled at rest from the cavity through the central tube; (b) calibrating a section of shaft for locating a working fluid - test fluid interface; (c) measuring temperature and pressure over the whole of the top of the cavity; (d) additional injection of working fluid through the central tube or the annular space to reach a set pressure; (e) during the previous stage, measuring the compressibility of the cavity; (f) injection of test fluid into the annular space so the working fluid - test fluid interface is at a level within the calibrated section; (g) first measuring of the interface at the end of injection; (h) during a set test period, measuring the pressures of the two fluids near the top of the shaft; (i) second measurement of the interface after the test period; (j) evaluation of the loss of test fluid during the test period from the measurement of the interface level, the shaft section, the calculated densities of the two fluids, the compressibility of the cavity and the pressure at the top of the shaft.

Leak test for an underground storage cavity for hydrocarbons

Les proprietes d'un massif rocheux sont en general differentes de celles mesurees au laboratoire sur un echantillon. On a cherche la pression d'equilibre a laquelle se stabilise la saumure contenue dans une cavite souterraine profonde fermee. Cet equilibre resulte de l'action du fluage du massif, qui tend a refermer la cavite, et de la permeation de la saumure a travers les parois de la cavite. On deduit de la valeur d'equilibre mesuree une permeabilite intrinseque moyenne de l'ordre de $K = 2·10^{-19}\text{m}^2$, sensiblement plus forte que les valeurs estimees anterieurement sur des echantillons ou dans un puits. Ce resultat permet de construire un scenario du comportement de la caverne apres abandon, qui exclut le risque de fracturation naturelle.

https://hal.archives-ouvertes.fr/hal-00111626/document

Benoit Guerber

Papers

A salt cavern abandonment test

Method of leak testing an underground fluid-storage cavity

Une mesure de la perméabilité et du fluage d'une caverne dans le sel : A measurement of creep and permeability of a salt cavern

Leak test for an underground storage cavity for hydrocarbons

Une mesure de la perméabilité et du fluage d'une caverne dans le sel