Probe material choice for nuclear magnetic resonance cryoporometry (NMRC) measurements of the nano-scale pore size distribution of unconventional reservoirs

Feng Zhu,Yifeng Liu,Chao Chang,Jian Cao,Wenxuan Hu,Biao Liu

doi:10.1177/0144598718802475

Abstract

Nuclear magnetic resonance cryoporometry is a newly developed technique that can characterize the pore size distribution of nano-scale porous materials. To date, this technique has scarcely been used for the testing of unconventional oil and gas reservoirs; thus, their micro- and nano-scale pore structures must still be investigated. The selection of the probe material for this technique has a key impact on the quality of the measurement results during the testing of geological samples. In this paper, we present details on the nuclear magnetic resonance cryoporometric procedure. Several types of probe materials were compared during the nuclear testing of standard nano-scale porous materials and unconventional reservoir geological samples from Sichuan Basin, Southwest China. Gas sorption experiments were also carried out on the same samples simultaneously. The KGT values of the probe materials octamethylcyclotetrasiloxane and calcium chloride hexahydrate were calibrated using standard nano-scale porous materials to reveal respective values of 149.3 Knm and 184 Knm. Water did not successfully wet the pore surfaces of the standard controlled pore glass samples; moreover, water damaged the pore structures of the geological samples, which was confirmed during two freeze-melting tests. The complex phase transition during the melting of cyclohexane introduced a nuclear magnetic resonance signal in addition to that from liquid in the pores, which led to an imprecise characterization of the pore size distribution. Octamethylcyclotetrasiloxane and calcium chloride hexahydrate have been rarely employed as nuclear magnetic resonance cryoporometric probe materials for the testing of an unconventional reservoir. Both of these materials were able to characterize pore sizes up to 1 μm, and they were more applicable than either water or cyclohexane.

Highlights

The exploration for and development of unconventional oil and gas resources is topic of considerable interest (Dai et al, 2012; Holditch, 2006)
Nuclear magnetic resonance cryoporometry (NMRC), which is based on radiation theory, is a relatively new technique for acquiring pore size distribution (PSD) measurements that uses intrusive materials, and it is classified as an intrusive method
If an appropriate probe material is chosen for NMRC, it will be absorbed into the pores spontaneously, thereby avoiding any damage to the pore structure similar to mercury injection techniques, and NMRC could be complementary to other methods due to its theoretical pore size distribution coverage from 2 nm to 1 lm

Summary

Introduction

The exploration for and development of unconventional oil and gas resources is topic of considerable interest (Dai et al, 2012; Holditch, 2006). NMRC has rarely been used for the testing of unconventional reservoir nano-scale pore sizes, and few studies have used water as the probe material during the testing of porous rocks such as coal and shale (Firouzi et al, 2014; Qian et al, 2016; Webber et al, 2013). Distinct probe materials have different effects on the wettability of a mineral surface and the NMR signal intensity and a disparate influence on the crystallization and melting of pore structures.

Results

Conclusion