FRENCH GAS-STORAGE PROJECT NEARING COMPLETION

Patrick de Laguerie
Geostock
Rueil-Malmaison, France
J. Gerard Durup
Gaz de France
La Pluine St. Denis

Geomethane, jointly formed by Gaz de France and Geostock, is currently converting 7 of 36 solution-mined salt cavities at Manosque in southeast France from liquid hydrocarbon storage to natural-gas storage.

The facility, used for liquid hydrocarbons since 1967, is operated by Geostock subsidiary Geosel. The conversions are part of wider development (Fig. 1),

The seven cavities lie 9001,500 in below ground and will operate over a pressure range of 6-18 MPa (60-180 bar). Aggregate storage will be 450 million normal cu in (MM cu in). The seven cavities are being commissioned over the period 1993-96.

In view of the large diameter (131/8 in.) of the original production wells and safety requirements, a unique high-capacity well completion has been developed for this project.

It will have two fail-safe valves and a flow crossover 30 in below ground to isolate the production well in the event of problems at the surface.

The project lies in the wooded Luberon Nature Reserve and due consideration has been given to locating the surface plant and blending it with the surroundings, The production wellheads are extra-low designs, the main plant was located outside the sensitive area, and the pipeline routes were landscaped.

20 YEARS' DEVELOPMENT

Since the first prototype of natural-gas storage in cavities mined in salt was completed in the U.S. in 1961, more than 30 sites have been developed worldwide, representing more than 200 individual cavities.

If all gaseous, liquid, and liquefied gas products are included, this total becomes more than 1,000 cavities.

Gaz de France began its salt storage at Tersanne in 1968, followed in 1977 by Etrez, which is still expanding. It now has 30 cavities in all representing I billion standard cu in (bscm) of active natural-gas storage capacity.

This is modest compared with the 17 bscm capacity in Gaz de France's aquifer storage, but salt-cavity storage offers a much higher peak delivery rate.

In 1967, Geostock (subsidiary of the BP France, Elf France, Societe des Petroles Shell, and C.R.D. Total France) decided to develop liquid hydrocarbon storage in solution-mined salt cavities at Manosque.1

Gaz de France's growing demand for storage capacity in southeast France combined with availability of some of the Manosque cavities resulted more than 20 years later in the setting-up in 1989 of Geomethane.

Gaz de France also built 70 km of 750 mm (30 in.) gas pipeline from Manosque to Cabries (between Marseilles and Aix) to link with the national bulk-transport system. 2

The Geosel cavities were built and operated under regulations governing underground storage of liquefied and liquid hydrocarbons embodied in Decree of Jan. 13, 1985, as amended by Decree No. 85-450 of Apr. 25, 1985.

Underground storage of combustible gas is subject to specific regulations set out in Decree of Nov. 6, 1962, as amended by Law of July 12, 1985, and Decree No. 88-220 (Mar. 8, 1988).

Geomethane's application procedure for a permit to store combustible gas was as follows:

Application accompanied by environmental impact-assessment and risk report: submitted June 1990.
Public enquiry: March 1991.

Two other enquiries, concerning the listed installations for the Gaude station and Cabries-Manosque pipeline were conducted at the same time by the same commission in order to submit the complete project to the public.

Local scrutiny by county government and local communities: April-September 1991.
National scrutiny by Interministry Council, French Public Health Council, and Mines General Council: October 1991-june 1992.
Order in Council: signed Mar. 24, 1993, with official publication of decision,Mar. 26, 1993.

From the very start of the project in 1989, considerable importance was given to keeping politicians and the public informed through a public meeting, press releases, handouts, and visits to the site and to other gas storage facilities.

Also, a local information committee was set up in 1991 and was chaired by the president of the regional council and attended by representatives of government, local communities, consumer associations, and the Nature Reserve.

It continues to meet regularly to keep watch on the progress of the project and express any special requests concerning safety and the environment.

