Combining pressure-controlled porosimetry and rate-controlled porosimetry to investigate the fractal characteristics of full-range pores in tight oil reservoirs

Tight oil reservoirs have the characteristics of wide pore size range and complex pore system. The understanding of pore characteristics is the basis for studying the accumulation and migration of oil and gas. To better study the fractal characteristics of full-range pores in Lucaogou tight oil reservoir, scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) were used to obtain the pore size. XRD was used to obtain the mineral composition. In addition, fractal analysis was performed on the samples based on MIP data. The results show fracture pore (FP), residual interparticle pore (RIP), dissolution pore (DP) and clay dominated pore (CDP) are the main pore types. The interparticle pores is commonly largest in size, followed by dissolution pores. Quartz and feldspar play dominant roles in the composition in Lucaogou tight oil reservoir. The full-range pore size distribution (PSD) curves ranged from 3.6 nm to 400 μm with two main distribution intervals. The fractal dimensions (D1, D2 and D3) reflect the complexity of CDPs, DPs and RIPs, respectively. The average values of fractal dimensions were 2.88, 2.8 and 2.56, the smaller pores have more complex pore system. The pore structure of clay dominated pores is most complex. The more complex the pore structure, the worse the reservoir performance. Quartz shows negative correlation with D3, Clay shows positive correlation with D1, and feldspar is positively correlated with D2. D1, D2 and D3 are negatively correlated to porosity, D1 shows a strong negative correlation with permeability.