The Application of Fractal Dimension on Capillary Pressure Curve to Evaluate the Tight Sandstone

Abstract

The rock of gas tight reservoir is more heterogeneous than that of conventional sandstone reservoir and is more prone to water-blockage because of the invasion of operation fluid. This paper presented a new approach for the analysis of the capillary pressure curve for tight gas reservoir. Herein, the saturation equation with fractal dimension proved the previous observation that the log-log plot of capillary pressure against water saturation is a straight line, which is quite different from the popular observation by Corey's correlation. How to transform the capillary pressure curve to relative permeability curve was also discussed with fractal dimension. The fractal dimension of capillary pressure, which is not only an indication of heterogeneity, can also reveal the potential water blocks in tight gas wells. If the rock has higher fractal dimension, being at the same water saturation, the capillary pressure will be higher and the relative permeability of water will be smaller, which means higher injection pressure is required to displace the trapped water in reservoir. It is suggested that for the tight gas pay zone with higher fractal dimension, more precautions should be taken to prevent the water trapping during drilling or stimulating operation.

Variable Dimension Fractal-Based Conversion Method between the Nuclear Magnetic Resonance T2 Spectrum and Capillary Pressure Curve

“…The existing studies show that fractal dimensions determined by the NMR T 2 spectrum and capillary pressure curve are considered as constants or segmental constants. − When there are several types of pores with different geometries in the rock system, the fractal dimension of segments must be used to accurately describe the changes of pore geometry. Assuming that the change rule of pore geometry is related to the pore size, then, the change rule of the fractal dimension is related to the pore size, and then, the pore structure of the rock meets the variable dimension fractal theory.…”

“…In the existing studies, fractal dimensions determined by the NMR T 2 spectrum and capillary pressure curve are considered as constants or segmental constants. − However, the constant and segmental constant fractal dimension cannot accurately reflect the pore geometry difference between large pores and small pores. Hence, it is necessary to study a fractal-based conversion method with a pore structure that considers a fractal dimension coefficient.…”

Investigating the Effects of Pore-Structure Characteristics on Porosity and Absolute Permeability for Unconventional Reservoirs

“…16 Through the fractal method, Guo et al suggested a straight line in the log−log plot of capillary pressure versus water saturation. 17 and tortuosity. 18 They further examined the relationship between fractal dimensions and pore structure parameters.…”

“…Miao et al proposed analytical models for seepage characteristics and extended the methodology to analyze the seepage characteristics of slug flow in fractal porous media . Through the fractal method, Guo et al suggested a straight line in the log–log plot of capillary pressure versus water saturation . This straight-line capillary function is quite different from the widely used Corey’s correlation.…”