Methods for evaluating reservoir oiliness include qualitative methods that rely on the experience of interpreters; rapid intuitive interpretation methods; and computer interpretation methods.
◎Qualitative interpretation methods: oil and gas layer minimum resistivity method, standard water layer comparison method, radial resistivity method, adjacent well curve comparison method, well log curve comparison method at different times (also called time Push logging method) etc.
◎Quick and intuitive explanation methods: intersection drawing method, curve overlapping method, etc.
◎Computer interpretation method: With the widespread application of computers, the quantification of well logging interpretation has made great progress. For pure sandstone and argillaceous sandstone (including dispersed mud, layered mud, etc.), their own interpretation models have been formed, and many interpretation procedures have been established. These research results and quantitative explanations of porosity, water saturation, permeability, etc. have laid a good foundation for accurate judgment of oil, gas, and water layers. The following focuses on explanation models in fast intuitive explanation methods and computer explanation methods.
(1) Cross-plot method
1. Resistivity-porosity cross-plot
Resistivity-porosity cross-plot is a method that applies Archie’s formula A commonly used quick and intuitive explanation technique. It is characterized by its intuitive image, which can not only qualitatively distinguish oil, gas, and water layers, but also semi-quantitatively determine the water saturation Sw. Convert Archie's formula:
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to:
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For a specific region and lithology For a certain interpretation layer, the coefficients a, b and exponents m, n can be regarded as constants. If the lithology and Rw are basically unchanged, then for a given water saturation Sw, there is a linear relationship.
It can be seen from the above formula that the horizontal axis φ of the cross plot can be on a linear scale, or can be replaced by the reading of any porosity log (such as △t), while the vertical axis Rt should be on a m scale. Figure 5-5 is made for the sandstone reservoir with a=0.62, b=1, m=2.15, n=2. Drawing method: The origin Y=0 corresponds to Rt=∞. The upper limit of the vertical axis is determined by the lowest resistivity of the reservoir. In this example, Rmin=0.5Ω·m, then Y=1.38, that is, the distance from Rt=0.5 to the origin Rt=∞ is 1.38 units. When Rt=1Ω·m, Y=1, then the distance from Rt=1 to the origin Rt=∞ is 1 unit. Other resistivity scales can be obtained in the same way.
The right side of the picture is an I-Sw slide rule (the left scale is I, the right scale is Sw). The ruler is parallel to the ordinate axis. The resistance increase coefficient I corresponding to the origin of the ordinate axis is ∞, and I=1 should be aligned with an integer of resistivity, which corresponds to a certain Rw and a certain rock with a water saturation of 100%. The resistivity value, other I values ??are calculated and marked according to Rt=I×R0. The numerical value of Sw is marked according to the I-Sw relationship (different a, b, m, n values ??intersect the plot with different scales, but the principle is the same).
After the above work is completed, the data of each reservoir in the interpretation well section will be marked on the resistivity-porosity cross plot. Then find water layers with pure lithology, sufficient thickness, reliable well logging readings, and no oil and gas indications. The water line (the line with 100% water saturation) should pass through these pure water layer points and the origin (porosity is zero). the horizontal axis point) of the straight line. The waterline thus determined should be verified with water data and other reliable data before use.
Figure 5-5 Resistivity-Porosity Intersection Plot
After determining the correct position of the water line, use the I-Sw slide rule on the right to draw the water saturation Sw line. Method: Find the point I=1 on the waterline, draw a vertical line through this point on the horizontal axis, and have an intersection point with a vertical line drawn on the vertical axis through a Sw point on the I-Sw slide rule. The straight line passing through the intersection point and the origin is the line with water saturation Sw. In this way, a set of Sw lines can be obtained.
After the water saturation line is drawn, its oil content can be visually judged based on the position of the interpretation layer data points on the cross plot, and the Sw value can also be obtained semi-quantitatively. Generally, the data points fall below the Sw=50% line, φ (porosity) gt; 10% of the reservoir is an oil layer, and φ≤10% of the reservoir is a dry layer.
