Underground Mined Type LGP Storange

7/29/2019 Underground Mined Type LGP Storange

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Chapter 5UNDERGROUND MINED TYPE LPG STORAGE

by D. F. McCarthy, President

McCarthy Engineering & Construction, Inc.Tulsa, Oklahoma

INTRODUCTIONMined underground chambers specifically designed for the purpose

of storing liquefied petroleum gas (LPG) at ambient temperatureshave been in use for over thirty years. The first caverns were de-veloped by Warren Petroleum of Tulsa, Oklahoma, in 1948, for itsplants in Texas.

The basic concept of an LPG-storage cavern is to create a spacein an impervious formation at a depth sufficiently below the watertable so that if any movement of fluid occurs the ground water willenter rather than the gas escape from the cavern.

In the past, the primary reason for LPG storage underground hasbeen its attractive cost advantages over steel storage on the sur-face. At current prices the cost of storing 7.949 ML (50,000 bar-rels) would be equivalent, whether stored on the surface or under-ground; whereas 79.494 ML (500,000 barrels) of storage undergroundcosts only one-fifth of surface storage. It is usually uneconomi-cal to excavate less than 79.494 m m (50,000 barrels) of storagespace. The fixed costs - engineering, mainshaft and ventilationshaft - are a substantial part of the total cost, whereas the in-cremental costs for excavating additional storage are relativelylow.

Recent concern over the hazards posed by the handling and stor-age of LPG and liquefied natural gas (LNG) has led to a new inter-est in the underground storage of these gases, Underground storageoffers the following advantages: safety, security, low initialcost, low maintenance cost, operating,cost, minimum land require-

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UNDERGROUND MINEDTYPE LPG STORAGE 87

ments, aesthetic value, efficient operation, and constant operatingtemperature. Part of the above predicates lower insurance rates.

SITE INVESTIGATIONIn the design of mined storage caverns, the primary concerns are

the geological conditions that might adversely affect personnel safe-ty and increased costs during the construction period, or limit thelife of the completed facility. A feasibility study, including coredrilling, is conducted to determine the suitability of a proposedsite. The information gathered from the cores is the most usefulmeans of obtaining data about the qualities of a rock mass essentialto the design and construction of the underground excavation.

The contractor gathers information that defines the site topo-graphy, means of ingress and egress, availability of drilling water,potential lost circulation zones, and the types of formations to becored. The more information available, the more realistic will bethe bids for the core drilling program.

Other helpful information includes source and availability ofpower, the number of available local subcontractors, suppliers,and restrictions on noise, and welding and cutting which are uniqueto each particular project. There is no substitute for meaningfuldata related to local geological and engineering conditions.

CORINGThe majority of all LPG-storage caverns have been constructed

at depths of less than 152 m (500 feet); therefore a 0.08 m (3inch) diameter core hole is ample. A portable drill rig is adapt-able to drill a 0.08 m (3 inch) hole and extract an NQ core inlengths up to 6.1 m (20 feet). An NQ diamond coring bit, which issatisfactory for conducting the necessary laboratory tests, cuts ahole 0.08m (3 inches) in diameter and obtains a core 0.05m (1,87inches) in diameter.

~~e drilling program should include at least one hole that iscored from the surface to its total depth. The remaining holesshould be cored a minimum of 6.1 m (20 feet) above and below theproposed cavern interval.

The recoverable core indicates the character of the intact rock,and the number and character of the natural discontinuities. Onemethod of quantifying the in-place rock mass quality is to recordthe fractures observed in the core. A rockmassof good qualitYwill have a Jow fracture frequency (approximately one fracture per0.30 m (1 foot) or less and therefore will have an in-place modu-


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88 1981 RETCPROCEEDINGSVOLUME 1lus of deformation that will approach the modulus of the intactspecimen. High fracture frequencies indicate a poorer quality rock.

As a means of obtaining a quantitative index for describing rockquality from the coring program, we have..usedthe RQD (rock qualitydesignation) method developed by Deere in 1964.

The RQD is a method of cataloging core recovery in which allthe pieces of sound core over 0.10 m (4 inches) in length arecounted as recovery. (The smaller pieces are attributed to shear-ing, jointing, fracturing, or weathering in the rock mass and arenot counted). The RQD provides a preliminary estimate of the vari-ation of the in-place rock mass properties of the sound portionof the rock core. An ROD approaching 100% denoted excellentquality rock mass as indicated by the scale:

RQD Description of rock qualityO-25 Very Poor25 - 50 Poor50 - 75 Fair75 - 90 Good90 - 100 Excellent

By using the RQD, the geologist quantifies his interpretation ofthe rock quality, which is useful when comparing rock quality fromprevious projects and core analyses by different geologists.