An engineering-cost evaluation of the project was conducted jointly in 1988-89 by Geostock and the underground reservoirs department and National Development Centre of Gaz de France to assess the suitability of the cavities for natural-gas storage and to design the leading features of the cavity conversions.

The findings were favorable and the Geomethane Group was incorporated in 1989.

Engineering design of the well equipment and platforms occurred in 1990 and 1991. Solution-mining of cavities which had not yet reached their full size resumed in 1990 and will be completed by 1996 for the last (seventh) cavity.

Equipment installation began in 1992 on the first three cavities commissioned in 1993 and will continue up to 1996 for the remainder.

There are plans to start a second stage of development by mining seven more cavities to bring total storage capacity to 6 million cu in.

They will be located within the authorized storage zone and be similar to the first group of cavities, although major earthmoving work will be required to build suitable platforms in the hilly terrain.

THE SITE

Currently, the gas facility consists of seven cavities, all originally mined in the 1970s for liquid hydrocarbons and now being converted to pressurized natural gas.

The leading features of the cavities are:

Unit mined volume 220,000-500,000 cu m
Aggregate geometrical volume for seven cavities 2.5 million cu m
Last cemented 131/8 in. casing shoe 900-1,200 m below ground
Cavity bottom depth 1,250-1,500 m below ground 0 Operating pressure range in-cavity 6-8 MPa (60180 bar)
Peak delivery (Gaz de France standard definition) of 1.7-3.7 bcmd (depending on cavity size)
Aggregate total gas volume 450 bcm
Aggregate total working gas volume 300 bcm.

Gas arrives at the central plant at Gaude from the Cabries-Manosque pipeline at 6-8 MPa (60-80 bar). Gaude has everything needed for metering and compressing the gas before it enters storage and for drying it and controlling its pressure when it is redelivered to the pipeline.

Although of conventional design, this surface plant is unusual in that it is some 2 km from the cavity site (Figs. I and 2), unlike the common arrangement of such a facility: in the middle of the storage area to keep distances between wellhead and station short. (The environmental reasons for this choice are described later.)

The only installations directly in the cavities area are those of the gathering station: metering and marshalling lines, methanol tanks, and a small electricity shed.

Gas is sent from the Gaude station to the marshalling lines over two pipelines at a maximum working pressure of 21 MPa (210 bar) of 30 and 8 in. OD, then passes on to individual cavities through 8-in. fines.

The brine expelled by the first filling of gas under pressure is recovered in the Geosel installations. Thereafter, the cavities operate by simple gas expansion and compression between the maximum and minimum working pressures.

The facility is operated from the Gaude control room by Gaz de France with assistance from Geosel during first filling to dispose of the brine and during certain production-well monitoring operations.

MANOSQUE GEOLOGY

The Manosque salt was laid down in the Oligocene after the Pyrenees-Provence orogenic phase in the Apt-Forcalquier basin. The Alpine mountain-building phase at the end of the Miocene gave it its present form extending along an anticline trending east-west (Fig. 3).

Tectonics subsequently deformed the evaporates, locally thickening the salt beds.

Today, the Manosque salt formation is 200-1,000 m thick with previously bedded insolubles completely broken up into pieces scattered throughout the mass of the salt.

They are mostly measured in decimeters but some may be up to several meters thick. The proportion of insolubles is on the order of 15%.

The upper anhydrite formation is several dozen to several hundred meters thick and provides a massive, gas-tight caprock over the salt. Above this are alternating clays, marls, and limestones, also dating from the Oligocene.

The top of the salt lies deeper in some places towards the east, which is why the Gontard area was chosen for the seven natural-gas storage cavities (Fig. 4): Maximum working pressure is substantially proportional to cavity depth, with 1,000 m generally considered the optimum.

MINING; EARLY STORAGE

Plans for underground storage of liquid hydrocarbons at Manosque were formed in the mid-1960s by Geostock's oil-company shareholders to meet their statutory obligations, following the Suez crisis, to hold strategic reserves.

