Mercury injection fractal characteristics and reservoir evaluation of heterogeneous sandstone reservoirs

Key words:  mercury injection curve  fractal model  reservoir evaluation  comprehensive fractal dimension  Baiyun Sag

Abstract:

By processing the mercury injection curve, the fractal geometry method and corresponding fractal dimension value can be used as a quantitative parameter to evaluate the physical properties of the reservoir. In previous studies, the fractal model was often calculated in two sections, but the corresponding geological meaning was unclear. In addition, the pore and permeability characteristics of rocks could not be expressed as a whole. In some studies, the concept of comprehensive fractal dimension has been applied, but the calculating weighted value lacks their theoretical support. An improved comprehensive fractal model is proposed. The new model was applied to process the mercury injection data of group 15 samples in Baiyun Sag of the Pearl River Mouth Basin. Combined with physical parameters such as porosity and permeability, the geological implications and influences of fractal dimension were analyzed. The results show that:the goodness of fit between mercury saturation and capillary pressure is generally greater than 0.85 in the logarithmic coordinate system, and the curve has fractal characteristics. The fractal dimension D₁ represents the degree of homogeneity of large pores, which is not directly related to reservoir physical properties. The fractal dimension D₂ is mainly affected by the pressure of inflection point. When calculating the comprehensive fractal dimension Dp, D₁ is used to represent the degree of homogeneity of pores. The weight of D₁ is controlled by the displacement pressure. Both D₁ and D₂ represent the contribution of large pores to the physical property of reservoirs. D₂ and its weight represent the contribution of small and medium-sized pores to physical properties, and D_p has a good correlation with physical properties parameters.