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    DOI: 10.1002/ente.201300155

    Scaling of Recovery by Cocurrent Spontaneous Imbibitionin Fractured Petroleum Reservoirs

    Abouzar Mirzaei-Paiaman*[a, b] and Mohsen Masihi[a]

    Introduction

    Spontaneous imbibition of an aqueous phase (i.e., the wet-

    ting phase, WP) into matrix blocks arising from capillary

    forces is an important mechanism to displace the nonwetting

    phase (NWP) from fractured reservoirs.[1] This can be either

    countercurrent or cocurrent displacement. In countercurrent

    spontaneous imbibition (COUCSI) all the permeable faces

    of a rock saturated with NWP are brought into contact with

    a WP and the WP flows in the opposite direction to the ex-

    pelled NWP.[2,3] However, in cocurrent spontaneous imbibi-

    tion (COCSI) only a portion of the permeable surfaces is in

    contact with a WP and, while keeping the remaining permea-

    ble surfaces covered by the NWP, both phases flow in thesame direction.

    In a typical COCSI experiment, matrix surfaces covered

    by WP and NWP are kept constant during the process.[4–14] In

    this case, level of the WP in the fracture system is kept con-

    stant at a certain level and there is no viscous force in the

    fracture system. We use the same COCSI case in this study.

    There are also other cases in which the recovery per-

    formance of a matrix block or a stack of matrix blocks under

    advancing WP level in the fracture system are studied.[15–20]

    Despite the fact that the COCSI can take place in frac-

    tured reservoir,[19,21,22] most attention has been received on

    scale-up of COUCSI.[23–25] Use of the scaled-up results of 

    COUCSI experiments leads to pessimistic forecasts regarding

    the rate of recovery and final recovery.[10,18,26] It has been em-

    phasized that COCSI is more efficient in terms of both final

    recovery and displacement rate than COUCSI.[7,9–11,18–20,26,27]

    Due to the significant differences between the recovery per-

    formances in these processes, the corresponding scaling

    equations cannot interchangeably be used. Several studies

    show that the scaling equations developed for the COUCSI

    process fail to scale up the COCSI data. [8,10,28–30]

    A few scaling equations have been proposed for recovery

    prediction of COCSI.[31–33] They have been applied to one-di-

    mensional displacement as no characteristic length has been

    defined for the COCSI. These scaling equations do not incor-

    porate all of the factors influencing the process; consequent-

    ly, they cannot properly describe the process. The scaling

    equation proposed by Rapoport[31] was developed by using

    the inspectional analysis of the main governing equations,

    making some simplifying assumptions; including that the

    prototype WP/NWP viscosity ratio must be duplicated in the

    model tests, initial fluid saturations in the prototype must be

    duplicated in the model tests, the relative permeability func-

    tions must be the same for both the model and the proto-

    type, and the capillary pressure functions for the both the

    model and the prototype must be related by direct propor-

    tionality. Several studies have attempted to develop the scal-ing equation of Rapoport (e.g., Ref. [34, 35]) to COUCSI

    data only. The scaling equations of Li[32] and Bourbiaux[33]

    were derived based on a restricting approximate solution to

    the main governing equations. They take the assumption of 

    piston-like displacement which is valid only for a few particu-

    lar cases. However, according to Mirzaei-Paiaman and

    Masihi,[25] the development of these two scaling equations is

    not consistent with common scaling practices.

    The main purpose of this study is to present universal scal-

    ing equations for one-dimensional COCSI based on the

    recent finding of Schmid[36] who notices that the analytical

    solution to unidirectional displacement given by McWhorter

    and Sunada[37] applies to COCSI with no artificial boundary

    conditions. We consider the consistency between the devel-

    opment of the new scaling equations and common practices

    as was considered for COUCSI in Mirzaei-Paiaman and

    Masihi.[25] These new scaling equations are rewritten in terms

    [a]  Dr. A. Mirzaei-Paiaman, Dr. M. Masihi

    Department of Chemical and Petroleum Engineering 

    Sharif University of Technology

    P.O. Box 11365-9465, Azadi Ave., Tehran (Iran)

    E-mail: [email protected]