Although quite remote from any large pipeline, the site was chosen because it was near Mediterranean refineries. The Geosel facility is linked to the refinery area and the port of Berre-Lavera by two 97 km, 20-in. pipelines (Fig. 5).

Work began in 1968 with 18 cavities and continued up to 1978, by which time there were 36. Each one was solution-mined over heights of up to 400 m approximately 400-1,500 m below ground.

Cavity sizes range from 100,000 to 500,000 cu m.

In the process, a well is used to circulate fresh water in the salt formation. As the salt dissolves, the cavity gradually grows. The water is recovered in holding ponds.

Its size is calculated from daily salt inventories, and its shape is controlled by proper positioning of the fresh water and brine-recovery pipes (Fig. 6). A periodic sonar scan provides a cross-check on the shape of cavity growth.

Salt solution at the cavity crown is controlled by a cushion of an inert substance at the top (at Manosque, this was diesel oil in the outer annulus).

The fresh water was taken from the tailrace canal from Durance hydroelectric dam and the Forcalquier reservoir.

Liquid hydrocarbons storage cavities operate on the brine-compensation principle to ensure cavity stability and facilitate pumping.

The cavity is always full of liquid, the heavier brine (1.2 sp gr) at the bottom and the lighter hydrocarbons (0.7 to 0.9 sp gr) at the top, with hydrocarbons expelling brine as they are pumped into storage, and vice versa (Fig. 7).

This type of operation involves ponding an amount of brine at least equivalent to the cavity's storage capacity. This is done mainly in the Lavalduc and Engrenier lakes in addition to two 100,000 cu m purpose-built ponds on site.

They are operated by Gisel, a joint subsidiary of Geosel and Compagnie des Salins du Midi et des Salines de l'Est, which owns the ponds and is responsible for brine management and marketing.

The surface installations at Manosque consist basically of a pumping station with capacity for simultaneously pumping 1,000-1,500 cu m/hr of brine and petroleum products, and more than 1,000 cu m/hr for solution-mining operations (Fig. 8).

Now that Geomethane has been set up, Geosel retains 28 cavities, 26 of which are operating with an aggregate capacity of 6.3 billion cu m, and the remaining two used for saturated brine production.

Although originally providing strategic crude oil reserves, Geosel has widened its interests to operational storage for crude and many other products such as diesel oil, motor spirit (gasoline), high-octane spirit, naphtha, and condensate.

The Geosel solution-mining installations are used for enlarging the Geomethane cavities and for removing the brine during intial gas fill.

SLUNG

Preliminary assessment of the 36 existing cavities prompted selection of the Gontard area for gas storage based on the following criteria:

Depths from 900 to 1,200 m permit storage pressures of the order of 18 MPa (180 bar).
Cavities could be enlarged because they had not reached their full size.
These cavities being some distance from the others permitted gas and liquid hydrocarbons activities to be kept separate.
There was room for extending the facility by mining new cavities in the area.

Unlike liquid hydrocarbons storage cavities which operate on the hydraulic-compensation principle at near-constant pressure (approximately 12 MPa at a depth of 1,000 m), gas cavities operate at variable pressure in the 6-18 MPa (60-180 bar) range.

It was necessary, therefore, to check that the geometry of the existing cavities (shape, depth, diameter, height, distance between cavities) and the mechanical properties of the surrounding salt were compatible with the design criteria associated with this type of operation.

The geomechanical analysis of cavity behavior focused on the following points:

Laboratory tests were made at the laboratories of the Ecole des Mines de Paris (Fontaibleneau) and the Ecole Polytechnique to supplement the data available on the elastic and viscoplastic (creep) properties of the Manosque salt.
In situ tests were performed in the cavities to test parameters.
A digital simulation was made of the response of the existing cavities to pressure variations representing normal operating conditions.3

These investigations confirmed that cavity shapes and dimensions and salt mechanics were compatible with variable-pressure natural gas storage.

To illustrate this, the Manosque salt can be said to resemble the Etrez salt although the cavity shapes may be less regular or optimum by reason of the insolubles beds which greatly hamper uniform growth (Fig. 9).

