rrd-gas-09 mraa gas pressure monito ring · louisiana department of natural resources. recommended...

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LOUISIANA DEPARTMENT OFNATURAL RESOURCES

RECOMMENDED MRAA PRESSURE MONITOR WELLS

BAYOU CORNE/NAPOLEONVILLE SALT DOMEEMERGENCY RESPONSE

FIGURENUMBER

1Shaw Environmental & Infrastructure, Inc.

A CB&I Company4171 ESSEN LANE

BATON ROUGE, LOUISIANA 70809www.CBI.com

Gas Area Legend

Known Gas AreaInitial Gas Thickness (ft)

0.0 - 0.5

0.6 - 1.0

1.1 - 2.0

2.1 - 3.0

3.1 - 4.0

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5.1 - 6.0

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May 10, 2013:


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RJC 5/2/13

1

NOTE: This material was developed to provide a quantitative framework for assessing methane gas behavior associated with the Bayou Corne sinkhole. In current form, the material is not based on any site-specific data.

Model Analysis of Methane Gas Behavior at Bayou Corne

Quantify Methane Quantity in the Gas Cap

One of the first quantities that must be determined is the amount of methane gas that is present in the gas cap under different environmental conditions including gas pressure, aquifer water pressure, and soil characteristics. This is difficult because methane is compressible and forms a non-wetting fluid. The approach developed calculates the “standard specific methane volume”, which corresponds to the volume of methane contained in the gas cap under conditions of standard atmospheric pressure, per unit plan area of the aquifer. The standard specific methane volume, designated Dm, has units of feet or meters (cubic feet per square foot plan area, or cubic meters per square meter plan area). For the purpose of this calculation, the soil is characterized by van Genuchten capillary pressure model parameters (α, N, M and Swr).

Figure 1: Pressure distribution for water (Pw) and methane (Pm) at the gas cap located adjacent to the top of aquifer

Figure 1 is presented as a basis for discussion. Pressure head (gage) is shown for water and methane as a function of elevation. The elevation ZP corresponds to the potentiometric surface (water pressure equals atmospheric pressure) for the confined aquifer (MRAA), and the water pressure head is zero at this elevation. Water pressure

Pressure head

Elevation

Pw/gw

hc

Pm/gw

ZP

ZAq

Zmw

Top of Aquifer

Clay

RJC 5/2/13

2

increases hydrostatically with depth, so that the slope of the pressure head-elevation line is equal to -1. The methane is assumed to have constant pressure Pm. It does not show a hydrostatic increase with depth because the methane density is so small (compared to liquid water). At elevation Zmw the methane and water pressures are the same. Pressures are normalized by the specific weight of water γw (pressure à head). The elevation ZAq corresponds to the top of the aquifer, with clay aquitard material located above. The methane gas cap is located between elevations Zmw and ZAq, where the methane-water capillary pressure is positive and does not exceed the entry pressure for methane into the overlying clay. Capillary pressure is the pressure difference between the nonwetting fluid (methane) and the wetting fluid (water). Expressed in terms of head, hc = Pm/γw – Pw/γw. The capillary pressure head hc is shown on Fig. 1 within the gas cap region. hc increases from zero at Zmw to a maximum value at the top of the aquifer (base of the clay), ZAq.

The pressure distribution and capillary pressure distribution are independent of soil material properties. However, soil material properties are essential for establishing the relationship between the capillary pressure distribution and the amount of the different fluids (methane and water) present within the soil pore space. The soil characteristic curve expresses the fundamental relationship between fluid saturation (fraction of pore space occupied) and capillary pressure. Figure 2 shows a representative example for sand texture soil, based on the USDA Rosetta database. The capillary pressure head is measured in feet. At zero capillary pressure the soil is 100-percent saturated with water, the wetting fluid. At a capillary pressure head of about 0.5-ft, the water saturation starts to decrease while the air (methane) saturation increases. At capillary pressure head exceeding about 3-ft, the water saturation approaches its irreducible value (residual water saturation) while the air saturation approaches its maximum value.

RJC 5/2/13

3

Figure 2. Representative soil characteristic curve for sand texture soil

The soil characteristic curve shown in Fig. 2 is expressed mathematically using the van Genuchten model as follows:

( )[ ] MNc

wr

wrwe h

SSSS

−+=

−−

= α11

(1)

Model parameter values used in Fig. 2 are α = 1.07 ft-1, N = 3.18, M = 1 – 1/N = 0.68, and Swr = 0.14.