    [b]  Dr. A. Mirzaei-Paiaman

    Department of Petroleum Engineering

    NISOC, Ahvaz (Iran)

    Cocurrent spontaneous imbibition (COCSI) of an aqueous

    phase into matrix blocks arising from capillary forces is an

    important mechanism for petroleum recovery from fractured

    petroleum reservoirs. In this work, the analytical solution to

    the COCSI is used to develop the appropriate scaling equa-

    tions. In particular, the backflow production of the nonwet-

    ting phase at the inlet face is considered. The resulting scal-

    ing equations incorporate all factors that influence the pro-

    cess and are found in terms of the Darcy number (Da) and

    capillary number, (Ca). The proposed scaling equations are

    validated against the published experimental data from the

    literature.

    166   2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim   Energy Technol.  2014, 2, 166–175

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    of two physically meaningful dimensionless numbers: the

    Darcy number (Da) and the capillary number (Ca). Then the

    new scaling equations are validated by using experimental

    data from the literature. Furthermore, we simplified the scal-

    ing equations, which can be useful in some certain applica-

    tions.

    The Analytical Solution

    The partial differential equation for one-dimensional hori-

    zontal displacement of two immiscible, incompressible fluids

    can be expressed by:[37]

    uw ¼ f Swð Þ ut  D Swð Þ@ Sw@  x

    ð1Þ

    This equation in combination with the material balance con-

    dition gives:[37]

    @ Sw@ t

     ¼ ut@  f Swð Þ@  x

      þ   @ @  x

      D Swð Þ@ Sw@  x

      ð2Þ

    in which the subscript w denotes the WP,  f   is the porosity,  S

    is the saturation,   t   is the time,  ut(=uw+unw) is the total volu-

    metric flux (ut=0 for countercurrent flow and  u t>0 for uni-

    directional displacement), the subscript nw denotes the

    NWP,   x   is the spatial coordinate,   f (Sw) is the fractional flow

    in the absence of capillary pressure defined as

     f Sw

    ð Þ ¼

      krw mnwk

    rw

     mnw þ

    krnw

     mw ð

    3

    Þand  D(Sw) is the capillary diffusion function defined as,

    D Swð Þ ¼  f Swð Þkkrnw mnwdP cdSw

    ð4Þ

    in which  kr  is the relative permeability,  m   is the dynamic vis-

    cosity,   k   is the absolute permeability, and   P c   is the capillary

    pressure.

    The appropriate initial and boundary conditions can be de-

    fined as,[37]

    Sw   x; 0ð Þ ¼ Swi   ð5ÞSw þ1; t ð Þ ¼ Swi   ð6Þ

    uw   0; t ð Þ ¼ At 1=2 ð7Þ

    The initial condition in Equation (5) states that at time

    zero the porous medium is at initial WP saturation,   Swi. The

    medium is assumed as a semi-infinite host at   Swi   at the far

    boundary for all times [Eq. (6)]. Equation (7) is an imposed

    inlet boundary condition defined by McWhorter and

    Sunada[37] to solve the problem given in Equation (2).

    McWhorter and Sunada[37] define the positive parameter   A

    as,

     A ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

    2 1  f iRð ÞZ 

      Sw;BC

    Swi

    Sw  Swið ÞD Swð ÞF Swð Þ  f n   Swð Þ   dSw

    s   ð8Þ

    in which   f i   is the WP fraction of the total efflux,  R  ¼   utuw   0;t ð Þ

    is the ratio of the total volumetric flux to the volumetric flux

    of the WP at the inlet (R=0) for countercurrent flow, andR=1 for unidirectional displacement,  Sw,BC   is the saturation

    of WP at the inlet open boundary,  f n is the normalized  f , and

    F   is the fractional flow function in the presence of capillary

    effects.[37]

     f n ¼  f   f ið ÞR

    1  f iR   ð9Þ

    F Swð Þ ¼ 1 R Sw;BC

    Sw

     bSwð ÞD  bð ÞF   bð Þ f n   bð Þ d b

    R Sw;BC

    Swi

    SwSwið ÞD Swð ÞF Swð Þ f n   Swð Þ  dSw

    ð10Þ

    The exact solution to Eqation (2) is given implicitly by

    McWhorter and Sunada[37] as:

     x Sw; t ð Þ ¼ 2 A  1  f iRð Þ

      F 0   Swð Þt 1=2 ð11Þ

    in which  F ’  is the derivate of  F  with respect to  Sw defined as:

    F 0   Swð Þ ¼R Sw;BC

    Sw

    D  bð ÞF  bð Þ f n   bð Þ d bR Sw;BC

    Swi

    SwSwið ÞD Swð ÞF Swð Þ f n   Swð Þ  dSw

    ð12Þ

    The solution expressed in Equation (11) can be used byfirst prescribing   Sw,BC   and calculating   F (Sw) from Equa-

    tion (10), and then finally computing   A   from Equation (8).

    However, the use of Equation (10) is indirect, as it has the

    form of an implicit functional equation, from which   F (Sw)

    has to be extracted. Therefore the computation of   F (Sw)

    from the integral Eq. (10) is performed by an iterative proce-

    dure. A convenient first trial is   F (Sw)=1 as suggested by

    McWhorter and Sunada.[37]

    By combining Equation (1) with the definition of   R   and

    the self-similar variable  l ¼ xt 1=2, the volumetric flux of theWP at the inlet boundary can be written as: [36]

    uw   0; t ð Þ ¼   D Sw;BC

    1  f Sw;BC

    R

    dSwd l   l¼0

    j   t 1=2 ¼ At 1=2 ð13Þ

    Using this equation, Schmid[36] noticed that the inlet boun-

    dary condition imposed by McWhorter and Sunada[37] to

    solve the original problem is redundant for the case of both

    COUCSI and COCSI and the solution given in Equa-

    tion (11) describes the standard situation found in the labora-

    tory experiments. Schmid and Geiger[23,24] and Mirzaei-Paia-

    man and Masihi[25] use this finding to develop the appropri-

    ate scaling equations for COUCSI cases. Mirzaei-Paiaman

    et al.[38]

    used the analytical solution for the COUCSI case to

    Energy Technol. 2014, 2, 166– 175   2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim   167

    Recovery in Fractured Petroleum Reservoirs

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    propose a mathematically based index for characterizing the

    wettability of reservoir rocks. In an another modeling ap-

    proach, Cai et al.[13,14] proposed analytical expressions for

    characterizing a spontaneous cocurrent imbibition process of 

    a wetting fluid into gas-saturated porous media based on the

    fractal character of the porous media. In their work, [13,14] the

    mass of imbibed liquid is expressed as a function of the frac-

    tal dimensions for pores and for tortuous capillaries, the min-

    imum and maximum hydraulic diameters of pores, and the

    ratio for minimum to maximum hydraulic diameters as well

    as the porosity, fluid properties, and the fluid–solid interac-

    tion.

     Application of the Analytic Solution to COCSIProcess

    McWhorter and Sunada[37] considered a special case of uni-

    directional displacement that could be realized in laboratory

    by using a semipermeable membrane that is permeable to

    only the WP (i.e.,  R=1). However in practice if a NWP is in-itially present and is being displaced by a WP, depending on

    the magnitude of the forced injection rate, three flow re-

    gimes may occur in a linear flow domain.[39,40] If there is no

    forced injection at the inlet boundary, there will be counter-

    current flow (with respect to the NWP) at the inlet face im-

    plying  R

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    common scaling practices should be considered. In practice,

    to scale imbibition data, recovery curves are normalized by

    the reference volume which can either be the ultimate recov-

    ery, pore volume, or initial the NWP in place. If the recovery

    in the scaling practice is normalized by the ultimate recovery,

    then the appropriate dimensionless time would be in the

    form of 

    t D ¼ 2 AF 0   Swið Þ  1  f iRð Þ

    L  t 1=2 ð23Þ

    If normalizing recovery to pore volume   V p   is required, the

    corresponding dimensionless time  t D;V p   should be defined as,

    t D;V p ¼ 2 A

    Lt 1=2 ð24Þ

    Similarly, for the case of normalizing the recovery to the

    initial NWP in place(V i), the corresponding dimensionless

    time  t D;V i  should be defined as,

    t D;V i ¼  2 A

    L  1 Swið Þ t 1=2 ð25Þ

    Moreover, these scaling equations can easily be extended

    to other wetting conditions by using the method proposed by

    Schmid and Geiger.[24]

