Seven samples in this experiment are from the Lucaogou Formation of four wells in Jimsar Sag. Their lithology is siltstone and dolomitic siltstone, which are the main lithology of shale oil reservoir in this area. The porosity of the sample ranges from 7.36 to 19.69%, the permeability ranges from 0.02 mD to 1.009 mD, and the oil-bearing grade ranges from oil stains to saturated oil (Table 1). The experimental oil is the actual crude oil of the Lucaogou Formation, with density of 0.85 g/cm³ at 25°C and viscosity of 110 mPa·s at 55°C.

The experiments were mainly completed on the displacement equipment MR-dd with high temperature and pressure system, and the experimental process was monitored on-line by NMR (MesoMR23-060H I). NMR parameters were set to TE = 0.2 ms, TW = 3000 ms, NECH = 12,000, and NS = 32. The environment temperature was 25°C and the humidity is 30%.

This study mainly carries out the simulation experiment of imbibition-expulsion oil and elastic-energy expulsion oil under high temperature and pressure. The experimental device is shown in Figure 1, and the process is as follows:

Experiment 1: The simulation experiment of imbibition-expulsion oil under high temperature and pressure: 1) Clean and dry the standard plunger sample of 2.5 cm × 5 cm and measure the nuclear magnetic signal T₂ spectrum. 2) Saturate the vacuumed sample with crude oil at 20 MPa and 60°C for 24 h. 3) Place the sample in the clamp holder and set the experimental temperature at 90°C and the confining pressure at 37 Mpa. Close valves 2 and 3, open valve 1, and fill crude oil with 35 MPa to supplement pore pressure. 4) Close valve 1 when the NMR signal is stable. 5) Open valve 2, fill D₂O at 37 MPa, and start the oil imbibition-expulsion simulation experiment under high temperature and pressure. Monitor the hydrogen signal of crude oil in the core based on NMR at certain intervals; the experiment ends until the NMR signal does not change.

Experiment 2: The simulation experiment of elastic-energy expulsion oil under different temperatures: 1) Cleane, dry, vacuum, and saturate the sample with crude oil. 2) Put the saturated oil sample in the holder and set the experiment temperature to 20°C. Close outlet 2 and 3, open valve 1, and fill crude oil to pore pressure of 35 Mpa. 3) Close valve 1 and open the outlet valve 3 and carry out the elastic-energy expulsion oil simulation experiment under the pressure difference of 35 Mpa. When the NMR signal is stable, the experiment ends. 4) Change the experiment temperature (35°C, 55°C, 75°C, and 90°C), and repeat steps 1∼3 to carry out the elastic-energy expulsion oil experiment at different temperatures.

Experiment 3: The simulation experiment of elastic-energy expulsion oil under different pressure: 1) Pretreat the sample as above, set the experiment temperature to 90°C, close valves 1 and 2, open valve 1, and fill crude oil to pore pressure of 35 Mpa. 2) Close valve 1 and set the outlet back-pressure valve to 32 Mpa when the NMR signal is stable. Open valve 3 and allow the sample to expulse oil at a pressure difference of 3 Mpa. 3) Repeat the abovementioned steps, set the pressure of the back-pressure valve to be 30 Mpa, 25 Mpa, 15 Mpa, 5 Mpa, and 0 Mpa, respectively, and carry out the elastic-energy expulsion oil experiment under the different production pressure.

In the process of imbibition-expulsion oil, water is imbibed from the fracture into the oil-bearing matrix under the action of capillary force, and the pore with water wetting drives out the crude oil through oil–water replacement (Zhu et al., 2002; Li et al., 2007). In total, four samples were subjected to the imbibition-expulsion oil experiments under high temperature and pressure in this study. The oil-expulsion efficiency is the ratio of the difference between the original saturated oil signal and the residual oil signal to the original saturated oil signal. The experimental results are shown in Figure 2.