The soil characteristic curve model is combined with the capillary pressure distribution to calculate the distribution of methane in the gas cap. The methane saturation Sm is calculated from

( ) ( )ewrwm SSSS −−=−= 111 (2)

With porosity n, the specific volume of methane-filled pore space is calculated from

( )⎟⎟

⎠

⎞

⎜⎜

⎝

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mw

Aq

mw

Z

ZemwAq

Z

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RJC 5/2/13

4

( )⎟⎟

⎠

⎞

⎜⎜

⎝

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−= ∫

Aq

mw

Z

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Atm

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PPSnD 1 (3)

From Fig. 1, one can see that hc = z – Zmw. Thus, in Eq. (3), one can use

( )[ ]∫ ∫−

−+=

Aq

mw

mwAqZ

Z

ZZMN

e dzzdzS0

1 α

Example One: Assume that ZP – ZAq = 110 ft and that Pm = 50 psig (Pm/γw = 115.4 ft gage). This corresponds to ZAq – Zmw = 5.4 ft, that is, the gas cap has thickness 5.4 ft. With n = 0.375, Pm = 9320 psfa, and Patm = 2120 psfa, Eq. (3) gives

( ) ( ) 64.542.14.52120

932014.01375.0=−×

×−×=MD ft

Thus within a circle of radius 200 feet, the gas cap would contain π * 2002 * 5.64 = 710,000 standard cubic feet of methane. Not all of this would be recoverable; roughly 1/3 would be trapped at residual saturation. The methane gas saturation within the gas cap is shown in Fig. 3, where the elevation datum represents the base of the clay confining bed. The red-dashed curve corresponds to residual methane saturation, where an f-factor = 0.3 is assumed for the sand-texture soil. The volume represented by the region between the saturation and residual curves corresponds to the methane volume that can be removed by venting.

Figure 3. Calculated gas cap saturation distribution based on a gas pressure = 50 psig

RJC 5/2/13

5

Change in Piezometric Surface Elevation

If the piezometric surface elevation changes, then both the methane pressure and thickness of the gas cap will also change. However, since the standard specific methane volume is a measure of the amount of methane present under standard conditions, it will not change. Dm is invariant to changes in ZP. In terms of absolute pressure,

Pm = PAtm + γw * (ZP – Zmw) = γw * (PAtm/γw + ZP – Zmw) (4)

Thus, one may write Eq. (3) as follows:

( ) wwr

AtmmZ

ZemwAqmwP

w

Atm

SnPDdzSZZZZP Aq

mwγγ −

=⎟⎟

⎠

⎞

⎜⎜

⎝

⎛−−⎟⎟

⎠

⎞⎜⎜⎝

⎛−+ ∫ 1

(5)

The only unknown in Eq. (5) is the elevation of the base of the gas cap, Zmw, and this can be found using various means.

Example Two. The elevation of the piezometric surface decreases by 10 feet over Example One. What impact does this have on the gas cap thickness and pressure? For these conditions, the right side of Eq. (5) equals 590. Taking the elevation of the top of the aquifer as the datum (ZAq = 0, ZP = 100 ft), one can find that Zmw = -5.65 ft. Thus the gas cap has thickness 5.66 feet and the gage methane pressure is (100 + 5.65) * 62.4 = 6590 psfg = 45.8 psig. There is a very slight increase in gas cap thickness while the gas cap (gage) pressure decreases by a small amount.

Methane Discharge to a Venting Well

The steady radial mass flux, fr, to a venting well over a gas cap thickness ba is given by

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drdPkkrb

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m

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m

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πµ

ππρρ =⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎠

⎞⎜⎝

⎛===

The discharge from the venting well to the atmosphere occurs under atmospheric pressure (standard conditions – ρm,Atm and PAtm) so the radial mass flux fr = ρm,Atm QAtm. Integrating this equation from the radius of the well rw where Pm = Pwell to the radius of influence R where Pm = Pmo gives

( ) ( )22, ln wellmo

m

rMawAtmAtmm PP

RTkkbrRQ −=µ

πρ

RJC 5/2/13

6

This may be written for the volumetric discharge of methane under atmospheric conditions

( )( )w

wellmo

mAtm

rmaAtm rR

PPPkkbQ

ln

22 −=

µπ

It is convenient to write this in terms of the aquifer hydraulic conductivity to water using Kw = ρw g k/µw as follows:

( )( )w

wellmo

mwwAtm

rmwaAtm rR

PPP

kKbQln

22 −=

µγπ (6)

In Eq. (6), µmw is the methane-water viscosity ratio (dimensionless), with magnitude approximately 0.0135.

