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Basic Petrophysics

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Basic Petrophysics

  1. 1. Day Two Basic Petrophysics
  2. 2. Petrophysicist Role • Petrophysics is the study of rock properties and their interactions with fluids (gases, liquid hydrocarbons, and aqueous solutions). • Petrophysicist is considered to be responsible for integration of all available data from wireline logging together with core analysis data in order to generate a complete picture for the reservoir in hand. • Petrophysicist is responsible for generating field study based on wireline logs to be provided further for the geologists and reservoir engineers to work on the static and dynamic modeling.
  3. 3. Archie Equation • Archie Equation is the most famous equation used by petrophysicists to determine the water saturation in the reservoir in hand. • Considered to be effective in clean reservoirs. • Used by petrophysicists and log analysts in manual interpretation to have a quick look evaluation. • Presence of any shale content within the reservoir decreases the reliability of the water saturation calculated by Archie Equation. • Using Archie in shaly reservoirs of high resistivity generates high water saturation values should be corrected further on the CPI log.
  4. 4. Extracting Archie Equation • Resistivity Index: • Considering Rt is the true resistivity of the reservoir, and Ro is the resistivity of the reservoir fully saturated with water, RI = Rt / Ro, Where RI is the resistivity Index. • If the reservoir is fully saturated with water, then the true resistivity should be the same as the Ro value. (SW)n = Ro/Rt (SW) n = 1 / RI (n) Is the saturation exponent
  5. 5. (SW)n = 𝑹 𝒐 𝑹𝒕 = 𝑭 𝑹𝒘 𝑹𝒕 Extracting Archie Equation • (F) The Formation Factor of the reservoir is characterized by two factors, the tortuosity and the porosity. F = 𝒂 𝝓 𝒎 Where (a) is Tortuosity Factor, (𝝓) is the porosity and (m) is the saturation exponent Where (Rw) is the water resistivity And (Rt) is the true resistivity Resistivity of Brine Water Ro = F * Rw The saturation equation will be
  6. 6. Extracting Archie Equation (SW)n = 𝒂 ∗𝑹𝒘 Ф 𝒎 ∗ 𝑹𝒕 • The generalized equation that had come from study of 152 core of oil producing sandstone reservoirs indicated that (m) = 2, (n) = 2 and (a) equal (0.81) in sand with porosity higher than 0.15 and (1) in carbonate and low porosity sand, while the other parameters are all could be detected from the logs, then finally the following general equation has been the standard Archie Equation. (SW)2 = 𝒂 ∗𝑹𝒘 Ф 𝟐 ∗ 𝑹𝒕 Or SW = 𝒂 ∗𝑹𝒘 Ф 𝟐 ∗𝑹𝒕 Simple Archie Equation Archie Equation
  7. 7. Humble Equation • Humble Plot was the first graphical attempt to solve the water saturation equation. • This equation was extracted based on core plugs, where F vs. ϕ were plotted on a chart, and the following formation factor parameters were obtained, F = 0.62 / ϕ2.15 • This Equation later simplified with the known formation factor, which is extensively used in sandstone formations F = 0.81 / ϕ2
  8. 8. Humble Equation (a) Values (m)Depends on the slope of the line.
  9. 9. Archie Parameters Rock Grains Porosity (Ф) (a) Is the tortuous Path between grains
  10. 10. Determining the Archie Equation Parameters • Total Porosity: The ratio of the volume of all pores to the bulk volume of the rock. (ϕt = Vp/VB) • Effective Porosity: The ratio of only the interconnected pores to the bulk volume of the rock. • (m) is the cementation exponent that differs according to the degree of cementation. • (n) is the saturation exponent that is determined from special core analysis. • (a) is the tortuosity factor depends on the path that the fluid will flow through.
  11. 11. Determining the Archie Equation Parameters Bulk Volume of the Rock Rock Matrix Porosity Fluids Bulk Volume = 1 VM+Φt =1 VM = 1-Фt BVW = Sw* Ф
  12. 12. Determining the Archie Equation Parameters Shale Volume What is the difference between water saturation (Sw) and Bulk Volume of Water (BVW)? Rock with shale content
  13. 13. Water Saturation and Bulk Volume Water • Water saturation is the amount of water in relation to the pore spaces only. • Bulk Volume of Water is volume of the water in relation to the total bulk volume of the rock. Sw= BVW/ϕeffФT = VP / VB BVW = Sw * ϕeff
  14. 14. Example • A rock with true resistivity around 20 ohm.m has been encounteredin Bahariya Formation in Western Desert with porosity ranging from 0.20 to 0.25, Determine the water saturation within this rock if the water resistivity is 0.035 ohm.m?
  15. 15. • A sandstone rock with 30% porosity and 10 ohm.m resistivity, What is the water saturation of this rock if the water resistivity is 0.12 and derive the bulk volume of water? Example
  16. 16. How to detect the lithology and the zone of interest?
  17. 17. Log Example RHOB NPHIRXO Sand L.St Dol. Anhydrite. ????
  18. 18. Detecting the Lithology from the D-N Crossplot Ρb = 2.4 g/cc ФN = 0.20 Ρb = 2.38 g/cc ФN = 0.165 0.20 L.St. 0.20 L.St or S.S ??
  19. 19. Calculating Porosity in the Formation • In order to calculate the porosity of the formation, it is necessary to have an idea about how to detect the lithology, how to read the logs, and the most important thing, How to feel the wireline logs and visualize the formation? • Moreover, the log analyst as well as the petrophysicist should have an idea about the equations that will lead him to give a quick look evaluation for the reservoir in hand.
