A Laboratory Approach Considering Salinity to Investigate Archie’s Parameters in Deep Shale: A Case Study in Longmaxi Formation, Southern Sichuan

Yanran Li,Zhiming Hu,Shuti Zeng,Xianggang Duan,Jingshu Guo,Xiangui Liu,Changhong Cai,Qingxiu Zhang,Bo Wu,Zhijin Pu,Ying Mu

doi:10.1155/2022/8341622

Abstract

Archie’s parameters are substantial to be investigated in the evaluation of water saturation. Many researchers adhered to the opinion that Archie’s equation can still be applied to water saturation calculation in shale if the accuracy of relative parameters could be improved. External conditions, such as temperature, confining pressure, water salinity, wettability, and displacement, may influence the determination of Archie’s parameters. The aim of the study is to investigate the effect of salinity on Archie’s parameters and their correlation with mineral composition and pore structure. The mineral contents and petrophysical properties were firstly acquired through X-ray diffraction (XRD) and basic measurements. Rock-electric experiments under different salinity were conducted on deep shale samples taken from Longmaxi (LMX) Formation in Luzhou (LZ). The results indicate that Archie’s parameters of “ a ,” “ b ,” “ m ,” and “ n ” under actual brine salinity are assigned to 1.47, 1, 1.26, and 1.6, respectively. Our cementation factor is lower than that in other studies including (shaly) sandstones, carbonates, and shales, probably due to the extremely low porosity in study area. Salinity has a positive effect on cementation factor ( m ) and saturation exponent ( n ), suggesting the traditional assignment to Archie’s parameters is inappropriate. It was concluded that the complicated pore structure and high porosity mainly associated with clay mineral may trigger the increased cementation factor. Further mathematical derivations confirmed that the rock resistivity is inextricable from pore system and establish a physical model. The paper provides a solid evidence base for further understanding and evaluation of water saturation in unconventional shale reservoirs, more significantly, innovatively unveils the research into Archie formula in deep shale reservoirs of southern Sichuan basin.

Highlights

Since Archie proposed his representative equation in 1942, the barrier between fluid saturation calculation and resistivity logging data has been broken, sparking a scientific sensation throughout the entire energy industry [1–4]
In the results of X-ray diffraction (XRD) measurement, the primary mineral composition of each sample was described in terms of mass fractions: quartz, feldspar, carbonate minerals, pyrite, and clay minerals
Quartz and clay minerals are dominant compositions accounting for 26%~60% and 5%~52%, respectively

Summary

Introduction

Since Archie proposed his representative equation in 1942, the barrier between fluid saturation calculation and resistivity logging data has been broken, sparking a scientific sensation throughout the entire energy industry [1–4]. Archie formula is still popular for determining water saturation in shale [5–7]. These researchers were convinced that Archie’s model was superior to other approaches if the accuracy of relative parameters could be improved. The Archie parameters (a, b, m, n) were acquired under ambient conditions. The deep shale reservoirs at depths of 3500-4500 m possess splendid hydrocarbon generation potential, allowing them to gradually become a vital developing field for further exploration [8–10]. The accuracy of Archie parameters considering external conditions in deep shale should be systematically investigated [11]

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Conclusion