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Saturation Determinatio

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  • Saturation Determination

  • INTRODUCTION

    Water saturation is the fraction (or percentage) of the pore volume of the reservoir rock that is filled with water.

    It is generally assumed, unless otherwise known, that the pore volume not filled with water is filled with hydrocarbons.

    Determining water and hydrocarbon saturation is one of the basic objectives of well logging.

  • DETERMINATION OF SATURATION IN CLEAN FORMATIONS

    In the determination of saturation, resistivity logs are those that are most depended on

    For a formations with homogeneous intergranular porosity saturation determination are based on Archies water saturation equation;

  • Archies equation is stated as

    Equation 1 (with n = 2) is solved, in nomograph form, in Chart Sw-1.

  • For Sxo, the water saturation in the flushed zone, a similar expression exists

  • The saturation exponent n is usually taken as 2;has shown to be good estimates

    The accuracy of the Archie equations depend on the accuracy of the input parameters (Rw(or Rmf), F, and Rt(or Rxo) )

    The formation water resistivity (Rw) should be verified in as many ways as possible:

    calculation from the SP curve, water catalogue, calculation from nearby water-bearing formation, and/or water sample measurement.

  • Formation Factor

    Formation factor F is usually obtained from the measured porosity of the formation through the relationship

    Where

    a is the tortuosity factor

    m is the cementation exponent

  • The formation factor is also defined by another relationship as the ratio of the resistivity of the rock completely saturated with brine to the resistivity of the brine and is given below.

    F=Ro/Rw

  • The values of a and m in Equation 3 are subject to more variation

    In carbonates,

    F = 1/ 2 is usually used;

    In sands,

    F = 0.62/ 2.5 (Humble formula), or

    F = 0.81/ 2 (a simpler form practically equivalent to the Humble formula).

  • While the Humble formula is satisfactory for sucrosic rocks, better results are obtained

    using F = 1/ 2.2 to 1/ 2.5 in compact or oolicastic rocks

    In some severely oolicastic rocks, m may even be as high as 3

  • True Formation Resistivity Rt

    This is measured by either a deep laterlog or a deep induction logging tool in the borehole

    The following factors must be taken into consideration whilst using Rt value in the Archie's equation

    it is important to evaluate the invasion profile and, if

    necessary, perform the necessary corrections The determination of Rt can be a problem in case of thin

    beds; resolution of tool should be taken in consideration

  • If there are clays present in the formation, the bound water in clays can act as a conductor and this can decrease the Rt value

    Also the overburden pressure at in-situ downhole can cause a significant increase in Rt.

  • Using the Humble Equation

    Step 1: Determine from acoustic, density or Neutron log.

    Step 2: Determine F from Humble Formula

    Step 3: Determine Rt from deep reading resistivity (induction or laterolog) log

    corrected for borehole, bed thickness and invasion.

  • Step 4: Determine Rw

    Step 5: Determine Sw from the Archie Equation (Equation 1) or from Chart SW-1

  • Flushed Zone in Humble Formula Procedure

    Step 1: Determine porosity from acoustic, density or Neutron log.

    Step 2: Determine F from Humble Formula

    Step 3: Determine and use Rxo from shallow resistivity log in place of Rt from deep reading resistivity log

  • Step 4: Determine and use Rmf in place of Rw

    Step 5: Determine Sw from the Archie Equation (Equation 1)

  • Saturation Determination SW-1

  • This nomograph solves the Archie water saturation equation

    If Ro (resistivity when 100% water saturated) is known, a straight line from the known Ro value through the

    measured Rt value gives water saturation, Sw. If Ro is unknown, It may be determined by connecting the formation

    water resistivity, Rw, with the formation resistivity factor, FR, or porosity, .

  • NB: Chart Sw-1 is constructed using the F = 1/ 2 porosity-to formation factor relationship. For any

    other porosity-to-formation factor relationship the nomograph should be entered with the formation factor

    Chart Sw-1 may also be used to solve Equation 2 for the flushed zone water saturation.

    To do this, the Rxo reading is inserted on the Rt leg of

    the nomograph and the Rmf value is inserted in the Rw leg

  • RESISTIVITY VS POROSITY CROSSPLOTS

    Combining Equation 1 and 3, the Archie saturation equation may be written

    If n and m are equal to 2, and a = 1, then Which can be written as

  • A plot of vs the inverse square root of Rt can be used to determine a number of parameters including Sw

    Rt value, is usually the log reading of the deep resistivity device provided the readings are not much influenced by invasion or other environmental factors

  • Resistivity vs porosity crossplot Example

  • Figure shows several points plotted over an interval in which formation-water resistivity is constant

    Assuming that at least some of the points are from 100% water bearing formations, a line drawn from the pivot point (f = 0, Rt = ) through the most north-westerly plotted points corresponds to Sw=1

    The slope of this line defines the value of Rw.