As shown in Figure 5-5, layers (4), (5), (9) and (10) are oil layers, layers (3) and (7) are dry layers, and layers (4), (5) and (9) are , the Sw of (10) layer are 43%, 47%, 26%, and 21% respectively.
The derivation methods of resistivity-porosity cross plot include: resistivity-sound wave cross plot, resistivity-density cross plot, etc. The conditions for the use of all these cross-plots are stability, the same lithology, less mud, and a sufficient number of water layers, and the porosity of the water layer preferably has a large variation range.
2. Rwa (apparent formation water resistivity) -SP (natural potential) cross plot
In the sand and mudstone section formation, if the formation water salinity changes greatly, the formation water The resistivity Rw is not easy to determine, which makes it difficult to judge the oil and water layers. In this case, the apparent formation water resistivity Rwa-SP cross plot can be used to estimate Rw and divide the oil and water layers.
The Rwa-SP intersection plot is shown in Figure 5-6. The logarithmic Rwa is the ordinate and the linear scale SP is the abscissa. Rwa is calculated from the deep sounding resistivity (Rwa=Rt/ F). In the figure, there are layer point numbers marked next to the formation points, and the GR value in API units is in parentheses. The Rwa line and Rw line (dashed line) are the interpretation reference lines, and Rmfe is the equivalent resistivity of the mud filtrate.
Figure 5-6 Rwa-SP intersection plot (T=150℉, Rmfe=0.7Ω·m)
The lowest points in the intersection plot are connected in a straight line (in the figure The dotted line) is the actual formation water resistivity line, which can be used to estimate the Rw of the interpretation layer. For example, the vertical axis parallel line drawn from the 14th layer point has an intersection with the Rw line, then the ordinate value of the intersection point is Rw=0.35Ω·m of the 14th layer.
The formation points located near the Rw line in the figure are water layers, such as formation points 2, 6, 7, 9, and 15 in the figure; the formation points above and far away from the Rw line are oil-bearing layers. Gas strata, such as stratigraphic points 14, 3, 5, and 11 in the figure; the remaining stratigraphic points need to be comprehensively analyzed. Coring from the well wall proved that there is oil at formation points 3 and 5, and gas at formation points 11 and 14.
Rwa-SP cross-plotting is suitable for sand and mudstone cross-section formations with large changes in formation water properties. It requires the reservoir to be relatively pure, because only when the mud content is small, the changes in SP in the cross plot can be considered to be mainly caused by changes in Rw.
The above are two commonly used intersection drawings. There are many other intersection drawings, so I won’t list them all here.
(2) Curve overlap method
The curve overlap method is also based on Archie's formula, generally using the same scale (same unit), the same baseline and the same horizontal ratio , the two curves are drawn together to form an overlap, and the oil and gas content of the reservoir is identified based on the difference in amplitude of the curves.
1. R0 overlaps with deep detection resistivity
For any formation with relatively pure lithology, whether it contains oil and gas or pure water, it can be determined by F- Use the φ relationship to determine the resistivity when the water saturation is 100%:
R0=FRw=aRw/Фm
In the formula: R0——when the water saturation is 100% Formation resistivity, Ω·m; Rw - formation water resistivity, Ω·m; φ - formation porosity, decimal; a - constant related to rock properties; m - cementation index.
By overlapping and drawing R0 and Rt together, the oil and gas layer can be identified based on the amplitude difference between the two curves.
If the R0 curve of the reservoir basically coincides with the deep detection resistivity curve (Figure 5-7, lower part of the figure), it means it is a water layer; if the deep detection resistivity value Rt is significantly greater than R0, such as Rt /R0≥3~5, it indicates obvious oil and gas content (Figure 5-7, upper part of the figure).
