Caracterizacion de Fracturas Asociadas a Las Perdidas de Fluidos

7/27/2019 Caracterizacion de Fracturas Asociadas a Las Perdidas de Fluidos

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Copyright 2000, Society of Petroleum Engineers Inc.

This paper was prepared for presentation at the 2000 SPE Annual Technical Conference andExhibition held in Dallas, Texas, 14 October 2000.

This paper was selected for presentation by an SPE Program Committee following review ofinformation contained in an abstract submitted by the author(s). Contents of the paper, aspresented, have not been reviewed by the Society of Petroleum Engineers and are subject tocorrection by the author(s). The material, as presented, does not necessarily reflect anyposition of the Society of Petroleum Engineers, its officers, or members. Papers presented atSPE meetings are subject to publication review by Editorial Committees of the Society of

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AbstractImaging log, sonic log, optical microscopy, NMR, MRI, andtomography have greatly enhanced the possibility of measuring fracture geometric characteristics. However, thefracture hydraulic width and permeability can only beevaluated from dynamic data revealing production oradsorption along the wellbore. Different analytical approachesaccounting for mud rheology and their application to severalfield cases where mud losses were recorded withelectromagnetic flowmeters are presented to evaluate thefracture hydraulic width from mud loss measurements.Observation of mud loss evolution in time also revealedwhether a single fracture or an intensely fractured zone hadbeen intercepted by the drilling bit. Results are discussed withrespect to imaging log data, core analyses, and well testinterpretation results. In general, a very good correspondencewas found between mud loss occurrence, fracture detectionfrom imaging log, core data, and productivity tests.

IntroductionFractured reservoirs are difficult to simulate due to thepresence of tectonic discontinuities, intersecting a generallyalmost tight matrix, that strongly influence the fluid flowpattern in the producing formation. Technologies such asimaging log coupled with microfracture analyses of conventional cores with optical microscopy, NMR, MRI, andtomography have greatly enhanced the possibility of measuring the fracture geometric characteristics.

However, the fracture hydraulic width and permeabilitycan only be evaluated from dynamic data revealing productionor adsorption along the wellbore. The use of high resolutionelectromagnetic flowmeters to monitor mud losses during

drilling appears very attractive for fracture detection ancharacterization in that flowmeters provide an accuratcontinuous recording of mud adsorption, thus giving a cleaindication of the formation conductive fractures or morpermeable zones intersected by the wellbore. (1,2) Only at time, production logs can indicate primary fluid entry pointand transient pressure analysis allow estimation of the fractureor fracture network permeability.

Appropriate data collection at early stages is crucial tidentify and characterize faults and fractures affecting threservoir productivity and performance, and integration of aavailable information probably represents the only effectivapproach to achieve a good understanding of fracturereservoirs in order to develop effective managemestrategies. (3,4)

Mud loss identificationThe exposure of open natural fractures by the drilling bproduces a sudden decrease of the outlet mud flow rate.

According to Dyke et al. (2) losses through mpermeability or into natural fractures can be distinguished bythe characteristics of the loss. Losses through pores staslowly and gradually increase as drilling proceeds wherealosses into natural fractures show a rapid initial increase iloss rate followed by a gradual decline in time. Such losprogressions with time give rise to the typical responses in thmud tank level, as shown in Fig. 1.

Fig. 1 - Type of loss zone from pit level: (a) Pore or fracturenetwork, and (b) Single natural fracture (after Dyke et al.modified)

The same loss evolution in time was detected when mulosses were recorded by the flowmeters, thus allowing tdiscriminate between single conductive fractures and porou

SPE 63266

Detection and Characterization of Fractures in Naturally Fractured ReservoirsF.M. Verga, SPE, Politecnico di Torino, C. Carugo, V. Chelini, and R. Maglione, SPE, ENI - Agip Division, and G. DeBacco, SPE, Politecnico di Torino

time M u d

l e v e

l (a)

time M u d

l e v e

l


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2 F.M. VERGA, C. CARUGO, V.CHELINI, R. MAGLIONE, G. DE BACCO SPE 63266

matrix or intensely fractured zones, both inducing very similarmud losses into the formation. Fig. 2 shows typicalelectromagnetic flowmeter responses in the two cases.