    The above scaling equations can also be generalized by

    presenting them in terms dimensionless numbers. Inserting

    Equation (7) into Equation (19) yields:

    Q

    Q1¼ 2 A

    2F 0   Swið Þ  1  f iRð ÞLuw

    ð26Þ

    Equation (20) gives the relationship between the Darcy ve-

    locity uw and the linear velocity vw as :

    uw ¼  vw

    F 0   Swið Þ  1  f iRð Þ t 1=2 ð27Þ

    Capillary pressure can be related to the Leverett J function

     J (Sw) as:[42]

    P c   Swð Þ ¼ s  ffiffiffi

    k

    r   J Swð Þ   ð28Þ

    Inserting Equations (8), (12), and (27) into Equation (26)

    and combining with Equations (4) and (28) yields:

    Q

    Q1¼

     ffiffik

    q L

    vw mw

    R Sw;BCSwi

     f Swð Þ mw mnw krnwdJ 

    dSw

    F Swð Þ f n   Swð Þ  dSw 2R Sw;BC

    Swi

    SwiSwð Þ f Swð Þ mw mnwkrnwdJ 

    dSw

    F Swð Þ f n   Swð Þ   dSw

    ð29Þ

    Similarly, using the relationship between different volumes,

    we can write:

    Q

    V p¼

     ffiffik

    q L

    vw mw

    R Swi

    Sw;BC

     f Swð Þ mw mnw krnwdJ 

    dSw

    F Swð Þ f n   Swð Þ  d b

    1  f iRð Þð30Þ

    Q

    V i¼

     ffiffik

    q L

    vw mw

    R Swi

    Sw;BC

     f Swð Þ mw mnw krnwdJ 

    dSw

    F Swð Þ f n   Swð Þ  d b

    1 Swið Þ  1  f iRð Þð31Þ

    The generalized Darcy number (Da) defined as the ratio

    of two characteristic lengths (of the pore and domain)[25,43]

    can be written as :

    Da ¼   kL2

      ð32Þ

    The generalized capillary number   Ca   representing the rela-

    tive effect of viscous forces versus interfacial tension can fur-

    ther be written as:

    CaQ1 ¼ vw mw

    s R Sw;BCSwi SwiSwð Þ f Swð Þ

     mw mnw

    krnwdJ 

    dSw

    F Swð Þ

     f n   Swð Þ

      dSwR Sw;BCSwi

     f Swð Þ mw mnwkrnwdJ 

    dSw

    F Swð Þ f n   Swð Þ  dSw 2   ð33Þ

    CaV p ¼ vw mw

    1  f iRð ÞR Swi

    Sw;BC

     f Swð Þ mw mnw krnwdJ 

    dSw

    F Swð Þ f n   Swð Þ  d bð34Þ

    CaV i ¼ vw mw

    1 Swið Þ  1  f iRð ÞR Swi

    Sw;BC

     f Swð Þ mw mnw krnwdJ 

    dSw

    F Swð Þ f n   Swð Þ  d bð35Þ

    The right-hand sides in Equations (29), (30), and (31) can

    be rewritten as  Da1=2

    Ca   implying that during COCSI, the recov-

    ery is controlled by the Darcy and Capillary numbers.

    Validation of the New Scaling Equations

    A comprehensive survey of the related literature was per-

    formed and eight sets of strongly “water-wet” experimental

    data from Hamon and Vidal,[6] Bourbiaux and Kalaydjian,[7]

    Akin et al.,[8] and Standnes[10] were found to fulfill the re-

    quirements of this study. Hamon and Vidal[6] performed four

    experiments on a synthetic porous medium by using water

    (WP) and a purified refined oil (NWP). In the experiment

    by Bourbiaux and Kalaydjian,[7] the porous medium was

    sandstone and brine and Soltrol 130 were used (WP andNWP, respectively). In the liquid–liquid and gas–liquid ex-

    periments performed by Akin et al.[8] the porous medium

    was diatomite with an initial zero saturation of the WP. In

    both liquid–liquid and gas–liquid experiments, the WP fluid

    was water and the NWP fluids were   n-Decane and air, re-

    spectively. In the experiment by Standnes[10] chalk was the

    porous medium, and water (WP) and  n-Decane (NWP) were

    used with an established initial WP saturation of zero. These

    experiments are summarized in Table 1.

