Shale primary porosimetry based on 2D nuclear magnetic resonance of T1-T2

Abstract

Porosity, a key parameter in assessing the physical properties of shale reservoirs and the reserves, is of great significance to the selection and evaluation of shale sweet spots. There are many methods at present to characterize shale porosity, most of which are aimed at post-core cleaning shale, such as those involving helium and saturated fluid (namely liquid-involved porosimetry). However, due to the low efficiency of shale core cleaning and the possible damage to pore structure during the core cleaning process, it's hard to guarantee the accuracy of porosity measurement. In this regard, we resort to the two-dimensional (2D) nuclear magnetic resonance (NMR) technology of T1-T2 in characterizing the primary shale porosity with samples taken from pressure coring in the 4th member of Shahejie Formation (Sha 4 Member) in well Fanxie 184 in the Jiyang Depression. Moreover, comparative experiments of shale porosity measurement by three methods, namely the simultaneous distillation extraction (SDE), helium and fluid measurement, are carried out simultaneously. The results show that the values obtained by SDE, gas and liquid measurement are similar, which are about 0.6 times of 2D NMR porosimetry. Core cleaning efficiency tends to seriously affect the results involving gas and liquid. In addition, the core cleaning treatment is bound to change the shale pore structure, and this is especially true in clay which tends to swell; consequently, porosity results are to be distorted by gas and liquid methods. It is thereby recommended to utilize the 2D NMR technology to characterize the primary total porosity of shale samples without core cleaning. The effective porosity of samples from pressure coring is determined by T2 cutoff value of around 0.3 ms.