Fig. 2 - Type of loss zone from electromagnetic flowmeterrecordings: (a) Fracture network, and (b) Single conductivefracture

Analytical approachesTo the authors' best knowledge according to the literature twoanalytical models have been developed to evaluate the fracture

hydraulic width from mud losses. Both models assume that thefracture can be regarded as a slot of constant width, h frac , andthat the mud propagates radially into the fracture.Furthermore, both models assume the validity of thePoiseuille's law and, therefore, that the fracture permeability,k frac , is proportional to fracture width, h frac , according to

(5):

12h

k 2frac

frac = .(1)

The main features of the models are briefly outlined anddiscussed in the following.

Method developed by Sanfilippo et al.The model developed by Sanfilippo et al. (6) is based on thediffusivity equation applied to the mud flow radiallypropagating into a fracture perperdicularly intersecting thewellbore:

tp

k

c

rp

r1

r

p

frac

mudfracmud2

2

=

+

.(2)

where p is the pressure, r is the radial distance from the wellaxis, mud is the mud Newtonian viscosity, frac is the fractureporosity, c mud is the mud compressibility, and t is time.

The solution to equation (2) assuming a constant terminalpressure boundary condition, namely the Van Everdingen-

Hurst solution(5)

, and substituting equation (1) is then:

0phrc2

)t(V

rc12

thln

rc12

th

cfrac

2wmudfrac

2wmudfracmud

2frac

2wmudfracmud

2frac

=

(3)

where c is a constant equal to 2.01, r w is the wellbore radius,and V(t) is the cumulative volume of mud lost in the fractureat time t.

The matrix is assumed to be totally impermeable antherefore, the fracture porosity is set equal to unity.

Method developed by Lietard et alThe model developed by Lietard et al. (7) is based on Dalaw. The description of the mud flow through a fracture iobtained by solution of the equation describing the locapressure drop due to the laminar flow of a plastic fluid in a sloof width h frac :

frac

y

2frac

p h

3

h

)t,r(v12

drdp += ....

where p is the pressure, y and p are the mud yield valueplastic viscosity, respectively, and v(r, t) is the local muvelocity for radial flow, equal to:

dt)t(dV

rh21

)t,r(vfrac

= ..

evaluated as a function of the distance from the well axis, rthe time, t, and the mud loss volume V(t). The latter is givenby:

]r)t(r[h)t(V 2w2

frac = ..where r(t) is the invasion radius at time t and r w is the weradius.

The solution to equation (4) is provided as a set dimensionless type-curves (Fig. 3) defined in terms of thdimensionless radius R and time T, respectively equal to:

wr)t(r

R = ...(

t3

pr

hT

P2w

2frac

= ....(

where p represents the difference between the bottom hopressure and the formation pressure and it is assumed to bconstant.

Fig. 3 - Type-curves to evaluate the fracture width (after Lietard etal. (7), modified)

time L o s s r a

t e

(a)

time L o s s r a

t e

(b)

3

4

5

6

7

8

9

10

3.5 4 4.5 5 5.5 6 6.5Log (R

2-1)

L o g ( T )

= 0

. 0 0 2

=

0 . 0

0 2

=

0 . 0

0 2

=

0 . 0

0 2

=

0 . 0

1

=

0 . 0

1

=

0 . 0

1

=

0 . 0

1


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SPE 63266 DETECTION AND CHARACTERIZATION OF FRACTURES IN NATURALLY FRACTURED RESERVOIRS 3

Each type-curve describes the mud loss volume as afunction of time and it is characterized by a value of theparameter , defined as:

phr3 y

frac

w

= ....(9)

In the case of Newtonian fluid = 0.Given that the following relations apply:

=

=

2w

frac2

p2w

2frac

r)t(V

h]1R[

t3

pr1

hT

..(10)

the fracture hydraulic width can be determined from thecoordinate shifts applied to obtain a satisfactory superpositionbetween the real data (Fig. 4) and a particular theoreticalcurve. Furthermore, the fracture width can be estimated fromthe value of the parameter characterizing such curve.

