Pore structure exponent of Archie's law in a dual-porosity medium: Vuggy reservoirs

The porosity exponent (m) is one of the most critical parameters of Archie's equation and plays a significant role in quantitative evaluation of petroleum reservoirs. In vuggy carbonate reservoirs, it is difficult to separately quantify the matrix pores and vugs in natural core samples; consequently, how the matrix pores and vugs influence m remains unclear. In this study, a novel method was developed for preparing artificial vuggy core samples with specific values for matrix and vug porosity. The optimal preparing parameters of vuggy core samples, for the material to be consistent with the porosity and permeability of actual core samples, were as follows: 0.25 mm dolomite particles and 0.18 mm glass beads as the rock skeleton, epoxy resin as the binder, 10 mm sodium thiosulfate to form the vugs (compacted and gasified), a component ratio of 1:3:3, and a compaction pressure of 20 MPa. Next, petrophysical and resistivity experiments were carried out to obtain the m and porosity value of 15 artificial vuggy cores. Based on a comprehensive analysis of the artificial core samples, actual core samples, and porosity model, the response of m to matrix and vug porosity was modeled further. The key results are as follows: (1) the larger the portion of vugs to total porosity (Φv/Φ), the greater the increase in m with a range from 2 to 3.5 was found; (2) with the increases of Φ, m increases first and then decreases, where the inflection point of artificial and actual samples is in the range of Φ ≈ 18%–20%; (3) the trend of variation of m with Φ is mainly controlled by Φv/Φ or Φm/Φ rather than Φv or Φm individually. What was revealed in this study, can provide theoretical support for using Archie's equation in carbonate reservoirs.