2. Radial resistivity overlay method
According to Archie’s formula (b=1, n=2):
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Divide the left and right sides of the two equations to get:
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In the formula: Sw - formation water saturation, decimal; Sxo - flushing Zone water saturation, decimal; Rt - formation resistivity, Ω·m; Rxo - flushing zone resistivity, Ω·m; Rw - formation water resistivity, Ω·m; Rmf - mud filtrate resistivity, Ω·m.
The above formula illustrates that the radial resistivity ratio Rxo/Rt is related to the radial water saturation ratio Sw/Sxo. Several situations are discussed below.
Figure 5-7 Overlay map of R0 and deep sounding resistivity
(1) Mudstone layer
The mudstone layer is an impermeable layer, and mud will not Intrusion occurs, so there should be Rt≈Rxo, that is, the Rt curve and the Rxo curve basically coincide. If the Rt curve of the mudstone section does not coincide with the Rxo curve, it will be deemed that there is an error in the Rxo curve. The Rt curve shall prevail and the Rxo curve shall be moved to coincide with the Rt curve.
(2) Pure water layer
The pure water layer is a permeable formation, which will cause mud intrusion, but Sw = Sxo. According to the relationship between mud filtrate and formation water properties, there can be three situations. When Rmf=Rw, it is obvious that Rxo=Rt, that is, the Rxo curve coincides with the Rt curve; when Rmfgt; Rw, then Rxogt; Rt; when Rmflt; Rw, then Rxo lt; Rt.
Figure 5-8 Overlay of radial resistivity curves
As shown in the lower part of Figure 5-8, in this example, the deep lateral resistivity RLLD is the Rt curve, and the micro lateral resistivity RMLL is the Rxo curve, Rmf/Rw=3.0, the lower part of the curve is Rxogt; Rt, so this section is a water layer.
(3) Oil and gas formations
For moderately intrusive formations, there is an empirical relationship
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If Rmf=Rw, then:
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The above formula shows that when Rmf=Rw, the pure water layer Sw=Sxo=1 , then Rxo and Rt coincide; if the oil and gas layer Sw lt; 1, then:
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Compared with water layers, oil and gas layers have relatively obvious drag reduction Intrusion, therefore, shows Rtgt at the reservoir containing oil and gas; the amplitude difference of Rxo can be used as a sign indicating the oil and gas layer (upper part of Figure 5-8).
3. Porosity overlay diagram
“Mobile oil and gas” refers to the oil and gas that can flow in the reservoir under a certain pressure difference. Well logging analysis of movable oil and gas is based on the difference in water saturation between the washout zone caused by mud invasion and the original formation. The difference is the movable oil and gas saturation. Generally speaking, when the well logging shows good oil content and movable oil and gas, it means that the reservoir has good production capacity; but when the oil content shows good but the movable oil shows poor, it should be analyzed carefully.
The movable oil and gas display is actually shown through the comparison of the oil and gas conditions between the original formation and the flushing zone. Therefore, the water-bearing porosity of the intact formation and the flushing zone is calculated separately, using an overlapping form. Can visually display movable oil and gas. Porosity overlap is an integral part of current computer interpretation results. It generally includes three porosity curves: formation porosity φ, original formation water-bearing porosity φw = φ·Sw, and washout zone water-bearing porosity φxo = φ·Sxo. Obviously there is the following relationship between them:
◎Oil and gas porosity: φh=φ-φw;
◎Residual oil and gas porosity: φhr=φ-φxo;
◎Mobile oil and gas porosity: φhm=φxo-φw.
In this way, three porosity curves drawn using the same baseline and the same lateral scale can effectively reflect the oiliness and movable oil and gas of the formation (Figure 5-9).
The favorable conditions for applying the porosity overlay method are: (1) The drilling fluid invasion must be shallow enough so that the deep detection resistivity basically reflects the true resistivity of the formation; (2) The interpretation well section should include many reservoirs. layers, especially those with obvious pure water layers; (3) the formation water properties within the interpretation well section are basically stable; (4) the lithology and mud content should be basically unchanged.
