According to the well logging data, after dividing the reservoir layers from the drilling geological section, it is further necessary to determine the oil and gas properties of each reservoir layer, which is to divide the oil, gas and water layers. There are two methods.
1) Well logging parameter curve overlay method (referred to as overlay method): Its characteristic is that it uses unified parameters (such as porosity, resistivity, etc.), unified lateral scale and unified baseline to draw Two (or more than two) logging parameter curves (measured curves or calculated curves) are used to evaluate the saturation properties of the reservoir according to the relationship between the drawn curves (coincidence or separation: positive amplitude difference or negative amplitude difference). The advantage of this method is that it is fast and intuitive, and can be used to explain the entire well section (or interpretation section); the disadvantage is that it is not conducive to the analysis of various influencing factors, especially the analysis of the influence of mud content.
2) Logging parameter cross plot method (referred to as cross plot method): Its characteristic is to cross two or three logging parameters that reflect the characteristics of oil, gas, and water from different angles, and Construct a cross diagram according to the well logging interpretation formula. The saturation properties of each reservoir are evaluated based on the distribution pattern of the data points representing each reservoir on the cross diagram and the graphic display characteristics of the cross diagram. The advantage of this method is that it is conducive to the analysis of various influencing factors, it is easy to find some problems in data quality, and it is also convenient for manual interpretation; its disadvantage is that it cannot analyze the entire well section (or interpret the well section), and it is possible Some hydrocarbon-bearing formations are missed.
Figure 6.7 is the processing result of a well using the dual porosity method. The reservoir has three sand layers. The ? and ?w curves of the lower two sand layers basically overlap, indicating that they belong to the water layer; while the upper sand layer Layer ?＞?w is interpreted as oil layer or gas layer.
Figure 6.7 Dual porosity overlay diagram [4]
The movable water index method is based on the water wettability of the gas reservoir rock and the reservoir above the gas-water interface. The porosity and water saturation have an approximately hyperbolic relationship (Figure 6.8) used. The irreducible water saturation Swi can be obtained by fitting the hyperbolic formula according to the actual gas layer ?-Sw relationship curve.
Figure 6.8 Example of water layer division using the movable water index method [3]
According to the relationship between porosity, resistivity and water saturation, the cross diagram can be Different areas are divided to distinguish oil, gas and water layers, and when conditions are favorable, more accurate water saturation values ??can be obtained. There are two commonly used methods, namely the special coordinate system (Figure 6.9) and the double-logarithmic coordinate system intersection diagram method (Figure 6.10).
Figure 6.9 Resistivity-Porosity Cross Diagram in Special Coordinate System [4]
Figure 6.10 Resistivity-Porosity Cross Diagram in Double Logarithmic Coordinate System[4]
When there is no well logging in the flushing zone or the quality of the logging data in the flushing zone is poor, shallow detection resistivity can be used instead of flushing zone resistivity. Figure 6.11 is an example of a cross plot of deep and shallow three lateral resistivity of a well in an oil field in my country.
Figure 6.11 Example of three-lateral resistivity cross plot of depth and shallowness [4]