From the experimental results, the effect of imbibition-expulsion oil under high temperature and pressure is not obvious. The oil expulsion phenomenon occurred only 60 h ago, and the final oil-expulsion efficiency is only 1.4–1.8% (Figure 2A). The oil-expulsion efficiency of different samples showed a significant negative correlation with the permeability of unit pore [Log (K)/φ] (Figure 2B). It reflects that the pore-throat system with a smaller size has larger capillary force, and it is more likely to have an imbibition-expulsion oil effect. Microscopically, NMR T₂ spectrum also show that the crude oil signal decreases gradually with the imbibition time in the low T₂ area (Figure 3), which also intuitively shows that the imbibition phenomenon mainly occurs in the small pore-throat system.

Different from the atmospheric pressure imbibition, when the matrix pore fluid is in a high-pressure state, the imbibition fluid is not only subjected to the capillary power to the matrix pore but also subjected to the outward expansion resistance of crude oil in the pore, which greatly reduces the efficiency of imbibition-expulsion oil. The author compared the results of imbibition-expulsion oil under high and atmospheric pressure. The crude oil signal of sample #12 decreased from 5926 to 5364 after 60 h under atmospheric pressure, and the oil-expulsion efficiency could reach about 10%, which was much higher than that under high pressure (Figure 4). It can also be confirmed that the existence of pore pressure will counteract the capillary force and prevent the occurrence of imbibition-expulsion oil. In this experiment, D₂O is used as the imbibition fluid. However, in the actual production process, the imbibition fluid is polymer fracturing fluid containing guar gum, and its percolation capacity is much lower than that of D₂O. Therefore, in the actual fracturing and soaking process, the efficiency of imbibition-expulsion oil may be even lower.

Under the condition of 35 MPa pore pressure, temperature has a significant effect on the oil-expulsion efficiency of elastic energy, which is mainly reflected in the oil-expulsion of large pores. Also taking #27 as an example, the amount of elastic-energy expulsion oil was less at 20°C, and the total NMR signal decreased from 12,848 to 12,270, corresponding to the oil-expulsion efficiency of 4.5%. As the temperature rose to 55°C, the total oil-expulsion efficiency reached 11.4%, and the oil expulsion mainly occurred in the region of T₂>10 ms (Figure 5A). With the further increase of temperature, the NMR signal in high T₂ area gradually decreases and the oil-expulsion efficiency further increased, reaching 24.4% at 90°C (Figure 5B).

In the elastic-energy expulsion oil stage, the driving force of oil-expulsion is the expansion energy of the fluid, which can be reflected as the fluid pressure within the matrix pores. The expulsion resistance is mainly the capillary force of the reservoir and the viscous force of the crude oil. With the same reservoir properties and pore pressure, the increase of temperature essentially reduces the crude oil viscosity (μ) and changes the viscous force of the crude oil, so as to improve the production efficiency of crude oil in matrix pores. The author measured the experimental oil viscosity at different temperatures, which presented a power function negative correlation (Figure 6A). It can be seen that with increase in temperature and decrease in crude oil viscosity, the expulsion efficiency of crude oil increases rapidly. Especially when the viscosity is below 100 mPa s, the improvement rate of oil-expulsion efficiency is further intensified (Figure 6B).

The elastic-energy oil expulsion experiments of seven samples were carried out at 90°C and different differential pressures (3 Mpa, 5 MPa, 10 MPa, 20 MPa, 30 MPa, and 35 MPa). There is a good linear positive correlation between the NMR signal of saturated oil sample and gas porosity (Figure 7), indicating that the sample has a high degree of saturation and meets the experimental requirements. The NMR data of the experimental samples at saturated oil state and after oil expulsion state are shown in Table 2.

Taking sample #41 as an example, the NMR T₂ spectrum shows bimodal characteristics, and the pore-throat size distribution ranges widely. With the increase of pressure difference, the NMR signal of crude oil in different size pores gradually decreases, which shows that crude oil is discharged from pores with different sizes (Figure 8A). As the production differential pressure rises from 5 to 35 MPa, the oil-expulsion efficiency increases from 4% to 26% in a power function (Figure 8B).