From F=aφ-m, three porosity curves can derive three formation factor curves, namely F curve, Fw curve and Fxo curve. By overlapping these three formation factor curves, we can get the Overlapping has the same effect. Therefore, the overlap of stratigraphic factors and the overlap of porosity are essentially the same.
4. Movable water analysis
“Mobile water” is the formation water that can flow in the reservoir. Movable water saturation refers to the difference between formation water saturation and irreducible water saturation. Using the concept of movable water can help determine whether the reservoir can produce oil and gas without water and predict the water content.
According to the concepts of movable water saturation and bound water saturation, it is obvious that Sw = Swi + Swm. According to the concepts of oil, gas and water layers, the conditions for judging them are:
◎Oil and gas layer: Sw≈Swi, Swm=0, Sw is low;
◎Water layer: Swgt; gt; Swi, Swmgt; gt; 0;
◎Oil and water layer: between oil, gas and water layer;
◎Dry layer: Sw≈Swi, Swm=0, Sw is higher.
Therefore, if you have independent sources of Sw and Swi, you can overlap Sw and Swi to visually display the changes in the movable water saturation of the formation (Figure 5-9): When Swgt; Swi, then two The difference in amplitude of the curves is the movable water saturation. If Swi and Sw basically overlap, it indicates that the formation does not contain movable water. If Sw is lower, it is an oil and gas layer, and if Swi is large, it may be a dry layer. When there is an amplitude difference between Sw lt;Swi, it is caused by the mismatch between the calculated Sw and Swi. At this time, the lower Sw is the oil and gas layer, and the higher Sw is the dry layer.
Figure 5-9 Reservoir movable oil and movable water analysis result diagram
5. Acoustic transit time-neutron gamma curve overlap to qualitatively determine the gas layer
The specific method of this method is: the vertical proportions of the two curves are the same, and the scales are in opposite directions. Find a water layer (or an oil layer with a low gas-oil ratio) that has the same lithology and similar porosity as the target layer in the interpretation well section, overlap the two curves, and draw the two curves overlapping.
Within the detection range, the presence of gas in the reservoir will increase the time difference of logging acoustic waves and increase the neutron gamma logging value. Therefore, on the overlay map, for the gas layer, there will be a "positive difference" (the neutron gamma curve is to the right of the sonic transit time curve); for the oil and water layer, the two curves coincide; for the mudstone, the overlay curve will have a "negative difference" (middle) The sub-gamma curve is to the left of the sonic transit time curve). The overlapping acoustic transit time-neutron gamma curves shown in Figure 5-10 intuitively indicate that layer A is a gas layer.
6. Neutron porosity-density logging curve overlapping method
Scale the two logging curves in the same record track. Since the hydrogen index and bulk density of natural gas are much smaller than those of oil or water, for gas-bearing reservoirs, neutron porosity logs show low porosity and density logs show reduced porosity. The overlay plot will There is an obvious amplitude difference and the image is a mirror image (Figure 5-11). However, the invasion of drilling fluid reduces the amplitude difference, while the opposite amplitude occurs when the muddy sandstone contains gas and water. Therefore, for sandstone gas layers with low mud content, shallow mud invasion, and medium to high porosity, the overlay map of neutron porosity-density log curves has the best application effect.
Figure 5-10 An example of using the sonic transit time-neutron gamma curve overlay method to determine gas layers
Figure 5-11 The overlapping of neutron porosity-density log curves indicates gas layers
(3) Method for calculating oil saturation
Oil saturation is the main indicator of reservoir oiliness and one of the important criteria for quantitatively judging oil, gas and water layers. , so whether the oil saturation can be accurately determined directly affects the judgment of oil, gas and layers.
The water saturation Sw is the percentage of the pore volume filled with water in the reservoir rock pores to the total pore volume. Therefore, 1-Sw is the oil and gas saturation So.
