Study on influencing factors and mechanism of pore compressibility of tight sandstone reservoir—A case study of upper carboniferous in ordos basin

Abstract

A series of studies were carried out on 11 tight sandstone samples of Upper Carboniferous in Ordos Basin. Firstly, the deposit composition and pore structure characteristics are investigated based on analysis and experiments including cast thin section scanning electron microscope high-pressure mercury intrusion and nuclear magnetic resonance Then, combined with DP-P test, the stress-dependent permeability change and pore compressibility characteristics of sandstone reservoirs were studied to reveal the influencing factors and mechanism of reservoir pore compressibility. The detrital particles of the sandstone reservoir in the study area are mainly quartz (75.8%–89%), followed by fragments (3%–16.1%), and almost no feldspar. The content of interstitial materials is 6.5%–11.2%. The type I reservoirs mainly consist of mesopores and macropores, accounting for 60.57% and 32.84% respectively. Mesopores are dominated in Type II reservoirs, accounting for 78.98% of the total pore volume. There are almost no macropores, while a similar proportion of mesopores, micro mesopores and micropores in the type Ⅲ reservoirs. The study of pore compressibility shows that the pore compressibility coefficient decreases with the increase of effective stress, and the reduction rate shows the two-stage characteristics of rapid in the early stage and slow in the later stage. The pressure turning point is between 3 and 10 MPa. The average pore compressibility coefficient increases from type I to type Ⅲ reservoirs. The compressibility coefficient is directly proportional to the changing rate of the pore volume. The higher the content of rigid detrital particles, quartz and carbonate cement in sandstone, the smaller the pore compressibility coefficient, while the higher the content of ductile components such as soft rock fragments and clay minerals, the greater the pore compression coefficient. The pore-throat structure is closely related to the pore compressibility, reservoirs with low displacement pressure, T2glm value, and large average pore-throat radius show lower compressibility coefficient. In addition, the compressibility coefficient of the reservoir is positively correlated with DL (dimension of large pores such as mesopores and macropores), and negatively correlated with DS (the fractal dimension of micropores and micro mesopores). It is considered the pore compression of sandstone including two stages, viscoplastic destructive deformation of ductile components for the first and then the small-scale non-ideal elastic deformation on rigid particles.

Analysis of Stress Sensitivity and Movable Fluid Characteristics of Tight Sandstone Based on NMR and Permeability Experiments

“…The average compressibility coefficients of samples NS1, NS2, and NS3 were 0.0108, 0.00104, and 0.00952 MPa –1 , respectively (Figure a–c), which were smaller than those of tight sandstones of Yangchang Formation in the Ordos Basin and similar to those of the tight clastic reservoirs in the Hailaer Basin, as previously studied. , These data indicate that sample NS2 was more difficult to compress than samples NS1 and NS3, which may be due to the high quartz content and low rock debris content of sample NS2.…”

“…The permeability stress sensitivity of a sample, based on the results of permeability tests, is generally expressed by the dimensionless parameter D k . 13,39 D k is the permeability loss rate as the effective stress changes and determined by the equation…”

“…The stress sensitivity of the samples is reported in the previous studies to be positively correlated with brittle minerals and grain size and negatively correlated with soft rock debris. 13,14 The low stress sensitivity of sample NS2 may be related to its high quartz content and low rock debris content (Table 1). It is noteworthy that the amount of signal in some of the T 2 relaxation time ranges of the samples increased with increasing pressure (Figure 6a,b).…”

“…Previous studies have evaluated reservoir stress sensitivity based on changes in porosity or permeability with pressure and modeled the compressibility coefficient of the reservoir. − It was found that permeability and porosity decrease linearly or exponentially with effective stress , and that factors such as grain size and lithology of the reservoir have an effect on the compressibility coefficient. , However, these methods can only macroscopically evaluate the compressibility of reservoirs and not the stress sensitivity of pores at different scales. In recent years, nuclear magnetic resonance (NMR) has been used as a convenient and nondestructive method to analyze the stress sensitivity of different pores in samples. − The results of previous studies show that the pore compressibility of samples from different coal ranks varies greatly and that macropores and microcracks are critical to the stress sensitivity of the samples. ,, For tight sandstones, samples with large-pore development are also strongly stress-sensitive .…”

“…9−11 It was found that permeability and porosity decrease linearly or exponentially with effective stress 7,12 and that factors such as grain size and lithology of the reservoir have an effect on the compressibility coefficient. 13,14 However, these methods can only macroscopically evaluate the compressibility of reservoirs and not the stress sensitivity of pores at different scales. In recent years, nuclear magnetic resonance (NMR) has been used as a convenient and nondestructive method to analyze the stress sensitivity of different pores in samples.…”