In view of the much higher pressures involved with gas storage, the lines, wellhead, and surface pipes will have to be replaced.

Attention has been on the last casing cemented whose integrity had to be demonstrated as an essential condition for cavity conversion. Its diameter (131/8 in.) is larger than at Etrez and Tersanne (91/s in.), offering a much greater delivery rate.

Before resuming mining, we therefore pressure-tested the cavity and production well to check gas-tightness at 110% maximum working pressure; that is, at 18-24 MPa (180-240 bar) in different cavities.

ENGINEERING, CONSTRUCTION

The Gontard cavities had not originally reached their full size, and some had only just started to be mined.

Once ultimate maximum dimensions had been confirmed by geomechanical modeling and containment performance had been demonstrated, plans progressed for mining over the period 1990-1995 with cavities becoming available from 1992 to 1995.

Energy costs dictated that the mining proceed from Apr. 1 to Oct. 31 of each year. The same techniques were used as for the liquid-storage cavities with stringent monitoring of cavity growth.

The seven cavities, originally aggregating some I million cu m in size, will eventually offer an aggregate effective gas-storage capacity of 2.5 million cu m by 1995.

As enlargement of a cavity is completed, it is again pressure-tested to 110% maximum working pressure.

Unlike pressure tests on steel storage tanks, there are secondary effects interfering with the pressure monitoring such as additional salt solution as the pressure rises, heat exchanges, and creep.

This means it is also necessary to test the production well, by filling it with diesel oil just under the cemented casing shoe and checking that there is no movement in the oil-brine interface.

COMPLETION; MONITORING

This method is very accurate in assessing well integrity.

The production-well equipment design is based on the following criteria:

Safety. Standard underground gas-storage practice in France requires fail-safe valves at and below the wellhead.

For salt cavities, this requirement concerns the brine phase during first filling and the gas phase.

The fact of the site lying in a wooded area with some earthquake potential makes such equipment all the more necessary.

Performance. These underground valves and associated difficulties with their maintenance passage of wire line tools, etc. had to be compatible with the requirement for high gas-delivery rates and brine rates during first filling.

Following is the production well configuration during first filling (Fig. 10):

13-3/8 in. cemented casing set in place during the drilling
10-3/4 in. casing called the "completion casing"
Central pipe made up of (from top downwards) 7-5/8 in. pipe for top 30 m, safety section with two fail-safe valves and crossover for 15 m approximately, and 4-1/2 in. pipe-to-bottom of cavity.

This unique completion, which has been patented, is designed to overcome the arduous problem of making an annular valve with the flow crossover section and two tubular valves. It was made by Guiberson-Ava, Houston.

The pioneering nature of this equipment for such a large diameters, which are unusual in the oil industry, is also worth noting. Nearly all the safety-critical items are accessible by wire line for all maintenance work. The central plug at the crossover site is also accessible by wire line to leave free passage for the scanning instruments during first filling.

Under operational conditions (Fig. 11), the 4 1/2. in. casing is removed by snubbing, and an isolating sleeve is used to prevent communication with the 10 3/4 in.-7 5/8 in. annulus.

The wellhead comprises (from bottom to top) a monitoring outlet from 13 3/8 in.-10 3/4 in. annulus, two 4 1/2 in. side outlets for brine with pressure-operated fail-safe valve, pressure-operated fail-safe master valve on the gas line, 7 7/16 in. side outlet for gas, and 7 1/16 in. valve.

The originality of this wellhead stack lies chiefly in its compactness, with a total height of 2.5 m, of which 1.5 m is below ground level. Figs. 12 and 13 show the old liquid hydrocarbons wellhead and the new gas wellhead, respectively.

The underground and wellhead fail-safe valves are operated from an hydraulic plant generating the oil pressure and providing all the remote-control and automatic opening and closing functions.

A skid-mounted unit meters the brine outflow during first filling and the fresh water inflow to prevent crystallization.

