Well Logging: 03 SP log 02

Well Logging: 03 SP log 02

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  PETE-303: Well Logging Spring2016 Week #02 Eng. Khaled Abdelgawad Department of Petroleum Engineering KFUPM ENGINEERING Well Logging 1
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  Spontaneous Potential Log
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  Well QATIF-46 Uses ofSP log 3 Identify permeable zones Define bed boundaries Compute shale content Depositional Environment from the SP Determine values of formation water resistivity
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  Well QATIF-46 Uses ofSP log 4 1.Identify permeable zones • The negative abnormal on SP curve usually indicates the permeable zone ; the higher abnormal range , the more permeable of the formation . • Since invasion can only occur in permeable formations, deflections of SP can be used to identify permeable formations.
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  Uses of SPlog 1.Identify permeable zones The negative abnormal on SP curve usually indicates the permeable zone ; the higher abnormal range , the more permeable of the formation . Since invasion can only occur in permeable formations, deflections of SP can be used to identify permeable formations. 5 SP alone can not tell the whole story
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  Uses of SPlog 2. Definition of bed boundaries Half of abnormal amplitude point will be boundaries of shale and sand. The bed thickness is the interval between two boundaries . The vertical resolution of SP is poor, and often the permeable bed must be 30 ft or more to achieve a static (flat baseline) SP 6
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  Uses of SPlog 2. Definition of bed boundaries Half of abnormal amplitude point will be boundaries of shale and sand. The bed thickness is the interval between two boundaries . The vertical resolution of SP is poor, and often the permeable bed must be 30 ft or more to achieve a static (flat baseline) SP 7
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  Uses of SPlog 3. Compute shale content The presence of shale in a permeable formation reduces the SP deflection. In water-bearing zones, the amount of SP reduction is related to the amount of shale in the formation. The SP response of shales is relatively constant and follows a straight line called a shale baseline. The SP value of the shale baseline is assumed to be zero, and SP curve deflections are measured from this baseline. 8
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  Base Line Drift Overlong intervals (several hundreds to thousands of feet), the SP baseline can drift, either in the positive or negative direction. This characteristic is commonly observed in shallow sections. Caused by increases in relative oxidation of the rocks that are close to the land surface. This may introduce errors if the SP magnitude is being calculated over that long interval, especially by means of a computer. The baseline drift can be removed (many programs have such editing routines) so that the SP baseline retains a constant value (usually set to zero) over the length of the logged interval. 9 A shallow section of the Dakota.
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  Shale Content Presence ofshale in the formation will reduce the static SP Shale lattice will slow the migration of chlorine ions and assist the flow of sodium ions, decreasing Ej This reduces SSP to a pseudo-static value, PSP 10 SSP  Max deflection possible for given Rmf/Rw SP  SP response due to presence of thins beds and/or gas presence PSP  Pseudostatic SP; SP when shale is present 𝑽 𝒔𝒉 = 𝟏 − 𝑷𝑺𝑷 𝑺𝑺𝑷 The volume of shale can be calculated:
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  Shale Content 11 Example-1 Determinethe volume of shale in zone B 𝑽 𝒔𝒉 = 𝟏 − 𝑷𝑺𝑷 𝑺𝑺𝑷 S𝑺𝑷 = 𝟏𝟎𝟎 𝒎𝑽 P𝑺𝑷 = 𝟕𝟔 𝒎𝑽 𝑽 𝒔𝒉 = 𝟏 − 𝟕𝟔 𝟏𝟎𝟎 = 𝟎. 𝟐𝟒 𝑽 𝒔𝒉 = 𝟐𝟒 % SSP and PSP are measured from shale baseline
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  Shale Content 12 Example-2 𝑽𝒔𝒉 = 𝟏 − 𝑷𝑺𝑷 𝑺𝑺𝑷 SSP S𝑺𝑷 = 𝟔𝟎 𝒎𝑽 P𝑺𝑷 = 𝟓𝟎 𝒎𝑽 𝑽 𝒔𝒉 = 𝟏 − 𝟓𝟎 𝟔𝟎 = 𝟎. 𝟏𝟔 𝑽 𝒔𝒉 = 𝟏𝟔 % PSP
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  Uses of SPlog 4. Determination of Formation Water Resistivity from SP Log 13 • The most direct way of finding water resistivity (Rw) is to obtain a sample of formation water and measure its resistivity • Formation water samples, if available, are invariably contaminated by mud filtrate. • Rw is therefore usually calculated.