Fig. 4 - Example of cumulative mud loss volume versus time

An example of superposition between real data and type-curve is shown in Fig. 5.

Fig. 5 - Type-curve matching with real data to determine thefracture width

DiscussionIn the case of drilling mud the assumption of Newtonian fluidis generally not feasible and, therefore, the rheological modeadopted by Sanfilippo et al. (6) does not appear torepresentative of the real fluid behavior. However, the modeis very flexible and can be applied to calculate the fracturwidth even if the loss evolution with time suggests thpresence of a locally intensely fractured rock rather than single conductive fracture. Obviously, in this latter case thcalculated fracture width must be regarded as an equivalenfracture width which accounts for the presence of a network omicrofracture.

The model presented by Lietard et al. (7) accounts forrheology and it is very useful for a first estimation of thfracture width. However, superposition of the real data to thmost representative theoretical curve poses some difficultiesand a slight shift to obtain the match between the real data anda particular type-curve can induce a significant difference ithe calculated fracture aperture value. Furthermore, whefractures characterized by small spacing values or fracturnetworks are exposed by the drilling bit, a departure of threcorded mud loss data from the type-curves occurs and thuthe fracture width can not be determined.

New analytical method developed to calculatefracture width from mud loss dataA new analytical model has been conceived by Maglione et al(8,9) assuming that the mud radial flow into a fracture of widthfrac can be described by the solution to the diffusiviequation [eq. (2)] under steady state conditions, namely thMuskat equation. The bottom hole drilling overpressurecan then be expressed as:

w

2 / 12w

frac3frac

mudloss

r

rh

)t(V

lnh

Q6)t(p

+

= ....

where Q loss represents the mud loss rate values recorded by tflowmeter during each time step, mud is the mud viscositthe wellbore radius, and V(t) is the cumulative mud volumlost in the fracture at time t.

It is assumed by the authors that the mud can be regardeas a Bingham fluid, thus the shear stress, , can be descby proper values for the yield point, y, and plastic viscop:

fracpy ' += .....(

where frac' is the shear rate in the fracture.Considering that frac' ranges between 10 4 and 10 7

fracture apertures ranging between 100 and 1000 m, thepoint value can be neglected, and the mud viscosity can bapproximated to the plastic viscosity:

pfrac

mud '

= .........(

7

8

9

10

11

12

13

14

0 0.5 1 1.5 2 2.5 3 3.5

Log [V(t)/ r w]

L o g

[ ( p t

) / ( 3

p r w

2 ) ]

Real Data

3

4

5

6

7

8

9

10

3.5 4 4.5 5 5.5 6 6.5 7Log (R

2-1)

L o g

( T )

9.3

10.3

11.3

12.3

13.3

14.3

15.3

16.3

0.2 0.7 1.2 1.7 2.2 2.7 3.2 3.7

Log[V(t)/ r w]

L o g [ p t ) / ( 3

pr w ) ]

= 0,002 = 0,002 = 0,002 = 0,002Real Data


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Fig. 9 shows an example of mud loss interpretation in thecase of a fractured zone, and the fracture hydraulic widthvalues calculated according to the models of Sanfilippo et al.and Maglione et al. are presented. The calculated fracturehydraulic width values are to be regarded as an equivalentvalue.

