GAS FIELD ENGINEERING

Gas Reserves Estimation

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CONTENTS

8.1 Introduction

8.2 Reserves and Reservoir Performance Predictions

8.3 Volumetric Estimates

8.4 Material Balance Estimates

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Lesson Learning Outcome

At the end of the session, students should be able to:

• Calculate Gas Reserves by Volumetric method• Calculate Gas Reserves by Material Balance method

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Introduction• Reserve Estimation Methods: more than one available.

• Different methods applicable at different stages of development.

• Data requirement different for each method, with some common

• Predominant methods:

• 1.Volumetric method

– Early stage of reservoir development

– Geology, Geophysics, Rock and fluid properties

– Recovery Factor(RF) assigned arbitrarily

– No time dependency, No Production data

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Introduction

• 2.Material Balance Method

– Later stage of development(after 20% of initial oil/gas is produced, or 10% of initial reservoir pressure has declined)

– Geological data, Rock and Fluid properties, Production data

– RF is calculated

– Time dependant

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Introduction

• 3.Decline Curve Analysis

– Later stage of development, when production rate undergoes natural decline

– Mostly production data

– RF is calculated

– Time dependant

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Introduction

• 4.Reservoir Simulation

– Can be applied at any stage but more useful and reliable for matured reservoirs

– Geological data, Rock and Fluid properties, Production data

– More useful as reservoir management tool

• Uncertainties associated with each method

• More than one method should be used when applicable

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Introduction• Natural gas reservoirs are reservoirs in which the contained

hydrocarbon fluids exist wholly as a vapor phase at pressure values equal to or less than the initial value.

• Unlike saturated crude oils and condensates, natural gases do not undergo phase changes upon reduction in reservoir pressure.

• Performance predictions are therefore relatively simple.

• Natural gas is commonly termed wet (or raw) gas.

• Cumulative gas produced (Gp) means separator gas plus vapor equivalent of the natural gas liquid (NGL) removed in the separator.

• Gas formation volume factor (Bg) and gas deviation factor (Z) refer to the properties of a sample of separator gas and liquid.

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Introduction

• Natural gas reserves are classified according to nature of their occurrence.

• Non-associated gas is free gas not in contact with crude oil in the reservoir.

• Associated gas is free gas in contact with crude oil in the reservoir.

• Dissolved gas is gas in solution with crude oil in the reservoir.

• This chapter address methods of estimating non-associated gas reserves.

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Gas in Place by Volumetric Equation

• To make reasonable recovery predictions, estimates of the initial gas in place in each reservoir must be made.

• Volumetric equation is a useful tool for calculating the gas in place at any time.

• Reservoir rock volume is usually obtained by planimetering isopacheous maps of productive reservoir rock.

• Gas initially in place (GIIP) is the product of three factors: reservoir pore volume, initial gas saturation, gas formation volume factor that converts reservoir volumes to volumes at standard, or base, conditions 60oF, and 14.7 psia.

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Gas in Place by Volumetric Equation

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(11.1)

Gas in Place by Volumetric EquationIf Bgi is in cu ft/scf,

Also:

Standard cubic feet of gas in place is given by:

•Volumetric equation is particularly applicable when a field is comparatively new, before gas have been produced to cause an appreciable drop in reservoir pressure. •If good data are available, volumetric will probably be reliable.

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(11.2)

(11.3)

(11.4)

Gas in Place by Volumetric Equation

• Omission of Z factor may affect reserve calculations up to 30% errors

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From gas laws,bb

b

ZpT

TZpBg

At standard conditions of 14.7 psia and 60°F,

p

TZ

p

TZBg 0283.0

]60460[

))(7.14(

(11.5)

(11.6)

Material Balance Equation

Assumptions

1. A reservoir may be treated as a constant-volume tank.2.Pressure equilibrium exists through out the reservoir.3.Laboratory PVT data apply to the reservoir gas at the average pressures used.4.Reliable production and injection data, and reservoir pressure measurements are available.5.Change in volume of the interstitial water with pressure, change in porosity with pressure, and the evolution of gas dissolved in the interstitial water with decrease in pressure are negligible.

The material balance is an expression of the law of conservation of mass.

.

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Derivation

Conservation of mass may be applied to a gas reservoir to yield mass and mole balances:

mp = mi – m (11.7)

andnp = ni - m

(11.8)where:

mp, np = cumulative gas produced in mass and mole units mi, ni = initial gas in place at initial pressure pim, n = gas remaining in reservoir at some subsequent pressure

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DerivationUsing constant volume tank concept,letVi = original hydrocarbon reservoir volume at the initial pressure pi

Assume -Wp stock tank barrels of water have been produced -We stock tank barrels of water have been encroached into the reservoir - V be the remaining gas volume in the reservoir

Vi = V + We – WpBw (11.9)

V = Vi - We + WpBw (11.10)

Vi , V, We and Wp Bw are in reservoir barrels; Bw is the water formation volume factor in reservoir barrels per stock-tank barrel.