The geologists ability to carefully supervise the drilling pro-gram minimizes fresh breaks in the core not related to the qualityof rock mass, which might occur during the drilling and handling ofthe core. The expertise of the driller will affect the amount ofbreakage and the core loss that will occur. Poor drilling tech-niques and incompetent supervision will penalize the rock massby lowering its apparent quality. Distinguishing between drillingbreaks and natural fractures that reflect the quality of the rockmass is difficult. A geologist present during the pulling ofcores can minimize improper logging.

The geologist assigned to logging the cores should give specialattention to the shale cores. They should be cleaned with a brinesolution, logged, and placed into plastic bags. In addition, goodpractice dictates the taking of close-up colored pictures of theentire length of the core. After removal from the core barrel,the cores should be baked and placed in dry, safe storage area.Drilling with air or clean water is desirable to obtain the mostaccurate results.


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UNDERGROUND MINED TYPE LPG STORAGE 89FIELD TESTING

Immediately after each hole is drilled, the formation should betested hydrostatically to determine the in-place permeability. Thisis done by applying a constant fluid pressure on an isolated zonein the formation, and measuring the rate at which fluid can be in-jected into the rock strata.

The core holes are tested using straddle packers spaced approxi-mately 7.01 M (23 feet) apart with a constant 517.11 kPa (75 psi)applied pressure.

An empirical guide that I developed some twenty-five years agohas been used in site selecting for almost all LPG mined typecaverns.

Formation Pressure TestWater Loss u m3/s (gph) Classificationo- 0.63 (0 - 0.6) Excellent0.63 - 1.89 (0.6- 1.8) Good1.89- 3.15 (1.8 - 3.0) Fair

3.15 ( 3.0) PoorOf the 87 LPG caverns constructed there have been three caverns

that were aborted immediately for reasons associated with fracturesand/or ground water problems. Two of the three caverns were com-pleted, but failed to pass the final air test conducted on thecavern. One cavern was terminated as the water inflow reached2.21 m cu m per sec. (35 gpm).

The drilled holes should not be cemented until the drilling pro-gram is complete. The holes should be cemented from the bottomback to the surface with neat cement.

LABORATORY TESTINGIntroduction

Laboratory tests are performed on cores to determine additionalrock properties than can be determined through field testing. Rou-tine analysis are performed to determine: (1) formation permeabili-ty; (2) porosity; (3) bulk density; and (4) unconfined compressivestrength, Water vapor exposure and propane immersion tests mayalso be valuable. A brief description of the individual testingprocedures follows.


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90 1981 RETCPROCEEDINGSVOLUME 1Permeabi1ity

Permeability is the rock property that indicates the ability offluid to pass through the rock. It is a property of the rock itself,and is not dependent upon the type of fluid flowing through it, ex-cept when chemical reaction occurs between the flowing fluid andthe rock.

Routine permeability measurements are usually made by flowingair or inert gas through a sample of rock of known, regular dimen-sions. The normal procedure is to cut a perm plug from a largercore and dress the faces of the plug to a smooth surface normal tothe axis of the plug. The plug is then placed in a holder thatseals the perimeter, and forces gas through the plug while measuringthe rate and pressure.

Even though permeability is a unique property of each rock sample,when it is being measured with a gas the indicated permeabilitywill vary with the pressure utilized to force the gas through thespecimen. The permeability constant is predicated on viscous flowthrough the sample, and elevated pressures create turbulent flowwithin the sample and thus distorts the permeability constant. Tocorrect for this effect, a plot of the indicated permeability con-stant can be made against the reciprocal of the median pressurewithin the sample. Extrapolation of this plot to infinite pressureor the value zero (0) of the reciprocal of pressure then will givea permeability constant which can be duplicated by flowing fluidsthrough the sample which are non-reactive with the sample beingtested.

The recommended practices developed by the American PetroleumInstitute, as set out in RP27 and RP40, are fol?owed throughout thepermeability determinations. Nitrogen gas is used as the testfluid,Porosity

Porosity is that portion of a rock unoccupied by solid minerals,The pore space will always be occupied by a fluid of some descrip-tion. There are two types of porosity: intergranular porosity,or the void spaces between discrete grains of the rock; and sec-ondary porosity, which has developed subsequent to the formation ofthe rock, taking the form of vugs OF solution channels in carbon-ate rocks. Normal laboratory measurements determine only the pri-mary or intergranular porosity.

