petroleum development geology 070_reserves estimate

8/13/2019 Petroleum Development Geology 070_reserves Estimate

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The most important role of a DG is to:

estimate the oil and gas reserves that may

be discovered in a particular venture.

keep track of the reserves in all pastventures.


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THE 4 BASIC RESERVES ESTIMATION ETHODS

1. Educated Guess and/or Comparisonwith nearby production.

2. Static Reserves EstimatesVolumetric Calculations

3. Dynamic Reserves Estimates

Decline Curve AnalysisMaterial balance calculationsReservoir Simulation


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THE EDUCATED GUESS and/orCOMPARISON OF NEARBY PRODUCTION

Consider a region where production is from ahighly fractured tight formation or whereporoperm heterogeneity is unpredictable.

Volumetric calculations are largelymeaningless.

A way to estimate potential production froma well is to consider those nearby.

Generally, such a wildcat well will notperform better than the nearest wells: best to

estimate cautiously


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VOLUMETRICS Most accurate and widely used methods of reserves

estimation.

Carried out by geologists as they are based ongeological structure and isopach maps.

Rock volumes are established that are assumed tocontain hydrocarbons (e.g. seismic bright spot).

Can be a simple volume calculation or a complex netgas or net oil isopach approach, determined bystructure contours modified by fluid contacts and net

isopachs (net reservoir thickness map). Accuracy of volumetrics depends on data for porosity,

saturation, net thickness, areal extent, formationvolume factor, integrity of those data within a reservoir.


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Volumetric Method

RR = 7758 x A.t x (1 Sw) x FVF x RF

Amount of oil in reservoir Amount of recoverable oil

RR = Recoverable Reserves7758 = conversion from acreft to barrels (if vol. in

m3. this conversion number is eliminated) A = area of porous rock, acre

t = thickness in feet = porosity,%

(1-Sw) = water saturation of reservoir

FVF = Formation Volume Factor (1/Bo & 1/Bg)Bo/Bg reservoir volume / surface volume (vr / vs )RF = Recovery Factor


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HOW TO DETERMINE

OCK VOLUME Most rock volumes established through use of

net gas and net oil isopachs (net pay map).

Constructed from superimposing of net isopach

map and structure contour maps then cut(reduced) it with well defined OWC and/or GOC.

Calculate the volume of net pay map byplanimeter (or digitizer table) and/or grid squarecounting


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HORIZON MAP(Superimposed Structure and Net Isopach Maps)

0 m

5 m

1 0 m

1 5 m

1 0 m

5 m

0 m


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NET PAY MAP


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Rock Volume Calculations

2 methods :1. PYRAMID

2. TRAPEZOIDS

A : area, m2 or acreh : isopach/contour interval, m or ftn : contour number (0 n)t : avg. thickness above the top of max. thickness


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FVF ormation Volume Factor


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RFRecovery Factor

Usually RF determination is carried out by

Reservoir Engineer. Mainly based on the reservoir drive, rock

properties and fluid properties. For oil with effective water drive the

primary recoveries are in 25 40 % range(max. 75%).

For gas with gravity drainage, water driveand depletion drive can provide RF > 80%.


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Average Oil RecoveryFactors,

% of OOIPDrive Mechanism

Range Average

Solution-gas drive 5 - 30 15Gas-cap drive 15 - 50 30Water drive 30 - 60 40

Gravity-drainagedrive 16 - 85 50

Average Gas Recovery

Factors,% of OGIPDrive Mechanism

Range AverageVolumetric reservoir (Gas expansion drive)

70 - 90 80

Water drive 35 - 65 50

Average Recovery Factors


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SOURCESOF

ATA


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Decline Curve Analysis (Reservoir Engineers jobs)

After wells have been producing for a while:

The rate of production is graphed Generally 6 months 1 year after start of

production

Good reserves estimates can be derived.Often compared with volumetric technique

results.

Can be done by well, by a group of well, by

block, by reservoir, by field


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Decline Analysis Results

Determine remaining recoverable reserves

under natural depletion rate.

To forecast production under existingconditions

Limitation: The degree of the accuracy is depend on the

reliability of the production data.


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DECLINECURVE

EQUATIONS


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Production Plots1. A plot of log(q) vs t is

Linear if decline is exponentialConcave upward if decline is hyperbolic (n>0) or harmonic

2. A plot of q vs Np isLinear if decline is exponential

Concave upward if decline is hyperbolic(n>0) or harmonic

3. A plot of log(q) vs Np isLinear if decline is harmonicConcave downward if decline is hyperbolic (n1 .

4. A plot of 1/q vs t isLinear if decline is harmonicConcave downward if decline is hyperbolic (n1.


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Example Exponential declinexample Exponential decline

Example. Exponential decline

q = 6049.1e -0.0524 t

100

1000

10000

0 10 20 30 40 50 60

time (quarter year)

R a

t e ,

s t b / d

.