There are various factors that affect water saturation in the reservoir, but the content and distribution form of mud in the reservoir are the most important factors affecting Sw. Different mud distribution forms have different effects on rock resistivity, so the methods of using resistivity to calculate water saturation are also different. The following introduces several methods currently used in conventional well log processing and interpretation programs.
1. Pure rock reservoir
In pure rock formations with uniform intergranular pores, according to the Archie water saturation formula:
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Generally, b=1, n=2, a=0.6~1.5, m=1.5~3.0.
2. Layered muddy sandstone reservoir
If pure sandstone and mudstone are interlayered in the rock layer, then the resistivity Rt of the rock layer and the resistivity Rlam of the mudstone layer and the pure The relationship between the resistivity Rsd of the sandstone layer is:
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In the formula: Rt——the resistivity of the rock layer, Ω·m; Rlam——the resistivity of the mudstone layer, Ω ·m; Rsd——Resistivity of pure sandstone layer, Ω·m; Vlam——relative content of layered mudstone, decimal.
Archie’s formula for pure sandstone layers:
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Where: φsd——porosity of pure sandstone part, decimal (φsd =φ/(1-Vlam), φ is the effective porosity of layered muddy sandstone).
So it can be deduced from the above formula:
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In the above formula, Rlam is usually replaced by the resistivity Rsh of the adjacent mudstone. This formula is applicable to both pure sandstone and layered argillaceous sandstone.
3. Dispersed muddy sandstone reservoir
The characteristic of this kind of reservoir is that mud is filled or bonded in the pore space of the rock, and more bound water is preserved.
For this type of reservoir, there are many methods for calculating water saturation, the main ones are as follows.
(1) "Indonesia" formula
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In the formula: Vsh - relative clay content, decimal; Rsh - clay resistivity ,Ω·m.
(2) Simandoux formula
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This formula is suitable for formation water with low salinity (less than 5000mg/L ) area.
(3) Two-water model formula
This model divides the water in the formation into two types: clay water (bound water) and free water (distal water), and believes that these The conductive properties of the two types of water are different, so the Archie water saturation formula becomes:
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In the formula: Ct——The conductivity of the uninvaded part of the untouched formation rate, mS/m; Cwe—equivalent conductivity of water in pore space, mS/m; φt—total porosity, decimal; Swt—total water saturation, decimal.
The equivalent conductivity of water is:
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Where: Vw, Vwb - are the volumes of formation water and bound water respectively Percentage of pore volume, decimal; Cw, Cwi - are the conductivity of formation water and bound water respectively, mS/m.
When expressed in terms of saturation, the above formula becomes:
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or
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Then the saturation equation becomes:
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The porosity and water saturation of the sandstone (pure formation) phase (i.e. non-clay phase), It can be obtained by subtracting the bound water volume (φt·Swi).
Therefore, the effective porosity is:
Ф=Фt(1-Swi)
The movable water saturation is:
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The parameter φt in the above formula is given by the neutron porosity-density cross plot, Swi can be determined according to various mud-sensitive measurement methods (SP, GR, φN, Rt, φN -ρb, △t -ρb, etc.) are derived. Rwi and Rw (Cwi and Cw) are usually used as input parameters.
4. Determination of m, n, a, b, Rw
(1) Determination of a and m
The a and m values ??of rock and the pores The size of the rock is related to the shape of the pores, and the porosity and pore shape determine the nature of the rock, the thickness of the rock particles, the quality of sorting, the nature of the cement, the cement content and the degree of cementation, etc. 1) The laboratory determines the F-φ relationship
By taking the logarithms on both sides of the formula, then:
lg(Ro/Rw)=lga-mlgФ
By A set of experimental data of F (Ro/Rw) and φ, on logarithmic graph paper, the relationship is a straight line. When φ=100%, the value of the straight line on the ordinate is a, and the slope of the straight line is m. For example, m=1.8369 and a=1.1466 obtained by a laboratory in a certain development zone.