Porosity determinations are usually performed on the specimenperm plugs. Fluids are extracted from the specimen by drying itto a constant weight of 105C. The specimen is weighed on ana-lytical balance and then placed in a Kolb-type porosimeter, utiliz-ing the Boyles law method of porosity determination, according


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UNDERGROUND MINED TYPE LPG STORAGE 91

to the techniques outlined in API RP40.In this technique, an extracted sample is subjected to an ele-

vated pressure with a gas. The change of gas volume is measuredwith the increase in pressure, which is representative of thetotal gas volume of the pore space within the sample and the con-taining vessel, The actual pore space is computed from the knownvolume of the containing vessel and the bulk volume of the rocksample.Specific Gravity

The determination of specific gravity of the rocks penetratedduring the coring has its primary utilization during the design ofthe cavern openings and pillar supports. It is used also to indi-cate the weight of material that must be handled during miningoperations.Unconfined Compressive Strength

Another parameter used to design a stable mine opening is theUnconfined Compressive Strength of the rock. Although it is sim-ple measurement in which a core specimen is loaded to collapse ina press, careful attention to details in the preparation of samplesfor a compressive test is important to ensure consistent results.

Recommended testing procedures are outlined inASTM D2938-71a.Each specimen is prepared by sawing its ends with a diamond saw,and measuring the diameter and height to insure that the ends areparallel to each other and at right angles to the longitudinalaxis. Until tested, the specimens are wrapped in plastic to pre-serve their moisture conditions.Water Vapor Exposure Test

To determine the effect of high humidity on the shales duringmining operations, a test was contrived that consists of placingspecimens in an oven for six days. The oven temperature is setat 40C and the specimens are subjected to alternating cycles oflow and high humidity as follows:

The specimens are accurately measured to determine their length,diameter, and bulk density before being placed in the oven to dryfor a 24-hour period. At the end of this period, they are removedfrom the oven, remeasured and reweighed. They are then returned tothe oven, along wf,tha pan of water that evaporates and raises therelative humidity in the oven atmosphere. At the end of this 24-hour period, the water and samples are removed from the oven, andthe samples are remeasured and reweighed a second time. The dryand wet cycles are repeated three times each to end the testingperiod. The effects on the samples at the end of the six cycles


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92 1981RETCPROCEEDINGSVOLUME 1are noted, and the results tabulated.Propane Immersion Test

For this test, samples are taken from each core hole located inthe proposed cavern interval. The samples are placed in a steelvessel, which is then filled with liquid propane. At the end offive days, the propane is vented from the vessel, the samples areremoved and examined, and the results tabulated.

CAVERN DESIGNThe size of the cavern openings are predicated upon maintaining

maximum structural support during the construction period.Permeability to air and propane tests are conducted oncore

samples taken from the region within 9.1 m (20 feet) above and be-low the proposed cavern interval. These tests indicate whetherthe rock mass is impervious to the transmissibility of the storedproduct. To be considered an ideal site, the measured permeabilityshould be less than 0.01 millidarcy. Careful judgment should bemade when examining small samples, as these tend to give a higherpermeability value than the in-place results.

The apparent porosity test is conducted to arrive at an esti-mate of the probable loss of product during the initial filling.These losses have been as high as 2%. The drying out and surfacefracturing of the formation will cause a loss of 1% while theother 1% is due to irregularities in the drainage and mining pat-terns plus the liquid that is not recoverable in the sumps. Lossesduring subsequent fillings have been negligible.

The results obtained from the water vapor exposure test are use-ful in determining the type of rock anchors used for supporting theroof and walls. The rate of deterioration (air slacking) of therock due to moisture is reflected in the design of the ventilationequipment, additional support in conjunction with the rock bolts,and the method of sealing off any inflow of water into a cavern.

The most reliable method of supporting the rock openings is touse rock bolts, which are installed in the roof as near to thework~ng face as possible. The depths and spacing of the bolts arepredicated upon the strength of the rock and the size of the open-ing. In soft shales, the length of the bolts should never be lessthan one-third of the roof span. The majority of rock bolts in-stalled in the past used a mechanical anchor. When subjected toair slacking (deterioration as the humidity changes), resin boltsor split sets give better support. Since the completed cavernoperates at approximately 0.60 MPa (100 psig) and the propane is in-ert to the rock, the supports have to be effective only during the


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UNDERGROUND MINED TYPE LPG STORAGE 93construction.

A list of the LPG mined type caverns constructed in the UnitedStates is shown in Table I.

COMPRESSED AIR STORAGEThe first phase of construction for an LPG cavern is the dril-

ling of the main shaft. The drill hole is usually a 1.9304m(76 inches) to 2.1336m (84 inches) diameter hole and cased with a60 inch diameter casing. After the casing is cemented, bailed anda very small cavity created directly beneath the 1.524 m (60 inch)cased hole, the shaft is closed in and the formation is air tested.

This formation test doesnt give any more data than what is ob-tained from the straddle test conducted on the core holes, but itdoes reconfirm the initial data.

For compressed air storage or reverse pump back storage wherethe depths and the excavations are much larger, I would reconnnendthat an access shaft be drilled as part of the testing program.At this time an extensive rock mechanic investigation could be un-dertaken.

This small diameter shaft could be incorporated into the finaldesign and used to expedite the underground construction.


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Underground Mined Type LGP Storange