Example Three: For the conditions of Example One with Kw = 5 ft/d, methane relative permeability = 0.66 and rw = 0.5 ft, what is the venting well discharge? From Eq. (6) the maximum discharge would occur with Pwell = PAtm. However, this corresponds to a pressure head loss of 115.4 ft across the flowing radius, and water upconing cuts the potential discharge. A well-choke is necessary to maintain sufficient pressure at the well and prevent detrimental water buildup. An allowable head loss of 3-ft is reasonable, resulting in a 3-ft water buildup. Thus the well pressure is Pwell = 9130 psfa. Equation (6) gives

( )( )

300,185.0200ln

913093200135.04.62212066.054.5 22

=−

××

×××=

πAtmQ ft3/d

Transient Analysis of Methane Venting

Within the area of capture Ac, the continuity equation for methane volume (under standard conditions) is

Atmmw

mw

mc

mc

m QdtdZ

dZdDA

dtdDA

dtdV

−=== (7)

To be useful, all variables must be expressed in terms of Zmw. From Eq. (5) one finds

( ) ( )⎥⎥⎦

⎤

⎢⎢⎣

⎡−⎟⎟

⎠

⎞⎜⎜⎝

⎛−++

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛−−⎟⎟

⎠

⎞⎜⎜⎝

⎛ −−= −

−

∫ mwAq

mwAq

ZZemwPw

AtmZZ

emwAqwAtm

wr

mw

m SZZPdzSZZP

SndZdD 11

0 γγ (8)

Equation (6) can be written

RJC 5/2/13

7

( )( )

( )( ) ( )( )( )w

wwellmwPwAtm

mwwMo

rmwmwAqMo rR

PZZPP

kKZZQ

ln

22 γγµγ

π −−+−= (9)

The resulting continuity equation gives

( ) ( )( ) ( )( ) ( )( )

( )( )( ) ⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−−++−−

−−+−⎟⎟⎠

⎞⎜⎜⎝

⎛

−=

∫ eTmwPwAtmemwAq

wwellmwPwAtmmwAqrm

wmwwrc

wmw

SZZPdzSZZPZZPZZk

rRSnAK

dtdZ

1ln1

22

γ

γγ

µπ (10)

Once assumptions are made concerning the time-variation of Pwell, Eq. (10) is integrated to find Zmw(t). A simple spreadsheet model is used. The gas cap pressure, thickness, venting rate, and cumulative venting volume can be calculated accordingly.

Example Four: Calculate the recovery rate and volume for a methane gas cap with initial pressure = 50 psig (based on Example One). It is assumed that a 3-ft pressure head loss is maintained across the radius of influence R so that both Pm and Pwell decrease at the same rate as methane is vented. During venting, the gas cap thickness and relative permeability decrease. The recovery volume and rate are shown in the Fig. 4. It takes more than 5 months to achieve venting of the gas cap.

Figure 4. Venting rate and cumulative volume

Groundwater Pumping to Control Water Buildup

Example Four shows control of water table buildup by limiting the pressure head loss across the radius of influence to 3-ft. Larger venting rates can be achieved through use of dual pump systems, with an individual intake to control the water table elevation. While not directly applicable, the API LNAPL Distribution and Recovery Model (LDRM) was used to evaluate this situation. The same soil parameters are assumed for a vacuum-enhanced recovery system with the vacuum screen section = 5.4 ft and vacuum pressure = 0.2 atm, corresponding to an increased head loss of 7-ft. A groundwater pumping rate of 3 gpm over a screened interval of 15 ft is used to control

RJC 5/2/13

8

water table buildup. Figure 5 shows negligible impact on the net water table elevation due to combined venting and groundwater production.