    The ability of the new equations to scale the recovery ex-

    periments is compared to the scaling equations of Ma

    et al.,[34]

    Mason et al.,[35]

    and Li.[32]

    An appropriate scaling

    Energy Technol. 2014, 2, 166– 175   2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim   169

    Recovery in Fractured Petroleum Reservoirs

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    equation should reduce the observed scatter in the recovery

    curves (Figure 1) to an acceptable limit. To use the scaling

    equations of Ma et al.[34] and Mason et al.[35] only the com-

    monly measured core and fluid property information are

    needed, as summarized in Table 1. However, to use the scal-ing equation of Li[32] the measured endpoint relative permea-

    bility and capillary pressure data at initial WP saturation are

    needed. Comprehensive studies reporting all required

    COCSI recovery data and capillary pressure and relative per-

    meability information are very rare in the literature. In this

    work we therefore assume that for “water-wet” materials,

    the relative permeability and capillary pressure information

    from a certain rock type is representative for a given materi-

    al. The relative permeability and capillary pressure data re-

    ported by Graue et al.[44] and Schembre and Kovscek[45] are

    used. However to use the new scaling equations, the relative

    permeability and capillary pressure data over the entire satu-ration range are needed (i.e.,   kr   and   P c   vs.   Sw   data are

    needed). Use of the aforementioned approach to run the

    model of Li[32] may not provide reliable estimates for  F ’(Swi)

    and   A. As the analytical solution presented by McWhorter

    and Sunada,[37] after consideration of backflow production of 

    NWP, describes exactly the COCSI, numerical values of 

    F ’(Swi) and   A   can be obtained by fitting the analytical solu-

    tion to experiments by using Equations (19) and (21) or

    Equations (19) and (22). Based on Equation (21), a plot of   Q

    V p

    versus  2

    L t 1=2 gives a straight line with slope   A   that can be

    computed by using a regression analysis. The numerical

    value of   A   can also be obtained by using plot of   Q

    V iversus

    2L  1Swið Þ t 

    1=2 and computing the slope of the resulting straightline [Eq. (22)]. After determination of  A, the numeral value

    of   F ’(Swi) can be computed by plotting  Q

    Q1 versus  2 A  1 f iRð Þ

    L   t 1=2

    and determining slope of the resulting straight line

    [Eq. (19)]. In the slope analyses, there may be some observa-

    tions deviating from linearity, particularly at early times,

    which besides the experimental reading errors can be related

    to heterogeneities at the pore level (i.e., pore shape and

    pore-level roughness) and/or some geometrical effects that

    result in the violation of the one-dimensional flow assump-

    tions such that the spatial gradient of the capillary pressure

    was not linear.[8,46] Buoyancy effects are assumed negligible

    for the strongly “water-wet” small-size rock samples. There is

    also some deviation from line-

    arity at very late times when

    the imbibition front reaches the

    far boundary. Obviously, the

    analytical solution considered

    in this study is not valid any-

    more when the WP front con-

    tacts the far boundary. Data

    points related to the nonlinear

    portions should therefore all be

    excluded from the regression

    analyses. The procedure used to

    determine the numeral values

    Table 1.  Summary of data for experiments performed by Hamon and Vidal[6] (HV1, HV2, HV3, and HV4), Bour-

    biaux and Kalaydjian[7] (BK1), Akin et al.[8] (AK1 and AK2), and Standnes[10] (ST1).

