Reservoir Characterization of Tight Sandstones Using Nuclear Magnetic Resonance and Incremental Pressure Mercury Injection Experiments: Implication for Tight Sand Gas Reservoir Quality

Abstract

A series of experiments including porosity and permeability measurements, thin section and scanning electron microscopy (SEM) observations, incremental pressure mercury injection (IPMI), and nuclear magnetic resonance (NMR) were conducted to systematically characterize the pore structure of tight sandstone from the Lower Shihezi Formation of Permian (P2x) in the northeastern Ordos Basin, China. The influences of pore types, pore size distribution, and fractal characteristics on reservoir quality of tight sandstones are also investigated. Results show that the studied tight sandstones generally possess poor quality and complex pore structure. The porosity and permeability range from 4.08% to 17.56% (average 9.22%) and from 0.05 to 16.66 mD (average 2.49 mD), respectively. Five pore types were observed in thin section and SEM images: primary intergranular pores, intergranular dissolution pores, intragranular dissolution pores, micropores within clay aggregates, and microfractures. The pore throats are mainly hairy/fibrous, inhibiting the connectivity between pores. Three types of pore structures were identified in the mercury-injection curves and pore size distribution curves from the IPMI experiment and in the T 2 relaxation time spectrum obtained by NMR. Both experiments yielded consistent classifications, and their combination was necessary to analyze the pore structure effectively. In general, permeability and porosity are positively related and depend on pore types. Large numbers of small pores confer high storage capacity, whereas small numbers of larger pores improve the flow capability. In the high porosity–permeability zone, larger pores also determine the storage capacity. The P2x tight sandstone is fractal, and macropores are more heterogeneous while micropores are more homogeneous. The fractal dimensions of macropores are good indicators of the reservoir quality of the P2x tight sandstone as larger fractal dimension values of macropores reflect poor reservoir quality.

Investigation of the controlling rock petrophysical factors on water phase trapping damage in tight gas reservoirs

“…Porosity is a basic physical parameter of formation rock and offers more reliable information with respect to pore volume than pore geometry. Once WPT damage has occurred and some pore space is occupied by water, less pore space is left to act as a flow channel . Since not all rock pore volume allows gas flow after water invasion, rock porosity does have some affect the WPT.…”

“…Once WPT damage has occurred and some pore space is occupied by water, less pore space is left to act as a flow channel. 32 Since not all rock pore volume allows gas flow after water invasion, rock porosity does have some affect the WPT. However, as the results in Figure 9 show the relationship between rock porosity, the degree of water drainage and gas permeability damage is not particularly significant and actually shows that porosity generally has a weak influence on WPT damage and is not a vital petrophysical factor affecting WPT damage.…”

Multi-Scale Quantitative Characterization of Pore Distribution Networks in Tight Sandstone by integrating FE-SEM, HPMI, and NMR with the Constrained Least Squares Algorithm

“…where dS hgi is the increment of total mercury intrusion corresponding to pores with a radius of r i [18]. The permeability contribution by HPMI is determined on the basis of the Purcell equation.…”

“…Many scholars have tried to integrate various techniques to describe the multi-scale PSD of tight sandstones [15][16][17]. Nuclear magnetic resonance (NMR) is an effective tool for non-destructively quantifying the multi-range pore structures of tight sandstone reservoirs [18][19][20], but the NMR-derived PSD related to the relaxation time (T 2 ) should be transformed to pore size (µm) through integration with other methods. The NMR-derived PSD has been determined with imaging methods (e.g., casting thin sections and field emission scanning electro microscopy (FE-SEM)) using a large amount of statistical multi-scale pore radii data [21].…”

Diagenetic characteristics and reservoir quality in tight gas sandstones: A case study of the Shanxi Formation in the north‐eastern Ordos Basin, China

“…Xie et al () suggested that the sandbodies develop extensively in the study area and the reservoir quality is controlled by a variety of factors. Some studies investigated the pore structure of the Shanxi Formation sandstones using nuclear magnetic resonance and pressure‐controlled mercury injection and found that the reservoir is tight and heterogeneous (Li et al, ; Shao, Pang, Jiang, et al, ; Shao, Pang, Li, & Zhang, ). Drilling results have shown that the Shanxi Formation tight sandstones in the north‐eastern Ordos Basin have more complex mineral compositions and diagenetic characteristics than those in the central part of the basin, leading to variable reservoir quality (porosity and permeability).…”