Figure 5. Water-table buildup associated with venting with 7-ft head loss plus groundwater pumping at 3 gpm

Example Five: Repeat Example Four with a venting well pressure corresponding to 0.2 atm (7-ft = 437 psf) below gas cap pressure. Figure 6 shows that venting rates are much larger and the required venting time exceeds 2 months.

Figure 6. Venting cumulative recovery volume and rate for well pressure combined venting and groundwater pumping system

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Innovations inWater Monitoring

Level TROLL® 700 InstrumentOptimized for aquifer characterization•Gauged (vented) and absolute (non-vented) instruments•Linear, fast linear, linear average, event, step linear, and true •logarithmic logging modesRugged titanium probe and sensor (OD: 1.83 cm; 0.72 in)•

Level TROLL® 500 InstrumentIdeal for groundwater and surface-water monitoring •Gauged and absolute instruments•Linear, fast linear, and event logging modes•Durable titanium probe and sensor (OD: 1.83 cm; 0.72 in)•

Level TROLL® 300 InstrumentSuitable for fresh water and industrial monitoring•Absolute instrument•Linear, fast linear, and event logging modes•Stainless steel probe and sensor (OD: 2.08 cm; 0.82 in)•

Powerful, Accurate, Reliable PerformanceSuperior accuracy• —For guaranteed accuracy under all operating conditions, instruments undergo extensive calibration procedures for pressure and temperature. Each instrument includes a serialized calibration report.Telemetry and SCADA integration• —Access data when you need it. No adapters or confusing proprietary protocols are required. Outputs include standard Modbus/RS485, SDI-12, and 4-20 mA.Low power consumption• —Batteries have a typical life of 10 years or 2 million readings. 8-36 VDC input is compatible with external batteries and solar power.Intuitive interface• —Win-Situ® 5 and Win-Situ® Mobile Software simplify data collection and management. Software features setup wizards, fast data download rates, multiple water level reference options, and more.

Level TROLL® Instruments

Applications

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Aquifer characterization•Coastal deployments—tide/harbor levels, storm •surge systems, and wetlands researchConstruction and mine dewatering•River, lake, and reservoir monitoring•Stormwater management•

Level TROLL® 300, 500 & 700 Instruments

General Level TROLL 300 Level TROLL 500 Level TROLL 700 BaroTROLL

Temperature ranges1 Operational: -20-80° C (-4-176° F)Storage: -40-80° C (-40-176° F)Calibrated: -5-50° C (23-122° F)

Operational: -20-80° C (-4-176° F)Storage: -40-80° C (-40-176° F)Calibrated: -5-50° C (23-122° F)



Diameter 2.08 cm (0.82 in) 1.83 cm (0.72 in) 1.83 cm (0.72 in) 1.83 cm (0.72 in)

Length 22.9 cm (9.0 in) 21.6 cm (8.5 in) 21.6 cm (8.5 in) 21.6 cm (8.5 in)

Weight 245 g (0.54 lb) 197 g (0.43 lb) 197 g (0.43 lb) 197 g (0.43 lb)

Materials Stainless steel body; Delrin® nose cone Titanium body; Delrin nose cone Titanium body; Delrin nose cone Titanium body; Delrin nose cone

Output options Modbus/RS485, SDI-12, 4-20 mA Modbus/RS485, SDI-12, 4-20 mA Modbus/RS485, SDI-12, 4-20 mA Modbus/RS485, SDI-12, 4-20 mA

Battery type & life2 3.6V lithium; 10 years or 2M readings 3.6V lithium; 10 years or 2M readings 3.6V lithium; 10 years or 2M readings 3.6V lithium; 10 years or 2M readings

External power 8-36 VDC 8-36 VDC 8-36 VDC 8-36 VDC

MemoryData records3

Data logs

1.0 MB65,0002

2.0 MB130,00050

4.0 MB260,00050

1.0 MB65,0002

Log types Linear, Fast Linear, and Event Linear, Fast Linear, and Event Linear, Fast Linear, Linear Average, Event, Step Linear, True Logarithmic

Linear

Fastest logging rate & Modbus rate

2 per second 2 per second 4 per second 1 per minute

Fastest SDI-12 & 4-20 mA output rate

1 per second 1 per second 1 per second 1 per second

Real-time clock Accurate to 1 second/24-hr period Accurate to 1 second/24-hr period Accurate to 1 second/24-hr period Accurate to 1 second/24-hr period

