Applying octamethylcyclotetrasiloxane as a probe liquid for characterizing the pore size distribution of oil-bearing tight sandstones by nuclear magnetic resonance cryoporometry

The characterization of tight sandstone pore structure is extremely important to the study of tight sandstone oil and gas. There are many techniques for the quantitative analysis of pore size distribution (PSD) in tight sandstone. Nuclear magnetic resonance cryoporometry (NMRC) is a relative new method that can be applied to obtain PSD of porous materials. However, the choice of the NMRC probe liquid is critical to the test result. In this study, we used a potential probe liquid named octamethylcyclotetrasiloxane (OMCTS) for NMRC. OMCTS, exhibiting excellent properties, such as amphiphilic wetting, a large KGT, and superior NMR behavior, has obvious advantages over water as a probe liquid for the PSD measurement of tight sandstone by NMRC, especially for oil-bearing tight sandstone. The applicability of this probe liquid for measuring nanometer pores is demonstrated through the accurate measurement of standard sample. The PSDs of four tight sandstone samples with different wettability from the Upper Triassic Yanchang Formation in Ordos Basin are measured by OMCTS. Mercury intrusion capillary pressure (MICP) is also performed on all samples for comparison. The experimental results show that OMCTS can measure pores in tight sandstone from 4 to 1400 nm. OMCTS can, as NMRC probe liquid, not only measure the PSD of hydrophilic tight sandstone, but also oil-wet tight sandstone. Furthermore, OMCTS does not damage the pore structure of tight sandstone. We think the NMRC with OMCTS as probe liquid will be proved to be a useful complement to current methods.