Controlling Effects of Tight Reservoir Micropore Structures on Seepage Ability: A Case Study of the Upper Paleozoic of the Eastern Ordos Basin, China

Abstract

In this study, the types of micropores in a reservoir are analyzed using casting thin section (CTS) observation and scanning electron microscopy (SEM) experiments. The high-pressure mercury injection (HPMI) and constant-rate mercury injection (CRMI) experiments are performed to study the micropore structure of the reservoir. Nuclear magnetic resonance (NMR), gas-water relative seepage, and gas-water two-phase displacement studies are performed to examine the seepage ability and parameters of the reservoir, and further analyses are done to confirm the controlling effects of reservoir micropore structures on seepage ability. The experimental results show that Benxi, Taiyuan, Shanxi, and Shihezi formations in the study area are typical ultra-low porosity and ultra-low permeability reservoirs. Owing to compaction and later diagenetic transformation, they contain few primary pores. Secondary pores are the main pore types of reservoirs in the study area. Six main types of secondary pores are: intergranular dissolved pores, intragranular dissolved pores, lithic dissolved pores, intercrystalline dissolved pores, micropores, and microfracture. The results show that reservoirs with small pore-throat radius, medium displacement pressure, and large differences in pore-throat structures are present in the study area. The four types of micropore structures observed are: lower displacement pressure and fine pores with medium-fine throats, low displacement pressure and fine micropores with fine throats, medium displacement pressure and micropores with micro-fine throats, and high displacement pressure and micropores with micro throats. The micropore structure is complex, and the reservoir seepage ability is poor in the study areas. The movable fluid saturation, range of the gas-water two-phase seepage zone, and displacement types are the three parameters that well represent the reservoir seepage ability. According to the characteristic parameters of microscopic pore structure and seepage characteristics, the reservoirs in the study area are classified into four types (I-IV), and types I, II, and III are the main types observed. From type I to type IV, the displacement pressure increases, and the movable fluid saturation and gas-water two-phase seepage zone decrease, and the displacement type changes from the reticulation-uniform displacement to dendritic and snake like. Key words: micro-pore structure, seepage ability, movable fluid saturation, the range of gas-water two phase seepage zone, displacement types Citation: Yang et al., 2020. Controlling Effects of Tight Reservoir Micropore Structures on Seepage Ability: A Case Study of the Upper

The Lithological Features of Sublacustrine Fans and Significance to Hydrocarbon Exploration: A Case Study of the Chang 7 Interval of the Yanchang Formation, Southeastern Ordos Basin, North China

“…The Ordos Basin extends from the central to the western part of the North China Craton and is surrounded by the Helan-Liupan Mountain north-south tectonic belt to the west, the Pacific tectonic region in the east, and the Qilian-Qinling fold tectonic belt and the Tianshan-Xingmeng fold tectonic belt in the north and south, respectively (Figure 1). It is the second largest onshore sedimentary basin in China, with an area of about 37 × 10 4 km 2 [58][59][60][61][62][63]. Under the influence of collision orogeny in Qinling area, the strata of the Middle and Late Triassic basin are generally high in the north and low in the south, thin in the north and thick in the south.…”

Pore-Throat Combination Characteristics and Their Effect on Gas–Water Seepage Behavior in Tight Sandstone Gas Reservoirs of the Dingbian Area, Ordos Basin, North China

“…The seepage characteristics of fluids in the pore-throat space of tight sandstone reservoirs have been a matter of concern for petroleum professionals. 26,28,35,36 Many scholars have conducted studies on the pore-throat structure and flow patterns of mobile fluids in tight sandstone gas reservoirs. They have concluded that the degree of pore-throat structure and connectivity significantly influence the interaction between rocks and fluids, as well as the reservoir capacity.…”

“…The seepage characteristics of fluids in the pore-throat space of tight sandstone reservoirs have been a matter of concern for petroleum professionals. ,,, Many scholars have conducted studies on the pore-throat structure and flow patterns of mobile fluids in tight sandstone gas reservoirs. They have concluded that the degree of pore-throat structure and connectivity significantly influence the interaction between rocks and fluids, as well as the reservoir capacity. − The distribution of fluids, relative permeability, and the phenomenon of water locking all reflect the influence of pore-throat structure on seepage characteristics. ,− In gas–water two-phase seepage, the relative permeability of the gas phase may be overestimated if the Klinkenberg effect is not taken into consideration. , Among the vast number of factors that influence fluid mobility, which one is the most crucial?…”

Fractal Characteristics and Model Applicability for Pores in Tight Gas Sandstone Reservoirs: A Case Study of the Upper Paleozoic in Ordos Basin

“…Tight reservoirs, widely developed in the world’s major oil and gas basins, have attracted considerable recent attention due to their significant oil and gas production potential. − Compared with conventional reservoirs, tight gas sandstone reservoirs are characterized by poor quality and strong heterogeneity, − which seriously limits the efficient and economical development of oil and gas resources. Pore structural characteristics closely related to oil and gas filling and migration, such as the size, shape, connectivity, distribution, and surface complexity of pores, − are important indexes for reservoir quality evaluation.…”