With the same fluid properties and production pressure difference, the oil-expulsion efficiency is mainly affected by the reservoir physical properties, especially the permeability (Huang et al., 2019). It is true that the high permeability of the reservoir is indeed conducive to the discharge of crude oil from the matrix pores, but in the process of elastic-energy oil expulsion, the pore volume and fluid elastic modulus are also the key factors restricting the oil-expulsion efficiency. Figure 9A shows that permeability is an important but not the only factor controlling oil-expulsion efficiency. For example, samples #12 and #31 have higher permeability than samples #13, but their oil-expulsion efficiency is lower due to their low porosity. Therefore, the pore-throat structure parameter Log (K)/Φ is constructed, which has a good positive correlation with elastic-energy oil-expulsion efficiency (Figure 9B).

The relationship between oil-expulsion efficiency and Log (K)/Φ of seven samples shows that they are linear under the same differential pressure (Figure 10). The general expression is y = Ax + B, and its characteristic performance parameters A and B increase with the increase of differential pressure (Figure 10).

Since there is a significant linear relationship between parameters A and B (B = 0.37A) (Figure 11A), the oil-expulsion efficiency with respect to the reservoir structure coefficient can be expressed as follows: E o = A× ( Log ( K ) / Φ + 0 . 37 ) . (1) Here E _o is the oil-expulsion efficiency, %; K is permeability, mD; and Φ is porosity, %.

Due to the same experimental temperature and experimental oil, parameter A should be a variable controlled by differential pressure. The relationship between parameter A and differential pressure reveals that they have a good power function relationship (Figure 11B), so Eq. 1 can be further expressed as follows: E o = 7 . 74 Δ Pa 0 . 62 × ( Log ( K ) / Φ + 0 . 37 ) , (2) where ΔPa represent the fluid differential pressure, MPa; Other parameters are the same as before.

According to Eq. 2, the oil-expulsion efficiency is negative when Log (K)/Φ is less than −0.37, which is obviously inconsistent with the fact. But it also reveals the lower physical property limit of elastic-energy expulsion oil, that is, Log (K)/Φ = −0.37. When the physical property is lower than this value, the oil-expulsion efficiency is 0.

In addition to fluid differential pressure and reservoir physical properties, temperature also has significant impact on shale oil-expulsion efficiency, which is manifested in the impact of temperature on oil viscosity. The elastic-energy expulsion oil experiment of sample #27 at different temperatures reveals that the oil-expulsion efficiency is negatively correlated with the logarithm of crude oil viscosity in a power function, which can be expressed as follows: E o = 82 ×Log ( μ ) - 2 . 7 , (3) where μ represent the viscosity of crude oil, Mpa; Other parameters are the same as before.

Therefore, considering the crude oil viscosity, fluid pressure difference, and reservoir physical properties, the oil-expulsion efficiency under the energy-depleted development mode can be expressed as follows: E o = C ×Log ( μ ) - 2 . 7 × Δ Pa 0 . 62 × ( Log ( K ) / Φ + 0 . 37 ) , (4) where C represent a comprehensive constant, dimensionless; Other parameters are the same as before.

Sample #27 has a porosity of 19.7% and a permeability of 0.63 mD; when the production differential pressure is 35 Mpa, the expression of ΔPa^0.62× [Log (K)/Φ+0.37] in Eq. 4 is equal to 3.26. Thus, the Eq. 4 can be expressed as follows: E o =   3 . 26  C ×log ( μ ) - 2 . 7 . (5)

Eq. 3, Eq. 5 are equal, so the comprehensive constant C is 25.2. Under the energy-depleted development mode, the evaluation model of shale oil production efficiency of the Lucaogou Formation in Jimsar Sag is expressed as follows: E o =   25 . 2 ×Log ( μ ) - 2 . 7 × Δ Pa 0 . 62 × ( Log ( K ) / Φ + 0 . 37 ) . (6)

According to Eq. 6, the energy-depleted oil-expulsion efficiency of the experimental samples at different temperatures and different differential pressure are calculated. The evaluation results are in good agreement with the experimental results, and the correlation coefficient R² is as high as 0.95 (Figure 12).