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  Uses of SPlog 4. Determination of Formation Water Resistivity from SP Log 14 1.Determine formation temperature. 2.Find Rmf at formation temperature 3.Convert Rmf at formation temperature to Rmfe value 4.Compute Rmfe / Rwe ratio from the SP 5.Compute the Rwe 6.Convert Rwe at formation temperature to Rw Induction electric log with an SP curve from a Pennsylvanian upper Morrow sandstone in Beaver County, Oklahoma.
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    TD TBHTFD TT s sf   SpontaneousPotential Log: Calculation of Rw • Resistivities are temperature dependent. • 1st step Find the formation temperature at the depth at which Rw is required. • Using linear interpolation between surface and bottom hole temperature. • The petroleum industry usually uses degF, not degC. 15 Data given on well header • Surface Temp (Ts), • Formation Depth (FD), • Bottom Hole Temp (BHT), and • Total Depth (TD) are usually.
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  Temperature Gradient 16 1oF/100ft = 1.823 oC/100m 1 oC/100m = 0.5486 oF/100ft • This chart can be used for estimating the temperature in the borehole opposite a formation at a specific depth.
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  Calculation of Rw17 The 2nd step: convert Rm & Rmf measured at the surface to Rmf at Tf, the formation temp. This is NOT a linear interpolation , The constant 6.77 is 0F. For Celsius it is 21.5   0 0 0 0 77.6 77.6    f mff mf T TR R The 3rd step: Find the ideal potential (SSP), correcting for bed thickness. • Bed thickness is determined from the SP log by measuring the distance between inflection points of the SP curve • Ri, from the SN (short reading resistivity) log next to the SP log • Calculate Ri/Rm (make sure you use the temperature corrected Rm), get the SP correction factor, then read off   0 0 0 0 77.6 77.6    f mf m T TR R
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  Well QATIF-46 Find Rmand Rmf at Tf 18
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  Correcting for bedthickness 19 The data necessary to Correct SP to SSP using Chart are: Bed thickness, Resistivity from the shallow-reading resistivity tool (Ri) Rm @ BHT
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  The 4th stepis to find the equivalent formation water resistivity, Rwe, using the following equation (remember Rmf is at formation temperature): The final step is to find the actual water resistivity, Rw, using the following equation: Calculation of Rw 20                   8.50log 0426.0 2 9.19log 1 105.0 10131.0 BHT we BHT we w R R R n t w w R RF S 1         Once Rw is calculated for the reservoir, use the Archie equation to calculate Sw.  BHT SSP mfwe RR 133.061 10  
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  Well QATIF-46 Equivalent FormationWater Resistivity 21 0.28
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  Well QATIF-46 Actual waterresistivity 22
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  Well QATIF-46 Actual waterresistivity 23 • Determine Rmf @Tf • 5.6(11+21.5)/(33+21.5) = 3.3m • Apply SP equation – -50 = - 0.24(33+273)log(3.3/Rwe) – Rwe = 0.68 – Rw = 1.3 ohm-m (See next page) 10mV -||+ Rmf = 5.6m @ 11º C Tf = 33º C • Determine Essp – Shale base line – Maximum deflection line – Calculate deflection -50mV Usually use charts, instead
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  Rw or Rmf RweorRmfe Rwe= 0.68 Rw= 1.3 24
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  Classical Method Review25 Rw From the SP-Classic Method The procedure for using the equations is as following: 1. Determine formation temperature. 2. Find Rmf at formation temperature 3. Convert Rmf at Tf to Rmfe value. 4. Compute The Rmfe/Rwe ratio from the SP. 5. Compute Rwe. 6. Convert Rwe at formation temperature to an Rw value.