    Experiment   S wi   L

    [cm]

    k  [m2]   f   mw[mPas]

     mnw[mPas]

    s  [mNm1] Endpoint

    k rw

    End

    point

    k rnw

    Maximum

    P c [kPa]

    F ’(S wi)   A [m ffiffi

     sp  ]

    HV1 0.35 9.7 1.8 10-15 0.27 1 11.5 49 0.14 0.98 480.1 4.7 9.5 10-6

    HV2 0.35 19.9 2 10-15

    0.28 1 11.5 49 0.12 0.97 461.2 5.2 1.7 10-5

    HV3 0.36 49.8 2 10-15 0.28 1 11.5 49 0.10 0.95 458.5 5.7 2.0 10-5

    HV4 0.36 84.8 1.6 10-15 0.27 1 11.5 49 0.12 0.95 512.8 5.4 1.7 10-5

    BK1 0.39 29 1.37 10-13 0.23 1.2 1.5 35 0.06 0.47 12.5 4.3 2.6 10-5

    AK1 0 9.5 7 10-15 0.69 1 0.0182 72 0.09 1.00 428.6 1.0 4.8 10-4

    AK2 0 9.5 7 10-15 0.69 1 0.84 51.4 0.15 1.00 200.4 1.6 4.6 10-5

    ST1 0 5.0 2 10-15 0.43 1 0.95 46 0.30 1.00 541.5 1.4 5.4 10-5

    Figure 1. Recovery in terms of different reference volumes: a) ultimate recov-

    ery, b) pore volume, and c) initial NWP in place versus time for the experi-

    ments collected from the literature.

    170   2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim   Energy Technol.  2014, 2, 166–175

     A. Mirzaei-Paiaman and M. Masihi

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    of  A  and  F ’(Swi) is shown in Figure 2 and the summarized re-

    sults are given in Table 1.

    As the scaling equations of Ma et al.[34] and Mason et al.[35]

    are based on the application of general principles of dimen-

    sional analysis by Rapoport,[31] rather than by a mathematical

    treatment, establishing an exact relationship between the ver-tical and horizontal axes in scaling plots is not possible.[25]

    Thus, in scaling plots, the vertical axis may be recovery nor-

    malized by any of the reference volumes. Figure 3 and

    Figure 4 show, respectively, the performance of the scaling

    equations of Ma et al.[34] and Mason et al.[35] after normaliz-

    ing the recovery data by the ultimate recovery, pore volume,

    or initial oil/gas in place, which is plotted against these scal-

    ing equations. These figures show that these equations

    cannot scale recovery curves as there is significant scatter in

    all plots. There may be several reasons for the poor scaling

    performance of the scaling equations of Ma et al.[34] and

    Mason et al..[35]

    These were derived based on the application

    of the general principles of di-

    mensional analysis by Rapo-

    port.[31] This approach causes

    the effects of some of the fac-

    tors influencing the process be

    systematically neglected. How-

    ever the main reason may be

    that these scaling equations

    have not been derived in a con-

    sistent way according to

    common scaling practices.[25]

    The ability of the scaling

    equation from Li[32] to plot the

    recovery data normalized by

    different reference volumes is

    shown in Figure 5 versus this

    scaling equation. The scaling

    result is not satisfactory and

    there still exists a nontrivial

    scatter in scaling plots. Oneshould note that this scaling

    equation has been specifically

    proposed for COCSI. There

    may be several reasons for such

    poor quality of the scaling rela-

    tion. This scaling equation has

    been derived based upon a re-

    stricting approximate solution

    to the main governing equation.

    Furthermore the development

    of this scaling equation is not

    consistent with common scalingpractices as highlighted by Mir-

    zaei-Paiaman and Masihi.[25] In

    Figure 5, the experiments re-

    ported by Hamon and Vidal[6]

    are not shown because the scal-

    ing equation of Li[32] predicts

    negative values if the viscosity

    of the NWP (11.5 mPas, in these experiments) becomes large

    compared to viscosity of WP (1 mPas, in these experiments).

    The ability of the new scaling equations to scale the ex-

    periments is shown in Figure 6, in which the recovery data

    normalized by different reference volumes are plotted

    against the appropriate scaling equations. Depending on thetype of normalization on the y-axis, the corresponding scaling

    equation should be put on the  x-axis. In each plot the curve

    given by the analytical solution is also included. The ability

    of the new equations to scale the experiments is much better

    than the existing scaling equations. However, for some ex-

    periments, some scatter around the analytical solution curve

    is noticeable which may be due to the quality of the reported

    experimental data. To be able to compare among the scaling

    equations (Figures 1, 3, 4, 5, 6, and 7) we use the same

    8 cycles on their horizontal axes.

