Integrating SANS and fluid-invasion methods to characterize pore structure of typical American shale oil reservoirs

Jianhua Zhao,Troy J Barber,Yuxiang Zhang,Markus Bleuel,Qinhong Hu,Zhijun Jin,Zhenkui Jin

doi:10.1038/s41598-017-15362-0

Abstract

An integration of small-angle neutron scattering (SANS), low-pressure N2 physisorption (LPNP), and mercury injection capillary pressure (MICP) methods was employed to study the pore structure of four oil shale samples from leading Niobrara, Wolfcamp, Bakken, and Utica Formations in USA. Porosity values obtained from SANS are higher than those from two fluid-invasion methods, due to the ability of neutrons to probe pore spaces inaccessible to N2 and mercury. However, SANS and LPNP methods exhibit a similar pore-size distribution, and both methods (in measuring total pore volume) show different results of porosity and pore-size distribution obtained from the MICP method (quantifying pore throats). Multi-scale (five pore-diameter intervals) inaccessible porosity to N2 was determined using SANS and LPNP data. Overall, a large value of inaccessible porosity occurs at pore diameters <10 nm, which we attribute to low connectivity of organic matter-hosted and clay-associated pores in these shales. While each method probes a unique aspect of complex pore structure of shale, the discrepancy between pore structure results from different methods is explained with respect to their difference in measurable ranges of pore diameter, pore space, pore type, sample size and associated pore connectivity, as well as theoretical base and interpretation.

Highlights

Fractal (g/cm3)Scattering length density (SLD) ( × 10−10 cm−2) Slope dimensionMore importantly, different techniques are based on different experimental conditions, principles and data interpretation approaches; some differences of shale pore structure are commonly observed
In order to provide more direct comparison, we calculate the mercury injection capillary pressure (MICP) porosity and cumulative pore volume in the pore-throat diameter range of 2.8–600 nm, and cumulative pore volume in the pore diameter range of 2–600 nm from the low-pressure N2 physisorption (LPNP) method, which are much lower than the small-angle neutron scattering (SANS) method (Table 3)
Inaccessible pores can occur as both intraparticle and interparticle, and both could be widely developed in shale, coal, carbonate and siltstone[2,3,20,30,37]

Summary

Introduction

Fractal (g/cm3)SLD ( × 10−10 cm−2) (cm−1) Slope dimensionMore importantly, different techniques are based on different experimental conditions, principles and data interpretation approaches; some differences of shale pore structure are commonly observed. It is challenging to obtain the results over a wide scale without modifying the original pore structure information of shale. Due to minimal contribution to hydrocarbon production, closed pores are often ignored in conventional oil and gas reservoirs. Closed pores can store an appreciable amount of oil and gas, especially within organic matter particles in unconventional shale reservoirs, which could be released after hydraulic fracturing[31]. Closed porosity is an essential factor to control oil/gas storage, transport pathway, and production behavior[23,28,32,33]. A clear understanding of the fundamental characteristics of pore structure, especially an assessment of the presence and contribution of closed pores, is crucial to sustainable shale oil/gas development

Methods

Results

Conclusion