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- Title
The Role of Pore‐Shape and Pore‐Space Heterogeneity in Non‐Archie Behavior of Resistivity Index Curves.
- Authors
Sun, Zhonghao; Torres‐Verdín, Carlos
- Abstract
Resistivity index curves describe the relationship between electrical resistivity and water saturation of porous media and are critical in formation evaluation and for the geophysical monitoring of subsurface processes. Archie's second equation enforces a linear relationship between resistivity index and water saturation in log‐log plots which has been widely used for the assessment of in situ hydrocarbon saturation. However, resistivity index curves that deviate from Archie's equation are ubiquitous in subsurface reservoirs, especially complex carbonates exhibiting bimodal pore‐size distributions, where the effects of pore‐scale controlling factors on rock resistivity remain unclear. We implement pore‐network models built under controlled conditions of pore shapes, bimodal pore‐size distributions, pore connectivity, micropore fractions, and anisotropy to investigate the effects of pore shapes and pore‐space heterogeneity on resistivity index curves. Results indicate that percolating wetting films associated with pore shapes decrease the resistivity index at low values of water saturation and cause a decrease in saturation exponent. In cases of bimodal pore‐size distributions, micropore fractions control the connectivity between macropores, thus affecting water drainage and resistivity index curves. At low fractions of micropores, the resistivity index curve is governed by the connected macropore system at high values of water saturation and is then determined by micropores, thereby resulting in non‐Archie behavior. Pore‐size distributions and spatial anisotropy also affect the resistivity index curves. We summarize the observed pore‐space heterogeneity effects on resistivity index curves and compare model predictions to numerical simulations; both the geometric mean model and effective medium theory provide acceptable estimates of the electrical properties of bimodal porous media. Plain Language Summary: Water/hydrocarbon saturation in subsurface rocks can be evaluated via resistivity measurements using electrical resistivity‐water saturation relationships—which are critical for petroleum engineers and earth scientists. The resistivity index is the ratio of the electrical resistivity of a partially saturated porous medium to that of the same medium saturated with electrically conducting water. The resistivity index curves—the relationship between the resistivity index and water saturation—are commonly described by Archie's equation. However, Archie's equation is empirically developed from rocks with relatively uniform pore spaces, and it fails to explain resistivity index curves for many reservoir rocks especially carbonates with complex pore geometries. Mechanisms causing non‐Archie behavior of resistivity index curves remain unclear, thus limiting the accurate water/hydrocarbon saturation assessment from resistivity measurements. We perform pore network simulations to investigate the role of pore geometry on the non‐Archie behavior of resistivity index curves. Pore geometry controls water drainage and distribution, thus determining the resistivity index curves: water films retained at pore corners form a percolating path even at low values of water saturation, and the drainage pattern correlating with the fraction of micropores causes non‐Archie behavior in bimodal porous media. The understanding of electrical conduction mechanisms in porous media leads to a better selection of predictive models. Key Points: Wetting films associated with pore corners and surface roughness cause a bending‐down of resistivity index curves at low water saturationsThe micropore fraction affects the connectivity of the macropore system, drainage mechanisms, and resistivity index curvesBoth the geometric mean model and effective medium theory provide acceptable estimates of the electrical properties of bimodal porous media
- Subjects
CARBONATE rocks; CURVES; EARTH scientists; POROUS materials; RESERVOIR rocks; ELECTRICAL resistivity; FRACTIONS
- Publication
Journal of Geophysical Research. Solid Earth, 2022, Vol 127, Issue 9, p1
- ISSN
2169-9313
- Publication type
Article
- DOI
10.1029/2022JB024792