    Figure 2. The procedure used to compute the numeral values of  A  and  F 0ðSwiÞ  from the experimental data: a) plotof 

      Q

    V pversus   2L t 

    1=2 gives a straight line with slope A  that can be computed by using a regression analysis, b) the nu-

    meral value of  F 0ðSwiÞ can be computed by plotting   QQ1 versus  2 A  1 f i Rð Þ

    L   t 1=2 and determining slope of the resulting

    straight line.

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    Derivation of a Simple Scaling Equation for “Water-Wet” Systems

    As the presented scaling equations incorporate the capillary

    pressure and relative permeability parameters, they are ap-

    plicable to systems with a wide range of wetting conditions.

    However, from a practical perspective, this information is

    generally not easily known, so there is still need for some

    utility in simpler scaling groups. By simple scaling groups we

    mean scaling equations that do not incorporate capillary

    pressure and relative permeability information and are

    mostly applicable to the systems with the same wettability

    conditions (e.g., all “water-wet”). These equations usually

    contain only simple-to-measure rock and fluid parameters

    such as porosity, absolute permeability, interfacial tension,

    geometrical dimensions, and the wetting and nonwetting

    phase viscosities. With the exception of scaling purposes,

    there exists significant interest in using simple equations,

    mainly in comparative studies, for the objective of studying

    the effect of aging time on the wettability alteration,[47,48] the

    effect of water adsorption and resulting microfractures in or-

    ganic shale rocks,[49,50] or the spontaneous imbibition charac-

    teristics of different porous media.[8,12]

    Figure 3. Recovery in terms of different reference volumes: a) ultimate recov-

    ery, b) pore volume, and c) initial NWP in place versus the scaling equation

    of Ma et al.[34]

    Figure 4. Recovery in terms of different reference volumes: a) ultimate recov-

    ery, b) pore volume, and c) initial NWP in place versus the scaling equation

    of Mason et al.[35]

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    Inserting Equations (3), (4), (8), (12), and (28) into the di-

    mensionless time equations brings them into the new formsas:

    t D ¼ t D;V p ¼  t D;V i ¼  G

     ffiffiffiffiffiffiffiffiffiffiffiffi2s 

     ffiffik

    q  mnwL

    2

    v uut  t 1=2

      ð36Þ

    in which  G, a group of variables, differs (Table 2).

    Our objective is to extract a simple scaling equation from

    Equation (36). We note that  G  is a function of wettability, in-

    itial wetting phase saturation, fluid viscosity, and pore struc-

    ture. Therefore the exclusion of  G  from the scaling equation

    [Eq. (36)] makes this equation   ¼  ffiffiffiffiffiffiffiffiffiffi2s   ffiffik

    p  mnwL2

    r   t 

    1=2 !  independent

    of wettability and thus applicable to “water-wet” systems.

    However, some parameters in the  G  variable are dependent

    on fluid viscosity, initial wetting phase saturation, and pore

    structure, and their overall effect should be considered, even

    after the exclusion of  G. In the case of the fluid viscosity de-

    pendence, this can be performed by replacing  mnw   in the sim-

    plified equation by an appropriate argument, called the vis-

    cosity group. To do this, several forms of the viscosity group

    were used and the scaling performance of the resulting sim-

    plified scaling equations was checked. The best scaling group

    was   mnw+  ffiffiffiffiffiffiffiffiffiffiffiffi mw mnwp 

      . Therefore the resulting simplified scaling

    equation can be written as:

    Figure 5. Recovery in terms of different reference volumes: a) ultimate recov-

    ery, b) pore volume, and c) initial NWP in place versus the scaling equation

    of Li.[32]Figure 6. Recovery in terms of different reference volumes: a) ultimate recov-

    ery, b) pore volume, and c) initial NWP in place versus the corresponding

    new scaling equations.