Fig. 7 - Typical mud loss evolution for a fracture network

Fig. 8 - Fracture hydraulic width values from mud lossinterpretation for a single conductive fracture

Fig. 9 - Fracture hydraulic width values from mud lossinterpretation for fracture zone

Fig. 10 shows a comparison between the fracture hydrauliwidth values obtained by application of the methodeveloped by Sanfilippo et al. (6) and by Maglione et al. (8

the mud losses detected during drilling of the three wells. Thassumption of Newtonian fluid in the model developed bSanfilippo et al. leads to greater fracture width values witrespect to the values calculated with the model developed byMaglione et al. In general, fracture width values calculatewith the model developed by Sanfilippo et al. are twofold thanthe values calculated with the model developed by Maglionet al.

Fig. 10 - Comparison between the fracture hydraulic width valuescalculated with the methods of Sanfilippo et al. and Maglione etal.

The comparison between the fracture hydraulic widvalues obtained by application of the methods developed bLietard et al. (7) and by Maglione et al. (8) for all the conductive fractures detected during drilling of the three wellis shown in Fig. 11. It can be noticed that there exists a vergood correspondence between the fracture width estimatewith the two methods.

Fig. 11 - Comparison between the fracture hydraulic widthcalculated with the methods of Lietard et al. and Maglione et al.

0

200

400

600

800

1000

1200

1400

0 200 400 600 800 1000 1200 1400

Fracture width-model of Maglione et al. ( m)

F r a c

t u r e w

i d t h - m o d e l o f

S a n f i l

i p p o e t a l . (

m

)

0

200

400

600

800

1000

1200

1400

0 200 400 600 800 1000 1200 1400

Fracture width-model of Maglione et al. ( m

F r a c

t u r e w

i d t h - m o d e l o f

L i e t a r d e t a l . (

m

)

0

100

200

300

400

500

600

0 500 1000 1500 2000 2500 3000 3500Time (s)

h f r a c

( m

)

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

0.0045

Ql o s s ( m

3 / s )

Sanfilippo et al.Maglione et al.Lietard et al.Mud Losses

0

0.001

0.002

0.003

0.004

0 500 1000 1500 2000 2500 3000Time (s)

Q l o s s

( m 3 /

s )

0

100

200

300

400

500

600

700

0 500 1000 1500 2000 2500 3000

Time (s)

h f r a c

( m

)

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

Ql o s s ( m

3 / s )

Sanfilippo et al.

Maglione et al.

Mud Losses


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Comparison between electrical and hydraulic fractureapertureThe fracture aperture values obtained from imaging logs are invery good agreement with the values calculated from mud lossdata in the case of relatively large fracture width values, i.e.when the aperture of the fracture is greater than 300-400 m.

Fig. 12, 13 and 14 show a comparison between the fractureaperture values obtained from interpretation of the imaginglogs and analyses of mud losses along three depth intervals of the examined wells where the presence of conductive fracturesor fracture network was detected. The first two columns showthe mud rates and imaging log as a function of depth,respectively. In the third column the calculated fractureelectrical apertures (circles) and hydraulic apertures(diamonds) are reported. The fracture hydraulic width valueswere calculated according to the method developed byMaglione et al.

It can be noticed that, as predicted by Luthi andSouhait (11) , in all cases the hydraulic fracture aperture valuesare greater than the electrical fracture aperture values.

Interpretation of imaging logs also allowed the detection of small open fractures (less than 100 m) that were not evidentfrom mud loss rate data (Fig. 13). Significant mud losses wererecorded only in the presence of fractures with hydraulic widthgreater than approximately 200 m.

Fig. 12 - Example of the good correspondence found between:fracture aperture values calculated from mud loss data analysisand imaging log interpretation for a single conductive fractureintercepted by the well.




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Small fluctuations in mud loss rates could not beconsidered indicative of the presence of open fractures bothbecause small fracture apertures do not produce detectablemud losses and because the electromagnetic flowmetersaccuracy can decrease to approximately 10 -3 m3 /s if transported solids in the outlet mud deposit on the flowmeterelectrods.