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  Example-2 The SP deflectionis –60 mV across a thick, water- bearing, clean zone. The value of Rmf at that temperature of 100 F is 0.5 ohm-m. Determine Rw at the same temperature (100 F) Rw from SP: Classical Method First, Determine the Rmfe (effective Rmf), since the resistivity is not an accurate determination of the ion activity that produces the SP. 26
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  Rmfe = 0.45ohm-m at 100 F. 1. Determine Rmfe 0.5,100F 0.45 ohm-m Rmf, 0.5 ohm-m 27
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  2. Determine Rwe fromRmfe Rmfe/Rwe = 7. Therefore, Rwe=0.45 ohm-m/7=0.064 ohm-m at 100 F 60, 100 7 SSP 28
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  (Rwe=0.064 ohm-m at 100F) 3.Finally, determine Rw •Rw=0.10 ohm-m at 100 F • Here, Rw<Rmf. This problem illustrates the fact that if Rw<Rmf, SP deflection is negative (0.1<0.45 ohm-m) (Normal SP) 0.064 mV 0.064, 100F 29
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  Rw from SP—Silva-BassiouniMethod The classical method is complicated to the novice. Simpler method is available and theoretically justified. The entire process is reduced to a single chart. 30 1.Enter the chart from below with Rmf at formation temperature. 2.Move up to intersect the temperature line at point A. 3. Move left to the SP axis, and then move down by an amount equal to the negative SP deflection. 4.Move right to the temperature line to point B. 5.From B proceed down to the resistivity scale, and read the value of at formation temperature.
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  Well QATIF-46 Rw fromSP—Silva-Bassiouni Method 31
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  Well QATIF-46 For thesame problem as before, ie Rmf=0.5 ohm.m at 100 F, determine Rw if the SP deflection is –60 mV. We see Rw=0.1 ohm-m, as shown with the classical method. 145 mV – 60 mV = 85mV Rw from SP—Silva-Bassiouni Method 32
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  Classical vs. Silva-BassiouniMethod The classical method requires 3 steps for the determination of Rw The Silva Bassiouni method gives you the same value of Rw. Hence it is easier to use 33
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  Example Given: Rm = 2.5Ω-m at 70°F Rmf = 2.0 Ω-m at 70°F Hole diameter = 8 in. Surface temperature = 60°F BHT = 164°F at 10,500 ft From the log: SP deflection = 70 mV Bed thickness = 24 ft Short normal resistivity = 65 Ω-m 34
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  Example Calculations 35 8114 8138 24ft Using BHT =164°F at 10,500 ft and surface temperature = 60°F: Tf at 8100 ft=140°F. Using Rm = 2.5 Ω-m at 70°F and Rmf = 2.0 Ω-m at 70°F: Rm = 1.3 Ω-m at 140°F Rmf = 1.0 Ω-m at 140°F. SP deflection =–70 mV. Ri/Rm = RSN/Rm = 65/1.3 = 50. The SP correction factor is 1.07 Corrected SP of 1.07 × –70 = –75 mV. From Chart 4, using SP corrected and Tf: Rmf/Rwe = 9.7. Rwe = Rmf/(Rmf/Rwe) = 1.0/9.7 = 0.103 Ω-m. Using Rwe and Tf. Rw = 0.103 Ω-m
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  SP shapes andDepositional Sequence 36 Since shales and clays are generally finer-grained than sands, a change in SP suggests a change in grain size. SP deflections can indicate depositional sequences, where either sorting, grain size or cementation change with depth and produce characteristic SP shapes. These shapes are referred to as bells, funnels, or cylinders .
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  Factors Affecting theSP Response Hydrocarbons: reduce the SP deflection Shaliness: reduces the SP deflection Bed thickness: thin beds do not develop a full SP deflection Permeability: low permeability zones will have a very high invasion diameter, so it may be impossible to read the Junction Potential, hence SP readings may be low 37
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  Passive Log Correlation GR,SP, and Caliper • Often correlate • Different measurements • Different reasons Correlation helps • GR instead of SP in oil base mud • Easier detection of shales • Facilitates “zonation” 38
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  Zonation Zonation - Definesintervals of similar properties Purpose • Well-to-well correlation • Evaluation of specific intervals Criteria • Lithology • Fluids • Porosity and permeability  Begin with coarse zonation oTypically • Well-to-well correlation 20 - 100 ft • Detail evaluation 10 ft thick or more oEasy lithologies first, e.g., shales Refine oMore subtle lithology changes oFluids in porous, perm intervals oDepends on measurements available 39
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  Well QATIF-46 Zonation andRw Effect 40
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  Limitations of SPLog The SP cannot be recorded in air or oil-base muds, since there is no conductive fluid in the borehole. Conductive mud is essential for generation of a spontaneous potential. In salt-mud, SP tends to be straight line (less salinity contrast). If bed is too thin, the full SP will not develop. Chart exist to correct for this effect, but only significant for bed thickness < 20ft. Hydrocarbon and shale in the formation reduce SP development. 41