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    t D;simplified ¼  ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2s   ffiffik

     mnw þ  ffiffiffiffiffiffiffiffiffiffiffiffi mw mnwp  L2v uut   t 1=2   ð37ÞWe should note that because this simplified scaling equa-

    tion contains the ffiffi

    k

    q   term, the dependence on the pore struc-

    ture may be assumed to be relaxed. However, the initial wet-

    ting phase saturation dependence cannot be removed. There-

    fore the simplified scaling equation presented above may not

    present very good scaling performance for systems with dif-

    ferent initial saturations of the wetting phase. As this scaling

    equation is a simplified form of the general scaling equations,

    the vertical axis in scaling plots can be recovery normalized

    by any reference volume. The ability of this equation to scaledata is shown in Figure 7, which shows an improvement as

    compared to Figure 3, Figure 4, and Figure 5. Moreover,

    Figure 7 (in comparison to Figure 6) shows acceptable accu-

    racy for the new simplified equation as compared to the new

    general scaling equations.

    Conclusions

    The following conclusions can be drawn from this work:

    *   As the analytical solution to unidirectional displacement

    given by McWhorter and Sunada[37] applies to COCSI,[36]

    suitable scaling equations for one-dimensional COCSI canbe found by using this solution.

    *  Backflow production of the NWP at the inlet open boun-

    dary is inherent to COCSI and its contribution to the pro-

    cess should be taken into account when using the analyti-

    cal solution given by McWhorter and Sunada[37] and when

    presenting new scaling equations.

    *   The strategy to account for the contribution of the back-

    flow production by assuming   R¼6 1 avoids the occurrenceof the possible instabilities in computing the integrals in

    the McWhorter and Sunada[37] solution.

    *   The new scaling equations presented in this study are uni-

    versal, incorporating all factors influencing the process, as

    the exact analytical solution to the problem without any

    assumption are used.

    *   Consistency between the development of the new scaling

    equations and common practices should be considered to

    obtain reliable results.

    *   The new scaling equations can be rewritten in terms of 

    two physically meaningful dimensionless numbers,   Da1/2/

    Ca (Da : Darcy number, Ca : capillary number).

    *  The ability of the new equations to scale the experiments

    was found to be much better than the existing scaling

    equations.

    Figure 7. Recovery in terms of different reference volumes: a) ultimate recov-

    ery, b) pore volume, and c) initial NWP in place versus the new simplified

    scaling equation.

    Table 2.  Expressions for G  for different dimensionless time equations.

    Dimensionless time Corresponding expression for G

    tD

    G ¼ R Sw;BC

    Swi

     f Swð Þkrnw dJ dSwF Swð Þ f n   Swð Þ dSw

     ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiR Sw;BCSwi

    Swi Swð Þ f Swð Þkrnw dJ dSwF Swð Þ f n   Swð Þ   dSw

    q t D;V p

    G ¼

     ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiR Sw;BC

    Swi

    Swi Swð Þ f Swð Þkrnw dJ dSwF Swð Þ f n   Swð Þ   dSw1 f i Rð Þ2

    t D;V i

    G ¼

     ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiR Sw;BC

    Swi

    Swi Swð Þ f Swð Þkrnw dJ dSwF Swð Þ f n   Swð Þ   dSw

    1Swið Þ2 1 f i Rð Þ2

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    *   For the literature scaling equations derived based on the

    application of the general principles of dimensional analy-

    sis, establishing an exact relationship between the vertical

    and horizontal axes in scaling plots is not possible. Thus,

    in scaling plots the vertical axis may be recovery normal-

    ized by any of the reference volumes.

    *   The scaling equations developed for COUCSI fail to scale

    up COCSI data.

    *   The former scaling equations proposed for COCSI, de-

    rived based on assumption of piston-like displacement,

    yield poor scaling results.

    *   The general scaling equations presented in this study can

    be used to present a simple scaling equation for “water-

    wet” systems. The ability of the new simplified scaling

    equation to scale “water-wet” data was found to be better

    than other existing scaling equations and also acceptable

    in comparison to new general scaling equations.

    Acknowledgements

    The authors thank Sharif University of Technology and the

    Research and Technology Departments of NIOC and NISOC 

     for permission to publish this paper.

    Keywords:  fluid dynamics  ·   fractured reservoirs  ·  petroleum ·

    scaling equations ·  wetting phase

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    Received: October 25, 2013

    Revised: December 5, 2013

    Published online on February 10, 2014

    Energy Technol. 2014, 2, 166– 175   2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim   175

    Recovery in Fractured Petroleum Reservoirs

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