Combining SEM and Mercury Intrusion Capillary Pressure in the characterization of pore-throat distribution in tight sandstone and its modification by diagenesis: A case study in the Yanchang Formation, Ordos Basin, China

Wei Wang,Shuang Xu,Lei Gao,Caili Yu,Le Zhao

doi:10.15446/esrj.v24n1.84838

Abstract

Determining the characteristics of pore-throat structures, including the space types present and the pore size distribution, is essential for the evaluation of reservoir quality in tight sandstones. In this study, the results of various testing methods, including scanning electron microscopy (SEM), pressure-controlled porosimetry (PCP) and rate-controlled porosimetry (RCP), were compared and integrated to characterize the pore size distribution and the effects of diagenesis upon it in tight sandstones from the Ordos Basin, China. The results showed that reservoir spaces in tight sandstones can be classified into those with three types of origins (compaction, dissolution, and clay-related) and that the sizes and shapes of pore space differ depending on origin. Considering the data obtained by mercury injection porosimetry and the overestimation of pore radii by pressure-controlled porosimetry, the full-range pore size distribution of tight sandstones can be determined by combining data from PCP with corrected RCP data. The pore-throat radii in tight sandstone vary from 36 nm to 200 μm, and the distribution curve is characterized by three peaks. The right peak remains similar across the sample set and corresponds to residual intergranular pores and dissolution pores. The middle and left peaks show variation between samples due to the heterogeneity and complexity of nano-scale throat bodies. The average micro-scale pore content is 33.49%, and nano-scale throats make up 66.54%. The nano-scale throat spaces thus dominate the reservoir space of the tight sandstones. Compaction, dissolution, carbonate cementation, and clay cementation have various effects on pore-throats. Compaction and carbonate cementation decrease pore body content. Pore-bridging clay cementation decreases throat space content. As pore-lining clay cementation preserves pore space.

Highlights

With the gradual decline of hydrocarbon reserves in conventional reservoirs and the increasing global energy demand, the great potential of unconventional reservoirs has made them the focus of global oil production (Ghanizadeh et al, 2015)
The pore size distribution (PSD) in tight sandstones can be characterized by many techniques, such as thin section analysis, scanning electron microscopy (SEM), X-ray computed tomography (CT), pressure-controlled porosimetry (PCP), rate-controlled porosimetry (RCP), nuclear magnetic resonance (NMR) and N2 adsorption, but these techniques all have limitations that hinder determination of the characteristics of pore-throats of a wide range of sizes
SEM, thin section, PCP and RCP analyses were performed on seven samples from the Upper Triassic Yanchang tight sandstone from the Ordos Basin so as to bring the combined strengths of a range of methods to bear on determining the full-range distribution of pore bodies and throats and the effects of diagenesis upon them

Summary

Introduction

With the gradual decline of hydrocarbon reserves in conventional reservoirs and the increasing global energy demand, the great potential of unconventional reservoirs has made them the focus of global oil production (Ghanizadeh et al, 2015). The pore size distribution (PSD) in tight sandstones can be characterized by many techniques, such as thin section analysis, scanning electron microscopy (SEM), X-ray computed tomography (CT), pressure-controlled porosimetry (PCP), rate-controlled porosimetry (RCP), nuclear magnetic resonance (NMR) and N2 adsorption, but these techniques all have limitations that hinder determination of the characteristics of pore-throats of a wide range of sizes. SEM (FE-SEM) and thin sections provide direct information on the morphological characteristics of reservoir space and can be used to determine the genetic origins of pore-throats, but they fail to provide quantitative data (Klaver et al, 2012; Loucks and Ruppel, 2007). PCP can be used to obtain quantitative data on pore-throat radius from samples and effectively identify nanopores but fails to measure large pores due to the shielding effect (Gane et al, 2004). Nitrogen adsorption can only obtain information on nanopores in tight sandstones (Yao and Liu, 2012)

Methods

Discussion

Conclusion