Comparison between imaging log and laboratorymeasurements on coresLaboratory measurements on cores with MRI, NMI, andtomography showed a network of microfractures through thecarbonatic rock but also the presence of few importantfractures.

Imaging log interpretation also allowed the detection of rock microfractures, and the aperture values of themicrofractures range between 20 and 60 m (Fig. 15).

Fig. 15 - Distribution of the open fracture apertures measuredfrom log data

The fracture aperture values obtained from imaging logsare in very good agreement with the distribution of thecemented fracture apertures measured on cores (Fig. 16).

Fig. 16 - Distribution of the cemented fracture apertures measuredon cores

Discussion of resultsThe fracture hydraulic width values determined from mud losrecordings and the fracture electrical aperture valucalculated from imaging logs are in very good agreement foall the examined wells, provided that a proper mud rheology iaccounted for. In the analytical method developed to evaluatthe fracture hydraulic width from mud loss data it is assumethat the drilling mud can be considered as a plastic fluid. Thmethod does not account for fracture plugging phenomena.

Mud losses into microfractures are not detectable antherefore, mud rate analysis proved to be an effective methodto estimate fracture width only when the fracture width rather large, i.e., greater than 200 m. Very similar rewere found by Dyke et al. (2), according to which mud loare detectable only when the fracture width is greater tha150-250 m.

Reliability of imaging log calibration and interpretatiowas also suggested by the very good agreement between thmicrofracture aperture values determined from logs and thcemented fracture aperture values measured on cores.

Based on the fracture or fracture network detection by mulosses and imaging logs the MDT tests could be properlpositioned along the wells and intervals for acid jobs could befficiently selected.

After the production was started a PLT was performealong one of the examined wells revealing that about 70% othe producing oil reaches the well through the main fracturdetected along the well. Evidence of this fracture is presentein Fig. 14 and the calculated hydraulic width is about 700

ConclusionsElectrical flowmeters proved to be very effective to monitomud loss rates to detect the presence of fracture or fracturnetwork during drilling.

The hydraulic fracture width values determined from muloss recordings and the electrical fracture aperture valuecalculated from imaging logs are in very good agreemenprovided that a non Newtonian fluid rheology is adopted.

Fracture width values significantly contributing to mulosses range approximately between 200 and 1000 msimilar results are reported by Dyke et al. (2), accordiwhich mud losses are detectable when fracture width valuerange between 200 and 700 m. When the fracture wexceeds about 1000 m a total loss of circulation occurs.

Based on the fracture or fracture network detection by mu

losses and imaging logs the MDT tests can be properlpositioned along the wells.

Nomenclaturea = tool constant for FMS or FMIA = FMS examination area, in 2

b = tool constant for FMS or FMIc = equation constant, equal to 2.01cmud = mud compressibility, Pa

-1

E = excess of conductivity with respect to the localmatrix values, mho/m

Classes of fracture aperture ( m)

F r e q u e n c y

Fracture aperture ( m)

F r e q u e n c y


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h = length of the local fracture trace, inhfrac = hydraulic fracture aperture, mk = conversion factor for electrical conductivity from

m/mho to V/ Ak frac = fracture permeability, m

2

p = pressure, PaQ loss = mud loss rate, m

3s-1

r = radial distance from well axis, mrw = wellbore radius, mR = dimensionless radiusRxo = background resistivity, Ohm mRmf = mud filtrate resistivity, Ohm mt = time, sT = dimensionless timev = local mud velocity, m s -1

V = cumulative mud volume lost in the fracture, m 3

wfrac = local electrical fracture aperture, mm

= dimensionless type-curve parameterfrac' = fracture shear rate, s -1

frac = fracture porositymud = mud viscosity, Pa sp = plastic viscosity, Pa s =the shear stress, Pay = yield point, Pa

AcknowledgementsThe authors wish to thank ENI AGIP Division and thePolitecnico di Torino for giving permission to publish the datapresented in this paper.

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Caracterizacion de Fracturas Asociadas a Las Perdidas de Fluidos

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