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geophysical_logs4.ppt

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This document discusses quantitative evaluation procedures to determine hydrocarbon saturation levels within a formation using well log data. The formation factor F is defined as the ratio of the resistivity of the total saturated formation to the resistivity of the water alone. This factor along with other resistivity measurements can be used to calculate the water saturation Sw within the formation. The Archie equation is presented as a way to estimate formation factor based on porosity. Threshold ratios of apparent water resistivity to true water resistivity are given to indicate hydrocarbon potential, above 4 suggesting oil production. An example output table is provided to calculate and display these values for evaluation.

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Quantitative evaluation Examine procedures in which data from 2 or more logs will be used to evaluate for the amount of hydrocarbon compared to pore water within a formation The proportion of pore space occupied by water is known as water saturation (Sw) e.g. if 65% of the formation is occupied by water and 35% by hydrocarbons the water saturation is 0.65 (percent water saturation is 65%) Definition of the formation factor (F) F = Ro/Rw Where Ro is the resistivity of the total formation (rock matrix + fluids) saturated with water Rw is the resistivity of the water itself
 F= Formation factor is used to help calculate water saturation – function of porosity  Definition F = Ro/Rw  Definition for water saturation Sw = √RO/RT RO = RT (for a 100% saturated sand)  Substituting in the equations and rewriting for water saturation we have: RO = F. RW SW = F. RW/RT
QUANTITATIVE EVALUATION FOR HYDROCARBON POTENTIAL Archie equation (1942) … F = a/φm a = constant which is lithology dependent m = empirically derived constant φ = porosity (e.g. 30% enter as 0.30) Actual values for a and m are lithology and region dependent Some examples F = 0.82/φ uncompacted sands F = 1/ φ2 compacted formations, chalk F = 1/ φ2.2 compacted clastic and oolitic rocks F = 0.62/ φ2.15 recrystallized carbonates (sucrosic texture) F = 1.13/φ1.73 generic value for large number of samples
Analysis for Hydrocarbon Potential Definition: Rwa = Rt / F  Rwa = apparent water resistivity  Rt is measured from a deeply focused induction log  F is calculated from the formula using log measured porosity value  est Sw (%) = √Rw/ Rwax 100  Rw = Rwa of a 100% water saturated formation. Measure from an obvious water-bearing formation  Rwa Analysis Rwa/ Rw  Ratio Rwa < 2 No oil production  Ratio Rwa > 4 Oil production expected
Hydrocarbon potential Locations SP mv Resistivity ohm-m φ F Rwa est Sw% Rwa analysis Rwa/Rw Hydrocarbon Production y/n sh dp dp in. 1 2 3 4 5 Use F = 0.83 / φ2
READINGS AND CALCULATIONS Levels SP mv Resistivity ohm-m φ F Rwa est Sw % Rwa/ Rw Prod. y/n sh dp dp in. 1 - 62 0.26 0.2 0.20 .30 9 0.02 100 1 N 2 - 45 7.50 6.0 6.70 .06 228 0.03 82 1.5 N 3 - 49 1.50 0.5 0.50 .23 16 0.03 82 1.5 N 4 - 18 1.08 0.8 0.98 .13 49 0.02 100 1 N 5 - 15 0.94 0.8 0.98 .26 12 0.08 50 4 Y

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geophysical_logs4.ppt

  • 1. Quantitative evaluation Examine procedures in which data from 2 or more logs will be used to evaluate for the amount of hydrocarbon compared to pore water within a formation The proportion of pore space occupied by water is known as water saturation (Sw) e.g. if 65% of the formation is occupied by water and 35% by hydrocarbons the water saturation is 0.65 (percent water saturation is 65%) Definition of the formation factor (F) F = Ro/Rw Where Ro is the resistivity of the total formation (rock matrix + fluids) saturated with water Rw is the resistivity of the water itself
  • 2.  F= Formation factor is used to help calculate water saturation – function of porosity  Definition F = Ro/Rw  Definition for water saturation Sw = √RO/RT RO = RT (for a 100% saturated sand)  Substituting in the equations and rewriting for water saturation we have: RO = F. RW SW = F. RW/RT
  • 3. QUANTITATIVE EVALUATION FOR HYDROCARBON POTENTIAL Archie equation (1942) … F = a/φm a = constant which is lithology dependent m = empirically derived constant φ = porosity (e.g. 30% enter as 0.30) Actual values for a and m are lithology and region dependent Some examples F = 0.82/φ uncompacted sands F = 1/ φ2 compacted formations, chalk F = 1/ φ2.2 compacted clastic and oolitic rocks F = 0.62/ φ2.15 recrystallized carbonates (sucrosic texture) F = 1.13/φ1.73 generic value for large number of samples
  • 4. Analysis for Hydrocarbon Potential Definition: Rwa = Rt / F  Rwa = apparent water resistivity  Rt is measured from a deeply focused induction log  F is calculated from the formula using log measured porosity value  est Sw (%) = √Rw/ Rwax 100  Rw = Rwa of a 100% water saturated formation. Measure from an obvious water-bearing formation  Rwa Analysis Rwa/ Rw  Ratio Rwa < 2 No oil production  Ratio Rwa > 4 Oil production expected
  • 5. Hydrocarbon potential Locations SP mv Resistivity ohm-m φ F Rwa est Sw% Rwa analysis Rwa/Rw Hydrocarbon Production y/n sh dp dp in. 1 2 3 4 5 Use F = 0.83 / φ2
  • 6. READINGS AND CALCULATIONS Levels SP mv Resistivity ohm-m φ F Rwa est Sw % Rwa/ Rw Prod. y/n sh dp dp in. 1 - 62 0.26 0.2 0.20 .30 9 0.02 100 1 N 2 - 45 7.50 6.0 6.70 .06 228 0.03 82 1.5 N 3 - 49 1.50 0.5 0.50 .23 16 0.03 82 1.5 N 4 - 18 1.08 0.8 0.98 .13 49 0.02 100 1 N 5 - 15 0.94 0.8 0.98 .26 12 0.08 50 4 Y