Slope=-D 1/quarter year


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Example Exponential declinexample Exponential decline

Example. Rate decline with production

q = -0.4301Np + 5768.7

0

1000

2000

3000

4000

5000

6000

7000

0 2000 4000 6000 8000 10000 12000 14000

Cum. prod, MSTB

q s t

b / d

q abondonment

Reserves


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Example Harmonic declinexample Harmonic decline

0

2000

4000

6000

8000

10000

12000

0 2 4 6 8 10 12 14 16

Time (years)

R a t e

( s t b / d )

0

5

10

15

20

25

30

35

40

C u m .

P r o

d u c

t i o n

( M M s t

b )


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Example Hyperbolic declinexample Hyperbolic declineHyperbolic Decline curve

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

0 50 100 150 200 250 300 350

days

q S T B /

MATERIAL BALANCEMATERIAL BALANCE


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General Concept of Material Balance.

From: Petroleum Reservoir Engineering Amyx, Bass, and Whiting (1960).

a. Initial reservoir condi tions. b. Conditions after producing Np STB of oil,and G

pSCF of gas, and W

pSTB of water.

Material Balance: Key IssuesMust have accurate production measurements (oil, water, gas).

Estimates of average reservoir pressure (from pressure tests).Suites of PVT data (oil, gas, water).Reservoir properties: saturations, formation compressibi lity, etc .

MATERIAL BALANCEMATERIAL BALANCEof a Petroleum Reservoir

(Mostly carried out by Reservoir Engineer)

( )RESERVOIR SIMULATION (RS)


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RESERVOIR SIMULATION (RS)RESERVOIR SIMULATION (RS)

Reservoir Modelling: primarily the reservoirengineers job. RS applies the concepts and techniques of math-

ematical modeling to the analysis of the behavior ofpetroleum reservoir systems. In a narrower sense refers only to the hydro-

dinamics of flow within reservoir. In a larger sense refer to the total petroleumsystem which includes the reservoir, the surfacefacilities, and any interrelated significant activity, andeconomic

The basic flow model the partial differentialequations using finite difference methods whichgovern the unsteady state flow of all fluid phases inthe reservoir medium.


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RESERVOIRRESERVOIRSIMULATORSIMULATOR

Rock data

Fluid data

Production data

Pressure data

Flow rate data

Mechanical &operational data

Miscellaneousdata

INPUTINPUT PROCESSEDPROCESSEDin the BLACK BOXin the BLACK BOX

OUTPUTOUTPUT

Reserves

Reservoir model

Plan of reservoirdepletion

Production

forecastOptimumproduction

RESERVOIR SIMULATIONESERVOIR SIMULATION


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Reservoir link with surface facility

The Role of DGThe Role of DG


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Prepare the array input data (maps) of individual flowunit : structure (top & bottom), isopach (net & gross),porosity, permeability, rock compressibility etc.

Advising to simulation engineer in the designing ofthe grid model and layer divisions.

Trace and established in the model grid theexistence of faults, horizontal and vertical barrierspermeability.

During the history matching of production, pressureetc., DG advises to simulation engineer in allowablegeological modification such as thickness, structure,rock properties and volumetric reserves.

The Role of DGThe Role of DG

in Reservoir Simulationin Reservoir Simulation

RESERVES CLASIFICATIONS


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RESERVES CLASIFICATIONS

PROVED : Estimated to reasonable certainty. Often based onwell logs but normally requires actual production orformation tests.

Proved developed reserves Reserves that are expected to be recovered from existing wells

Proved undeveloped reserves To be recovered by new drilling, deepening wells to a new

reservoir or where additional finance is required to produce PROBABLE RESERVES

Less certain than proved but can be assessed to

some degree of certainty. May include loggingestimates, improved recovery technique estimates POSSIBLE RESERVES

Not as certain as probable reserves and can only beestimated to a low degree of confidence.

UNPROVED RESERVES Resources

RESERVES CLASSIFICATIONS


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Decision Making: protocol Despite these defined terms, there is stil l some latitude in their

application. In general, we use this:

Proved Reserves = minimum case economics. Financialinvestment is based on proved reserves.

Proved + Probable Reserves = most likely caseeconomics. Internal company decisions usually based on this.

Proved +Probable + Possible Reserves = maximumcase economics. This is the best that could reasonably happenfor a venture. Companies try to sell ventures based on this.


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MM DARISSALAM, YOGYAKARTA JUN. 08

petroleum development geology 070_reserves estimate

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