  • Rt values on this line corresponds to Ro values at the reference porosities. Thus for = 10%, Ro = 6.5 ohm-m.

    Rw can also be determined using any with its corresponding Ro. Thus, for f = 10%, F = 100. Since Rw = Ro/F, Rw = 6.5/100 = 0.065 ohm-m as shown.

    For other Sw values, Rt and Ro are related by the equation Rt = Ro/SW

    2. For Sw = 50%, 1/SW2 = 4, and Rt =

    4 Ro. This relation establishes the line for Sw = 50%.

  • Plots of the values of Ro for Sw = 10%, 20%, 30% and 50%, using the above relationship, for a set of porosities can be determined

    Sw in the zones of interest can be determined from the position of the plotted points in relation plot saturation lines

  • Rwa Comparison

    The term Rwa is apparent formation water resistivity

    It is only equal to Rw in 100% water-bearing formations

    In hydrocarbon-bearing formations, Rwa computed from will be greater than Rw

    The relationship between Sw, Rwa, and Rw Is given as

  • At 100% water saturation, the Archie equation can be written as

    The Rwa technique can, therefore, be useful for identifying potential hydrocarbon bearing zones and for obtaining Rw values

  • Since the Rwa technique, as normally applied, requires that deep resistivity (Rdeep) Rt, invasion must be shallow enough that the deep resistivity reads essentially the same as the true resistivity.

    Calculated Rwa values will approximate Rw values in clean water-bearing sands

    Usually, an Rwa value at least three times that of Rw is needed to indicate possible hydrocarbon potential

  • A continuous log of Rwa can be recorded at the wellsite using resistivity and porosity logs.

    Figure 4 is an example computed from the BHC sonic log and induction-SFL log combination

    The Rwa combination indicates the lower sand to be predominately water bearing with a good show of hydrocarbons at its top

  • RESISTIVITY RATIO METHODS

    In resistivity ratio methods, it is assumed that a formation is divided into two distinct a flushed or invaded zone and a non-invaded zone

    The resistivities of the two zones must be measurable or derivable from logs, and methods for determining the resistivity of the water in each zone must be available.

  • Flushed Zone Method

    If n = 2 is assumed, Archies equations for Sw and Sxo can be compared to give

    This equation gives the ratio of Sw to Sxo, and no knowledge of formation factor or porosity is needed

    Rxo may be found from a MicroSFL log, Rt from an induction or laterolog, and Rmf /Rw from a measured values or from the SP curve

  • The ratio Sw/Sxo,is valuable in itself as an index of oil movability.

    If Sw/Sxo = 1, then no hydrocarbons have been moved by invasion, whether or not the formation contains hydrocarbons.

    If Sw/Sxo is about 0.7 or less, movable hydrocarbons are indicated.

    The value of Sw/Sxo along with and Sw, is useful in evaluating reservoirs

  • To determine Sw from Equation , Sxo must be known

    For moderate invasion and average residual oil saturation, an empirical relation between Sxo and Sw has been found useful: Sxo = Sw1/5. giving

  • DETERMINATION OF RW

    In addition to formation factor or porosity, values of formation water resistivity, Rw, and mud filtrate resistivity, Rmf, are needed for the water saturation calculations

    Mud resistivity, Rm, mudcake resistivity, Rmc, and Rmf are generally measured at the time of the survey on a mud sample from the flowline or mud pit

    These values are recorded on the log heading

  • Rw can be determined in a number of ways

    From the SP Log

    From water saturation equation in a 100% water-bearing formation

    From produced water samples

    From water catalogues

  • Rw from water saturation equation in a 100% water-bearing formation

    When the water saturation is assumed to be 100%, the Archie water saturation equation

    reduces to: Where Rt is from a deep-investigation resistivity log,

    and F is computed from a porosity log reading

    The values of Rwa can be calculated for a number of water bearing zones and if the same then the value of Rwa can be assumed to be equal to Rw

  • Procedure

    Step 1: Identify a permeable, water bearing zone near to the hydrocarbon bearing zone of interest.

    Step 2: Read the resistivity of the zone of interest from a deep investigation resistivity log.

    Step 3: Determine the Formation Resistivity Calculate F from the Humble formula.

  • Step 4: Determine Rwa from Equation above Step 5: Compare values of Rwa:

    The values in the water bearing zone should be similar.

    The lowest value of Rwa is generally the value of Rw.

    If the calculated value of Rwa at a particular depth is over

    three times that of Rw determined in a definite water bearing zone then the zone is potentially hydrocarbon bearing