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Derivation

Gp = cumulative gas produced from pi to p, scfR =gas constant, 10.732 cu ft-psi/lb mole-oR.

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Derivation• For reservoirs with no water influx and no water production:

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Application

• Material balance equation applied to estimate initial gas in place, determine existence and estimate effectiveness of any natural water drive, assist in predicting performance and reserves.

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Reserves and Reservoir Performance Predictions

• Energy required for gas production is usually derived either from gas expansion or a combination of gas expansion and water influx.

• Volumetric estimation, curve are methods which may be used to estimate gas reserves in place.

• Abandonment pressure is determined by economic conditions

- future market value of gas- cost of operating and maintaining wells- cost of compressing - transporting gas to consumers.

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Volumetric Estimates• Volumetric equation is useful in estimating gas in place at any

stage of depletion.

• During the development period, it is convenient to calculate gas in place per acre-foot of bulk reservoir rock.

• Multiplication by estimate of bulk reservoir volume then gives gas in place for the lease, when reservoir volume is defined and performance data are available, volumetric calculations provide valuable checks on estimates obtained from material balance methods:

GIIP (scf/acre-ft)

Bulk reservoir volume = (Ah) acre-ft

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Volumetric Estimates

• Some gas pipeline companies use an abandonment pressure of 100 psi/1000ft of depth.

• If the abandonment pressure is known, recovery factor can be calculated.

Eg

Eg = recovery factor, fraction of initial gas in place to be recovered

For water drive reservoir:

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Example 11.2

• A proposed gas well is being evaluated. Well spacing is 640 acres and it appears that the entire 640 acres attributed to this well will be productive. Geological estimates indicate 30 ft of net effective pay, 15% porosity, and 30% interstitial water saturation. The initial pressure is 3000 psia and reservoir temperature is 150oF. The abandonment pressure is estimated to be 500 psia. The gas gravity is expected to be 0.60. Base temperature and pressure are 60oF, and 14.65 psia respectively. An estimate of the gas reserve is required.

Solution

The first step calculation of Bgi which requires pseudo-critical T and P, pseudo-reduced T.

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Referring to Fig. Zi is found to be 0.83. Using Eq. 11.5

Second step is to calculate the recovery factor, Eg. Abandonment pressure being 500 psia, pseudo-reduced pressure = 500/668 = 0.75. Using this value together with the pseudo-reduced temperature. Za is found to be 0.94. Hence from Eq. 11.19:

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Third step is use Eq. 11.18 to calculate reserve in scf/acre-ft

Final step is to multiply the above figure by the net acre-feet; hence estimated reserve:

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• In some cases the porosity, connate water, and/or effective reservoir volumes are not known with reasonable precision, and volumetric method may be used to calculate the initial gas in place.

• However, this method applies only to reservoir as a whole.

• Accurate pressure-production data are essential for reliable material balance calculations.

• Most likely source of error is estimating average reservoir pressure, especially during the early history period.

Material Balance Estimates

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Material Balance Estimates

• If there is no water encroachment, only information required is production data, pressure data, gas specific gravity for obtaining Z factors, and reservoir temperature.

• However, early in the producing life of a reservoir the denominator of right-hand side of material balance equation is very small, numerator is relatively large.

• A small change in the denominator will result in a large discrepancy in the calculated value of initial gas in place.

• Therefore, material balance equation should not be relied upon early in the producing life of the reservoir.

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Example 11.3

(a) Calculate the initial gas in place in a closed gas reservoir if, after producing 500 MMscf, the reservoir pressure had declined to 2900 psia from an initial pressure of 3000 psia. Reservoir temperature is 175oF., and the gas gravity is 0.60.

(b) If the reservoir pressure measurement were incorrect and should have been 2800 psia instead of 2900 psia, what would have been the true value of initial gas in place and what is the percentage error involved?

Solution

(a) Using a gas gravity of 0.60 and referring to the Z-factor correlation charts (Figs. 2.4 and 2.5), Z at 3000 psia is computed to be 0.88 and Z at 2900 psia is determined to be 0.87. Next step is to calculate the two values of Bg;

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Eq. 11.23 is in bbl/scf, Eq. 11.6 is in cu ft/ scf;

Equation 11.22 is next used to compute initial gas in place:

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(b) If the pressure measurements were incorrect and the true average pressure is 2800 psia, then the material balance equation will be solved using the true pressure. Z-factor at 2800 psia is determined to be 0.87:

Next, initial gas in place is calculated by the material balance equation:

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Q & A

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Thank You

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