2) Determine the F-φ relationship using pure water layer data
Select a pure water layer in which there are several relatively pure lithology and physical properties (mainly referring to Porosity) has changed and the well logging does not show the reservoir. Through analysis of logging data such as natural potential, it was confirmed that the formation water resistivity in this section is stable. The most ideal thing is to have formation water salinity analysis data in this interval. Determine Ro/Rw and φ of each layer within this layer section, and use the above method to determine a and m.
(2) Determination of b and n
1) Laboratory method of determining b and n
Take the logarithm of both sides:
lg(Rt/Ro)=lgb-nlgSw
Given several pairs of Rt/Ro, Sw data, use regression method to find b and n. The laboratory can use different methods to determine Rt/Ro and Sw. The early methods used were "water loss method" and "air blowing method". Currently, the "semi-permeable partition method" is a better method.
2) Use the oil layer logging data to calculate b and n.
The sum of the oil saturation and irreducible water saturation of a pure oil layer is 100%. That is:
Swi=(1-Sh), Sw-Swi
Under the condition of knowing Rw, find a set of Rt, φ. The oil layer Ro is obtained from Rw and φ. Place the Rt/Ro and Swi points of a set of oil layers on the double coordinates, draw a straight line according to the point distribution rule, and obtain the slope n of the straight line and the ordinate intercept b.
3) Use the oil saturation of core analysis to find b and n
Use oil-based drilling fluid and sealed core cores to obtain the core oil saturation and water saturation, Use the above method to find b and n.
(3) Determination of formation water resistivity Rw
The formation water resistivity in the saturation equation can be determined by the following four methods.
1) Convert the formation water salinity obtained from oil testing
Convert the formation water salinity obtained from oil testing data into equivalent total salinity, and then determine the well depth conditions The formation water resistivity underneath. The calculation formula is Schlumberger's diagram of the relationship between total salinity, resistivity, and temperature of equivalent NaCl solution. The formula is as follows:
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Where: P——equivalent total NaCl salinity, mg/L; T——formation temperature, ℉; Rw—— Formation water resistivity, Ω·m.
2) Calculate the formation water resistivity by combining porosity and resistivity
Find the standard water layer for determining the formation water resistivity in the interpretation well section. It should be completely water-bearing and rock-solid. For a water layer with uniform properties, low mud content, and sufficient thickness, the calculation formula of formation water resistivity is Rw=Ro·φm/a.
3) Use the natural potential curve amplitude to calculate the formation water resistivity
The diffusion adsorption electromotive force in the well can be expressed as:
For pure sandstone:
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For pure mudstone:
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Where: Ed, Eda - diffusion adsorption electromotive force, mV; Kd , Kda - diffusion adsorption electromotive force coefficient (at t = 18°C, the pure sandstone layer is -11.6mV, the pure mudstone layer is 58mV, and other rock layers are between the above two); Cw - formation water and salt concentration (mineralization degree), mg/L; Cwf——salt concentration (salinity) of the mud filtrate, mg/L.
Static natural potential:
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When the salt concentration in the formation water or drilling fluid is high, the equivalent resistivity Rwe of the formation water is introduced and the equivalent resistivity Rmfe of the mud filtrate, then:
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The above relationship is the theoretical basis for natural potential logging to determine the resistivity of formation water. When there is a problem, first use SP to calibrate the SP-3 plate to get SSP, then use the mud resistivity Rm to get Rmf through the plate, then use the plate to correct it to get Rmfe, and then use SSP and Rmfe to get Rwe from the SP-1 plate. Find Rw from the SP-2 plate.
4) Use the ratio of deep and shallow resistivity to calculate the formation water resistivity
Determine the pure water layer in the interpretation well section. For the water saturation of the pure water layer and the water saturation of the flushing zone The degree is 100%, through Archie's formula:
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Then
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In the past Under the condition that Rmf is known, Ro can be represented by the deep induction (or deep lateral) value, and Rxo can be represented by the eight lateral (or micro lateral) value.