From a geological point of view, the single-well productivity of shale oil is controlled by the physical properties, oil content, and movable efficiency of the reservoir. NMR logging has unique advantages in reservoir evaluation, which can effectively evaluate the porosity, permeability, and oil saturation of the reservoir. Wang et al. (2019) combined experimental measurement results and NMR relaxation time of bound-water of different types of clay and determined the starting NMR time of effective porosity of shale oil reservoir of the Lucaogou Formation in Jimusar is 1.7 ms. Xiao et al. (2013) and Chen et al. (2017) considered the strong heterogeneity of tight reservoir and quantitatively classified the reservoir according to the flow unit index (FZI) from small to large using NMR logging data. The porosity and permeability of each type of flow unit have good correlation, so the permeability can be evaluated according to NMR porosity. Based on the simulation experiment and nuclear magnetic logging, Mao and Shen (2021) and Yan et al. (2021), respectively, used 1D and 2D NMR logging to realize the quantitative evaluation of oil saturation of tight reservoir/shale reservoir.

The current geothermal gradient of the Lucaogou Formation in Jimsar Sag is about 3.1°C/hm and the formation pressure coefficient is around 1.36. Based on the relationship between temperature and crude oil viscosity (Figure 6A), as well as the difference between formation pressure and bottom-hole pressure, the shale oil production efficiency of the target formation has been evaluated combined with the porosity and permeability calculated by the above method. The evaluation parameter movable oil porosity (Φ _om ), which integrates reservoir physical properties, oil-bearing properties and mobility (Φ _om = Φ×S _o × E _o ), was further constructed, and the quantitative evaluation of this parameter was carried out for the target interval. The results show that there is a good positive correlation between Φ _om and oil production intensity in the perforated interval of a single well (Figure 13). According to the oil production conclusion, the Φ _om of dry layer is basically lower than 0.5%, that of poor oil layer is between 0.5%–1.5%, and that of oil layer is greater than 1.5%. Further, according to the productivity difference of oil layers, the oil layer with Φ _om > 2.5% is determined as type I oil layer, and the oil layer with Φ _om = 1.5–2.5% is determined as type II oil layer (Figure 13).

Application results of reservoir classification and evaluation method reveal that, type I and II shale oil layers of Lucaogou Formation are mainly distributed in P ₂ l ₂ ²⁻¹∼ P ₂ l ₂ ²⁻³, and P ₂ l ₁ ²⁻¹∼ P ₂ l ₁ ²⁻³, which are the upper and lower shale oil “sweet spot” sections in Jimsar Sag (Figure 14). The lithology of the “sweet spot” section is mainly feldspathic-lithic siltstone and dolomitic sandstone. Under the syn-quasi-synthetic leaching and organic acid dissolution, the reservoir performance is effectively improved (Wang et al., 2020), providing a large amount of storage space for shale oil in the “sweet spot” section. The sections of P ₂ l ₂ ²⁻⁴ and P ₂ l ₁ ²⁻⁴∼ P ₂ l ₁ ²⁻⁷ corresponds to the lacustrine transgressive and high-stand system tracts of third-order sedimentary cycle, developing two sets of high-quality hydrocarbon source rocks (Yang et al., 2018; Zhang et al., 2021). They provide sufficient oil sources for the upper and lower sweet sections, making the oil saturation of the sweet sections average around 70% and locally up to 90%. Based on frozen argon ion polishing SEM, Wang et al. (2021) revealed that the heavy components of crude oil mainly exist in the pores below 300 nm and the pore walls above 300 nm, which are mostly in the adsorbed and bound states and difficult to be used. The light-medium components are mainly stored in the center of pores above 300 nm, which is also the main contributor to the available shale oil. In the feldspathic-lithic siltstone and dolomitic siltstone of the study area, the pores with size above 300 nm accounts for about 75 and 60% respectively (Wang et al., 2021), which also ensure that the shale oil from the upper and lower “sweet spot” sections has high mobility efficiency.

It can be seen that the upper and lower “sweet spot” sections have superior shale oil enrichment conditions, and the dissolution and expansion porosity effects of the two main lithologies also make the shale oil in these two “sweet spot” sections have good mobility. In addition, the evaluation results based on this study further confirm that P ₂ l ₂ ²⁻¹∼ P ₂ l ₂ ²⁻³ and P ₂ l ₁ ²⁻¹∼ P ₂ l ₁ ²⁻³ have high movable oil porosity and are favorable sections for shale oil development in this area.