Sensor Type/Material Piezoresistive; stainless steel Piezoresistive; titanium Piezoresistive; titanium Piezoresistive; titanium

Range Absolute (non-vented)30 psia: 10.9 m (35.8 ft)100 psia: 60.1 m (197.3 ft)300 psia: 200.7 m (658.7 ft)

Absolute (non-vented)30 psia: 10.9 m (35.8 ft)100 psia: 60.1 m (197.3 ft)300 psia: 200.7 m (658.7 ft)500 psia: 341.3 m (1120 ft)

Gauged (vented)5 psig: 3.5 m (11.5 ft)15 psig: 11 m (35 ft)30 psig: 21 m (69 ft)100 psig: 70 m (231 ft)300 psig: 210 m (692 ft)500 psig: 351 m (1153 ft)

Absolute (non-vented)30 psia: 10.9 m (35.8 ft)100 psia: 60.1 m (197.3 ft)300 psia: 200.7 m (658.7 ft)500 psia: 341.3 m (1120 ft)1000 psia: 703 m (2306.4 ft)Gauged (vented)5 psig: 3.5 m (11.5 ft)15 psig: 11 m (35 ft)30 psig: 21 m (69 ft)100 psig: 70 m (231 ft)300 psig: 210 m (692 ft)500 psig: 351 m (1153 ft)

0 to 16.5 psi; 0 to 1.14 bar

Burst pressure Max. 2x range; burst > 3x range Max. 2x range; burst > 3x range Max. 2x range; burst > 3x range Vaccum/over-pressure above 16.5 psi damages sensor

Accuracy @ 15° C4 ±0.1% full scale (FS) ±0.05% FS ±0.05% FS ±0.1% FS

Accuracy (FS)5 ±0.2% FS ±0.1% FS ±0.1% FS ±0.2% FS

Resolution ±0.01% FS or better ±0.005% FS or better ±0.005% FS or better ±0.005% FS or better

Units of measure Pressure: psi, kPa, bar, mbar, mmHg, inHg, cmH2O, inH2OLevel: in, ft, mm, cm, m

Pressure: psi, kPa, bar, mbar, mmHg, inHg, cmH2O, inH2OLevel: in, ft, mm, cm, m

Pressure: psi, kPa, bar, mbar, mmHg, inHg, cmH2O, inH2OLevel: in, ft, mm, cm, m

Pressure: psi, kPa, bar, mbar, mmHg, inHg, cmH2O, inH2O

Temperature Sensor

Accuracy & resolution ±0.1° C; 0.01° C or better ±0.1° C; 0.01° C or better ±0.1° C; 0.01° C or better ±0.1° C; 0.01° C or better

Units of measure Celsius or Fahrenheit Celsius or Fahrenheit Celsius or Fahrenheit Celsius or Fahrenheit

Warranty 1 year 2 years 2 years 2 years

Up to 5-year extended warranties are available for all instruments—call for details

BaroTROLL® InstrumentThe titanium BaroTROLL measures and logs barometric pressure and temperature. Use the BaroTROLL in conjunction with Level TROLL Instruments.

Win-Situ® Baro Merge™ Software simplifies post-correction of water level data. Barometric readings are automatically subtracted from data collected by an absolute Level TROLL to compensate for changes in pressure due to barometric fluctuations.

24/7 SupportIn-Situ technical support specialists assist with instrument setup, application support, and troubleshooting. Call for free technical support.

1 Temperature range for non-freezing liquids 2 Typical battery life when used within the factory-calibrated temperature range.3 1 data record = date/time plus 2 parameters logged (no wrapping) from device within the factory-calibrated temperature range4 Across factory-calibrated pressure range5 Across factory-calibrated pressure and temperature rangesSpecifications are subject to change without notice. Delrin is a registered trademark of E.I. du Pont de Nemours and Company.

Call to purchase or rent—www.in-situ.com221 East Lincoln Avenue, Fort Collins, Colorado, U.S.A. 805241-800-446-7488 (toll-free in U.S.A. and Canada)1-970-498-1500 (U.S.A. and international)Copyright © 2012 In-Situ Inc. All rights reserved. Mar. 2012 (T3; web)

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In-Situ High Pressure Transducer Cable Pass-Through Fitting

rrd-gas-09 mraa gas pressure monito ring · louisiana department of natural resources. recommended...

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