FRACTAL CHARACTERIZATION OF PORE-THROAT STRUCTURE OF TIGHT SANDSTONES

The pore-throat structure is one of the most vital properties of tight sandstones, and it provides a basis for the evaluation of favorable plays. Fractal theory is an effective method for quantifying the regularity, complexity and integrated characteristics of pores and throats in tight sandstones, whereas the pore-throat-related parameters are not well coupled with fractal dimension in a broad range of pore-throat size distribution. To remedy this situation, mercury intrusion technique was conducted to obtain the intrusion/extrusion capillary pressure curves of typical tight sandstones samples, and a multitype voids space model was introduced to assist with distinguishing between the pore-throats of various types and scales. The results show that the tight sandstones can be classified into six types based on the optical observations and mercury intrusion analysis: intergranular pore-dominated, dissolved pore-dominated, poly-genetic pore-dominant, clay-related pore-dominated, throat dominated and microcrack-induced type. The pore-throats derived from fractal dimensions of intrusion and extrusion capillary pressure curves corresponding to three and two pore-throats radius ranges, respectively, namely D_i1, D_i2, D_i3, D_e1 and D_e2. The voids space of the tight sandstones can be subdivided into interparticle-, intercrystalline- and intracrystalline-scale by the combination of microscopic identification and capillary pressure curves with various-segment fractal characteristics on the basis of multitype voids space model. With the tight sandstones from intergranular pore-dominated to microcrack-induced type, the fractal dimensions of interparticle-scale spaces increasingly, while the fractal dimensions of intercrystalline and intracrystalline-scale decrease gradually, which indicating that the heterogeneity of interparticle-scale voids is relatively stronger due to the effect of interstitial materials, whereas that of intercrystalline and intracrystalline-scale voids are gradually decreased on account of intensive mechanical compaction. This work provides insights into targeting the petroliferous intervals for tight sandstones by discerning the large-size and homogenous pore-throats spaces and helps to determine the preferred areas for economic exploitation goals.