Pore structure characteristic parameter is a quantitative index to describe the pore structure characteristics of rock. Normal distribution method and moment method (also known as geological mixed empirical distribution method) are commonly used to determine quantitative characteristic parameters of reservoir rock pore structure, such as average pore throat, pore sorting coefficient and skewness, which have been widely used in oilfield development.

For some sandstone reservoirs with high porosity and permeability and low original intergranular porosity, the pore throat size of most reservoirs obeys normal distribution, so the normal distribution method can be used to determine the characteristic parameters of rock pore structure. However, the porosity and permeability of low permeability sandstone reservoirs are very small. Under the action of various diagenesis and tectonic stress, rocks have complex pore structure, including flaky micro-cracks, and most of them have no normal distribution law. It is unrealistic to study the pore structure of low permeability reservoir with a single pore throat size distribution model, and it is unreasonable to determine the pore structure characteristic parameters of low permeability reservoir with normal distribution method. The moment method considers the influence of diagenesis and epigenesis on the pore structure of rocks, and describes the characteristic parameters of pore structure by using the digital characteristics of geological mixed experience distribution. Its principle and treatment method are in line with the actual situation of pore structure of low permeability reservoirs (Changcheng Wang et al., 2003). Therefore, the moment method should be a reasonable method to determine the pore structure characteristic parameters of low porosity and low permeability reservoirs in southeast Sichuan.

This method divides the observed values of pore throat radius and mercury saturation into 10 ~ 15 according to the throat size, and summarizes the information about the observed values into a mathematical method that can be processed by mathematical language. The important mathematical characteristic parameters of rock pore throat are as follows.

(1) average

Mean value is one of the position characteristic parameters, which describes the average position of experimental data. For the pore structure of reservoir rock, it represents the average position of the whole pore throat distribution. The average value can be obtained by the weighted average value of the observed values, that is

Identification and prediction of complex reservoirs

Where xi is the initial value of the interval (median and final value are also acceptable), and the reservoir rock is represented by φ value; △Si is the observed value.

(2) Standard deviation (σ)

The standard deviation is a discrete characteristic parameter, which describes the dispersion degree centered on the mean value-X. In the study of pore structure, the standard deviation is used to describe the sorting degree of pore throat size, which can also be called pore throat sorting coefficient (Sp). The standard deviation can be used to describe the dispersion degree of experimental data on the whole number axis. For pore system, the better the pore throat sorting, the smaller the sorting coefficient. The standard deviation σ can be expressed as:

Identification and prediction of complex reservoirs

(3) coefficient of variation (c)

Coefficient of variation is the ratio of standard deviation to average, and it is a very useful measure of relative variability of observed values. Used to describe the comparison between average pore throat and sorting degree. If the average pore throat (φ value) is large (there are many pores), the sorting is good (all pores) and the C value is small. In a certain range, the value of C can reflect the pore structure of reservoir rocks. Generally speaking, the greater the C value, the better the pore structure of reservoir rocks. The coefficient of variation is expressed as:

Identification and prediction of complex reservoirs

(4) Skewness

Skewness is one of the characteristic parameters of distribution. It is a measure of distribution asymmetry, which can also be called skewness. Skewness indicates whether the distribution is biased towards large holes or small holes relative to the average value, generally between +2 and -2, which can be expressed as:

Identification and prediction of complex reservoirs

2.2. 1.2 Determination of lower limit of reservoir throat

The pore structure of carbonate rocks is not only affected by the epigenetic change of diagenesis, but also by the development of fractures of different scales, which makes its classification evaluation and determination of lower limit more complicated.

In an oil and gas reservoir, there are reservoir rocks with different lithology on the continuous oil and gas profile. At this time, it is difficult to completely divide reservoir rocks into two types: reservoir and non-reservoir. Because the pore space of reservoir rocks is very uneven, especially carbonate oil and gas reservoirs. A simple method is to use the throat size as the lower limit standard of reservoir rocks.

According to the capillary pressure data of the sample, the contribution of throat radius to permeability is calculated. When the cumulative contribution of permeability reaches 99.9%, the corresponding throat radius is equivalent to the critical throat radius where fluid is difficult to flow in rocks (Ye Qingquan et al., 2002). At this time, throat radius less than this value has little significance for permeability. Therefore, the pore throat radius corresponding to the cumulative contribution of permeability of 99.9% can be regarded as the lower throat limit Rc.