  20. 20. Calculating Porosity in the Formation • Calculating porosity from the density curve ϕD = STUV W SX STUV W SY 𝜌mat is the matrix density S.S = 2.65 L.St = 2.71 Dol. = 2.847 Bulk Density Porosity From Density Fluid Density Fresh = 1 Salt = 1.1-1.2
  21. 21. Calculating Porosity in the Formation 27.5 % Determine the porosity from density curve if the ρb curve reads around 2.4 g/cc?
  22. 22. Calculating Porosity in the Formation • Calculating porosity from the Sonic curve ϕS = `VTUV W`Vabc `VTUVW`VY 𝛥𝑇mat is the matrix Transit Time S.S = 55.5 µs/ft. L.St = 47 µs/ft. Dol. = 43 µs/ft. Sonic Reading Porosity From Sonic • Primary Porosity Could be calculated from sonic curve in conjunctionwith the density and neutron curves. Fluid Transit time Fresh = 189 µs/ft. Salt = 185 µs/ft.
  23. 23. Calculating Porosity in the Formation 10.7 % Determine the porosity from sonic curve if the 𝛥tlog curve reads around 70 µs/ft.?
  24. 24. Calculating Porosity in the Formation • Calculating porosity from the Neutron curve ϕN = Reading of the neutron curvePorosity From Neutron • The higher the porosity of the formation, as well as the hydrogen content, the higher the neutron curve value. • Interpret the high value of the neutron curve in the shale beds?
  25. 25. Calculating Porosity in the Formation 26 % Determine the porosity in sandstone from neutron curve if the ФN curve reads around 18% in limestone scale?
  26. 26. Calculating Porosity in the Formation • Calculating the total porosity from the curves within hand φT = ϕklϕmlϕn o Total Porosity φT = ϕklϕn p Or Or φT = ϕklϕn p For Gas Intervals
  27. 27. Example • In the following log example, determine the interested zones, calculate the porosity, detect the resistivity, then determine the water saturation within the zones of interest if the water resistivity of the same formation was detected from water sample and the results were around 0.03 ohm.m?
  28. 28. Log Example RHOB NPHIRXO
  29. 29. Detecting the Water Resistivity • Water resistivity is the most important parameter in Archie equation, and plays very important role in the equation. • Inaccurate water resistivity value results in totally unreliable water saturation and wrong true bulk volume of water. • Water resistivity could be determined through different equations and methods, the most easiest and fastest one is from the porosity with resistivity value in the clean water zone.
  30. 30. Methods of Detecting the Water Resistivity • Form Archie Equation, in a clean 100% water interval with high porosity. • If the cementation exponent is known from core analysis, this will typically enhance the accuracy of water resistivity value. Otherwise use the standard (m) value as usual. Rw = ФT ∗qV U
  31. 31. Methods of Detecting the Water Resistivity • Form Picket Plot, which is considered to be an integration between the reversed Archie Equation and the crossplot. • This crossplot has to be in coherent with the value calculated from the reversed Archie Equation (more or less the same). Rw = ФT ∗qV U
  32. 32. Methods of Detecting the Water Resistivity • Form Picket Plot, which is considered to be an integration for the reversed Archie Equation with crossplot. Rt
  33. 33. Methods of Detecting the Water Resistivity • From a water sample salinity value measured in the lab. This is considered the best method. • Water resistivity calculated from the logs using true resistivity. There could be some errors due to the influence of the mud on log readings. • Analogues if no water zone available in the well.
  34. 34. Methods of Detecting the Water Resistivity
  35. 35. Methods of Detecting the Water Resistivity • From SP Log, we can determine the Water Resistivity value using the following equation • Kc = Temperature coefficient = - (61 + 0.133 T degF) • Rmfe= Resistivity of mud filtrate (When Rmf @ 75° F > 0.1, then Rmfe = Rmf x 0.85. • When Rmf @ 75°F < 0.1, use Schlumberger Chart SP-2 to find Rmfe. • SSP = Static Spontaneous Potential. SSP = -Kc Log qTYr qsr
  36. 36. Methods of Detecting the Water Resistivity
  37. 37. Methods of Detecting the Cementation Exponent Value • Form Picket Plot, (m) is the slope of the line that detecting the Rw value.
  38. 38. Methods of Detecting the (m) Value • From Special Core Analysis. • A log-log plot of F vs. ϕ yields a straight line that can be extrapolated to ϕ =1 to find (a) value. The slope of the line is –m (Due to that Log ϕ always will be with (– value) . • A best fit line drawn through the points intersects the representing ϕ =1. at a value of 1.4 then a=1.4 .The slope of the line is -1.8, so m=1.8. • (a) value is detected at the point of the intersection between the drawn line and (X = 1) a value m= -Y/X
  39. 39. • These methods were discussed extensively and finally it is found that in order to use a specific (m) value but the standard one, there must be a core taken in the field under observation. • Different (m) values affect the water saturation dramatically and lead to completely unreliable petrophysical interpretation, while difference in (a) or (n) values does not affect the water saturation as much as (m). Methods of Detecting the (m) Value
  40. 40. • References: • SLB log interpretation principles & application • SLB Chart book 2009 • Zaki Bassiouni, Theory,Measurement,and Interpretation of Well Logs
  41. 41. Thank You