Insights into Pore Types and Wettability of a Shale Gas Reservoir by Nuclear Magnetic Resonance: Longmaxi Formation, Sichuan Basin, China

Abstract

Reservoir properties, such as pore types and wettability, are essential for shale gas reservoir evaluation. However, advanced nuclear magnetic resonance (NMR), which has been routinely used in petrophysical characterization of reservoirs, is barely used to estimate these properties in shale gas reservoirs. In this study, several sets of specially designed NMR measurements, together with total organic carbon (TOC), X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and contact angle tests, are used to study the pore types and wettability of Longmaxi shale gas reservoir. Results show that the NMR transversal relaxation time (T 2 ) spectrum can be used to characterize pore types and wettability of gas shale. Three identified T 2 spectral peaks (0.01−0.4 ms, 0.4−15 ms, and >15 ms) separately correspond to organic pores, inorganic pores, and microfractures, which is consistent with the results of FE-SEM. The T 2 spectra in as-received and water/ oil-imbibition states qualitatively prove that the microwettabilities of organic pores, inorganic pores, and microfractures are oilwet, water-wet, and mixed-wet, respectively. In addition, a novel wettability index is defined to reflect the microwettability of pores quantitatively. The dominant minerals and TOC jointly affect the pore wettability index; they show good correlations with the wettability index of inorganic and organic pores, whereas there is no obvious correlation with the wettability index of microfracture. In contact angle measurements, water and oil droplets show different behaviors on the surface of shale specimens, which qualitatively indicates that the macrowettability of Longmaxi gas shale is mixed-wet (both water-wet and oil-wet) and more prone to be oil-wet. After analysis of the theory of characterizing macrowettability by NMR, a new NMR-based model is proposed to characterize the macrowettability quantitatively. In summary, this study proposes novel methods and models to characterize the pore types and wettability, expanding the use of NMR in shale gas reservoirs.

Measurement of Shale Wettability Using Calorimetry: Experimental Results and Model

“…Si–O can also interact with water molecules to produce silanol groups (Si–OH), and O-position silanol groups can be further dehydrated to form siloxane bonds (Si–O–Si), which are easily subjected to hydrophilic carboxylation, thus leading to the hydrophilicity of quartz. , Tabrizy et al, (2011) also considered that quartz is a hydrophilic mineral according to the isotherm morphology of quartz obtained from microcalorimetry experiments. In addition, under strong compaction, the contact types of rigid quartz particles are lines or points which protect some fractures and pores with clay to form a better connective water pathway ,− (Figure ).…”

“…The organic pores are dispersed in the OM, which show ellipsoid or bubble shapes (Figure 4a), and the formation of OM shrinkage cracks are related to abnormal pressure during the hydrocarbon process 61 (Figure 4b). Oil more easily enters the organic pores and makes shale more oil wet.…”

Complexities Driving Wettability Evaluation of Shales toward Unconventional Approaches: A Comprehensive Review

“…Pore connectivity is mainly controlled by the developed organic and inorganic pores and the extent of their connection. 82 Wang et al 83 observed that oil-wet organic and water-wet inorganic pores jointly contribute to the macrowettability of Longmaxi gas shale, which was found to be mixed-wet based on the contact angle experiments. Tinni et al 84 used the sequential spontaneous imbibition of brine and dodecane to study pore network connectivity in shales using NMR.…”

Pore Structure Evolution of Cemented Paste Backfill Observed with Two-Dimensional NMR Relaxation Correlation Measurements

“…Following eqs –, this ratio may therefore be expressed as which at a fixed NMR field strength depends only on the surface correlation times τ m and τ s and is independent of factors such as pore structure or the surface density of paramagnetic species . This metric is therefore considered a direct and noninvasive metric of molecular mobility within the adsorbed surface layer and has been extended elsewhere to the interpretation of wettability and adsorbate surface affinity (see e.g., refs − ).…”