First-filling with gas will proceed as follows:

Fill well with gas and test completion for leaks,
Continue to inject gas while simultaneously discharging brine to the Geosel installations.

On completion of the operation, as soon as gas starts to rise up the central pipe, the safety valves close automatically to prevent gas escaping to the brine system.

Under operating conditions, gas is pumped in and withdrawn to meet demand, and the cavity inside pressure is allowed to vary between the maximum and minimum design limits.

Cavity stability and creep (if any) are monitored from the surface by a leveling system with 200 survey points spread over the Gontard area. Periodic surveys will detect any subsidence.

In addition, the geophone-based seismic monitoring system at the Geosel facility will be extended to Geomethane.

NATURE RESERVE

The Geosel and Geomethane sites lie in an area where the salt formation investigated is suitable for cavity storage in the eastern part of the Luberon Nature Reserve set up in 1973.

The Gontard valley lies at the edge of the "nature and silence" area of the reserve, and the companies decided to locate the central station away from it, at Gaude, where there is a lignite mine which closed in the 1960s.

The platforms for the cavities have not been changed and the gas equipment which has replaced the hydrocarbons equipment is equally as discreet.

Only one new platform has been built, for the marshalling point. It occupies an area of 3,500 sq m along a minor road and has only such equipment as valves, filters, and a meter,

Screening and planting for concealment of the platforms and reinstatement of the tracks made when the gas pipelines were laid was designed in close consultation with the Reserve's landscape experts.

As soon as work is completed, the Gontard valley will be as peaceful as before and as at the Geosel site. The entire Geosel and Geomethane area is in fact a wildlife sanctuary.

FIRE, EARTHQUAKE

Fire in this Mediterranean region is a major hazard, and the woods at Gontard meant considering the risk from the outset (in terms of risk both to and from the installations).

The redundant safety systems previously described keep any risk of accidental leakage of gas to a minimum. There is an extensive fire-fighting system with fire mains and hydrants by each platform, duplicated supply, and mobile plants available at Gaude and Geosel.

Forest management, including scrub removal in liaison with the French National Forestry Board, is also essential in combating forest fires.

The Manosque region is particularly sensitive to seismic hazard, as shown by the historical earthquake record. The area is in fact in a Zone II area according to the new French seismic zoning.

Earthquakes were therefore one of the major natural risks considered in the design.

In fact, siting installations underground is in itself a form of protection against earthquake damage, which is mainly concentrated at ground level where reflected seismic waves amplify the energy released.

Underground, on the other hand, the cavities move with the ground and the waves "wash" over them without causing any dangerous stresses or strains.

This is widely confirmed from both modeling and experience of recent earthquakes in California and Armenia where there was no damage to underground works.

Thorough seismic analyses were made for all the underground and surface works. A seismo-tectonic approach was taken with specialists from the French atomic energy authority CEA and consulting specialists.

An historically probable maximum earthquake (HPME) was defined from local historical seismicity (positions and effects of historical earthquakes in the region).

Then the design earthquake is the "factored earthquake," found by taking the next intensity up from the HPME in order to simulate even more severe conditions.

In the absence of any strictly applicable regulations, the favorable underground conditions enabled us to use the approach recommended by the French Fundamental Safety Rules in Nuclear Engineering,

These calculations confirmed the absence of and, major earthquake effects on the cavities and the wells linking them to the surface.

The surface installations, especially the buildings at Gaude, were designed to PS 69/82 earthquake design rules and AFPS 90 guidelines on the basis of a special site study (site earthquake response).

REFERENCES

Dubois, D., "Underground Storage of Hydrocarbons at Manosque," Annales de Mines, December 19,2 (in French).
Chapon, M., and Rossignol, C., "Cabries-Manosque Gas Pipeline in Regional Environment," 110th Gaz ATG Congress, Nantes, 1993 (in French), Congress Proceedings
Vouille, G., Bergues, J., Durup, G., and You, T., "Study of Stability of Solution-Mined Cavities in Salt," presented to Eurock, Lisbon, 1993 (in French).

Issue date: 12/12/94