Simulation of Production and Injection Performance of Gas Storage Caverns in Salt Formations
- Jacques Hagoort (Hagoort & Assocs. B.V.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- November 1994
- Document Type
- Journal Paper
- 278 - 282
- 1994. Society of Petroleum Engineers
- 5.8.8 Gas-condensate reservoirs, 4.3.1 Hydrates, 4.6 Natural Gas, 5.10.2 Natural Gas Storage
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This paper presents a simple yet comprehensive mathematical model forsimulation of injection and production performance of gas storage caverns insalt formations. The model predicts the pressure and temperature of the gas inthe cavern and at the wellhead for an arbitrary sequence of production andinjection cycles. The model incorporates nonideal gas properties, thermodynamicheat effects associated with gas expansion and compression in the cavern andtubing, heat exchange with the surrounding salt formation, and nonuniforminitial temperatures but does not include rock-mechanical effects. The model isbased on a mass and energy balance for the gas-filled cavern and on theBernoulli equation and energy balance for flow in the wellbore. Cavernequations are solved iteratively at successive timesteps, and wellboreequations are solved within an iteration cycle of the cavern equations. Gasproperties are calculated internally with generally accepted correlations andbasic thermodynamic relations. Example calculations show that the initialtemperature distribution has a strong effect on production performance of atypical gas storage cavern. The primary application of the model is in thedesign, planning, and operation of gas storage projects.
Subsurface caverns in rock salt formations are being increasingly used forstorage of natural gas. Gas-filled caverns provide high deliverability andsecure gas sources at relatively low costs and with little environmentalimpact. Their primary use is supply of gas during periods with short-term(days) peak demands in gas. Of course, a large number of caverns may also beused to meet seasonal variations in gas demand. The first salt cavern for gasstorage was put into operation in the early 1960's in the U.S. Since that time,many more salt caverns have been constructed in the U.S., Canada, and Europeand have become vital parts of gas distribution systems.
Nowadays, a typical gas cavern in a salt formation is located between 1000and 2000 m deep and has an internal volume between 1x105 and 5x105 m3, amaximum operating pressure of 15 to 25 MPa, and a total storage capacityvarying from 15 to 150x106 m3 of natural gas.
The open technical literature on gas storage in salt caverns deals primarilywith the engineering geologic, rock-mechanical, and solution-mining aspects ofcaverns. To the best of our knowledge, no detailed studies have been publishedon the production and injection behavior of gas caverns.
In this paper, we present a mathematical model for the simulation of theproduction and injection performance of gas storage caverns. The model emphasisis on pressure and temperature behavior during loading and unloading of thecavern; rock-mechanical aspects are not addressed. Features that are includedin the model are nonideal gas properties, thermal interaction of the cavernwith the surrounding rock salt, and thermodynamic heat effects that areassociated with expansion and compression of gas in both cavern and wellbore.This model may find application in the planning and operation of gas caverns bygas distribution and transmission companies.
Fig. 1 shows the physical system that we wish to model. It consists of asubsurface cavern within an infinite salt formation connected to a surfacewellhead assembly by a straight, vertical, cased borehole equipped withproduction/injection tubing. The cavern contains a dry natural gas. We assume auniform gas pressure and temperature in the cavern at all times. The cavernpressure and temperature changes with time as a result of gasproduction/injection and heat exchange with the surrounding salt formation.During production, the gas expands; this is accompanied by a decrease in gastemperature. Likewise, the gas temperature increases when gas is compressedduring injection.
Initially (i.e., at the start of the gas storage operation), the temperaturein the rock salt in the vicinity of the cavern is lower than far away from thecavern. This is the result of the solution-mining process that is used tocreate a cavern. In this process, cold fresh water is injected continuouslyinto the cavern to dissolve the rock salt. The dissolution of salt into wateris an endothermic process that further reduces the water temperature. Duringsolution mining, therefore, the salt formation is cooled down continuously bycirculating cold water, giving rise to a cold zone around the cavern. In themodel, this cold zone is approximated by a zone of uniform temperature that isdifferent from the temperature farther from the cavern.
The two mechanisms that govern heat exchange with the salt formation are (1)transfer of heat at the cavern wall by natural convection and (2)non-steady-state conduction of heat in the salt surrounding the cavern. Theheat transfer at the wall by convection is represented by an empiricalheat-transfer coefficient.
Flow within the tubing is assumed to be steady state and adiabatic.Therefore, temperature variations within the wellbore owing to compression andexpansion of the gas are incorporated, but heat exchange with the wellenvironment is ignored. The latter simplification is justified because of thehigh flow rates of the gas in the wellbore during injection and production.
The gas in the cavern is assumed to be a real gas. That is, the pressure,volume, and temperature behavior of the gas are described by the real gas law,which includes the gas deviation, or z factor. The z factor depends on thecomposition of the gas and is a function of pressure and temperature.
The well constraints that are included in the model are a prescribedproduction (or injection) rate and a maximum (or minimum) wellhead pressure. Toallow a realistic representation of a practical operational sequence, the wellconstraints are to be specified as a function of time. During a productioncycle, the production rate is prescribed together with a minimum wellheadpressure, which is determined by the minimum intake pressure of the surface gasplant. If this minimum pressure is reached, the well becomes pressureconstrained and produces at declining rates. During an injection cycle, theinjection rate, the temperature of the injection gas at the wellhead, and amaximum injection pressure are prescribed. The last is determined by themaximum permissible pressure of the wellhead and casing assembly.
The pressure and temperature within the cavern are governed by the massbalance and the energy balance applied to the cavern at large.
Mass Balance. The mass balance states that, at any time, the amount of gasin the cavern must be equal to the amount of gas initially present minus theamount of gas produced.
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