1. Introduction

2. samples and experiment

2.2. Experiment

(1) Gas drive water displacement—Magnetic Resonance Imaging (GWD-MRI)

Displacement-nuclear magnetic resonance imaging combined equipment can be used to visualize the distribution of formation water during the displacement process. The displacement instrument uses the MR-HTHP type high temperature and high pressure displacement device. The confining pressure of the holder is 0~30 MPa, and the driving pressure is 0~5 MPa. Magnetic imaging is detected by MacroMR12-150H nuclear magnetic resonance instrument. The diameter of the coil is 150mm, which can form a stable magnetic field in the area of 100mm, the frequency is 12.80 MHz, the magnet strength is 0.3 T, and the imaging time is about 40min each time. The displacement needs to be stopped during the imaging process. The nuclear magnetic resonance imaging (MRI) instrument can also perform NMR T2 spectrum detection at the same time, the echo interval is set to 0.15 ms, the measurement time is 8000 ms, and the CPMG sequence (Glorioso, 2003) can be used for quantitative evaluation of water content (Figure 2).

(2) Semi-Permeable Baffle (SPB) charging experiment

Use a semi-permeable baffle that only allows water to pass through but cannot allow natural gas to pass through as the compartment of the displacement chamber. When natural gas exists in the high pressure chamber, the natural gas enters the core under the action of gas pressure, and discharges a part of the water. Water seeps out into the measure cup through the semi-permeable baffle, while the natural gas will not escape from high pressure chamber, thus simulating the accumulation characteristics of the reservoir. The instrument uses the GMD-III type high temperature and high pressure capillary pressure resistivity continuous measuring instrument, the driving pressure is 0~1.4 MPa, the pressure stability accuracy is 0.01 MPa, and the formation water volume measurement accuracy is 0.005 ml.

Figure 3. Semi-permeable baffle experiment device (SYT 5346-2005 industry standard).

Figure 3. Semi-permeable baffle experiment device (SYT 5346-2005 industry standard).

(3) Centrifugal- Nuclear Magnetic Resonance (Cen-NMR) experiment

NMR is detected by MacroMR12-150H, frequency is 12.80 MHz, magnet strength is 0.3 T, coil diameter is 150mm, echo interval is set to 0.15 ms, measurement time is 8000 ms, CPMG sequence (Glorioso, 2003) is used, same as MRI device in Figure 2. Water saturating uses the vacuum-pressurization saturation, which vacuum 4–6h, water pressure 30 MPa, and saturation time 24 h. The centrifugal speed is generally 1500 r/min, 3000 r/min, 4000 r/min, and the maximum centrifugal speed is 7000 r/min.

3. result

3.1. migration layer charging result

The main feature of the migration layer charging is that, the natural gas and the formation water are discharged from the formation together, the gas is mainly passing through the core, and the displacement experiment is most fit with the characteristics of the migration layer charging. This paper analysis the Nuclear Magnetic Resonance Imaging (MRI) while Gas driven Water Displacement (GWD), and completes 3 samples. The driving pressure is 0~2 MPa, the total displacement time is 60~90 min, and each displacement time is 1~10 min. After one displacement is completed, weighing and MRI are carried out, and one single MRI is about 40 minutes, NMR T2 spectrum detection at the same time.

(1) Sample A10 experiment result

A sample of A10 (4320.9 m from Well A-2) has a permeability of 0.34 mD and a porosity of 8%. After a drive pressure of 0.3 MPa for 5 minutes and a drive pressure of 0.6 MPa for 20 minutes, the cumulative driving time is 25 minutes, Nuclear Magnetic Imaging (MRI) shows that the residual water is evenly distributed, indicating that natural gas cannot charge low-permeability cores under the condition of 0.6 MPa. After 0.3 MPa and 0.6 MPa driving, the water saturation of the samples decreased, which is believed to be the result of water evaporation caused by the heating of the core within 40 minutes of imaging.

Figure 4. sample of A10 (4320.9m in Well A-2) low-permeability GWD-MRI experiment results.

Figure 4. sample of A10 (4320.9m in Well A-2) low-permeability GWD-MRI experiment results.

The name of the photo represents “Sample Number-Driving Pressure-Cumulative Driving Time-Water Saturation”.

The drive pressure increased to 1 Mpa for 10 minutes, the cumulative driving time was 35 minutes. The water content in the front of the core is decreased, the center and the end of the core did not change, and the water saturation was 93%, indicating that 1 Mpa can charge into the core. Continue to use 1 Mpa driving pressure for 15 minutes, the cumulative driving time is 50 minutes, water content in the front of the core continues to decrease, and the water content in the center and end of the core begins to decrease, with a water saturation of 87%; cumulative driving time is 60 min, 70 min, 80 min, water content of different core positions all decreased; the cumulative driving time was 95 minutes, the water content at the front of the core was very low, and the water content at the end of the core also decreased significantly, with a water saturation of 75%.

(2) Sample A25 experiment result

The sample of A25 (4324.9 m from Well D-1) has a permeability of 1.39 mD and a porosity of 8.2%, was tested with one only driving pressure of 0.3 Mpa. After 5 minutes of driven, the water content in the front of the core decreased significantly, and the water content in the center and the end of the core began to decrease, with a water saturation of 93%, indicating that 0.3 Mpa can charge into a medium-permeability core. Continue driving for 5 minutes, that is, the cumulative driving time is 10 minutes. The water content in the center and the end of the core decreases slowly, and the water content of the front end of the core decreases significantly, with a water saturation of 87%. The water content at the end of the core is similar to that of the original sample, but the water content in the center and the front of the core has decreased significantly, reflecting that the natural gas has driven the water at the front of the core to the end of the core. When the cumulative driving time 25 minutes, the water content of the core is equal, indicating that the natural gas has completely passed through the core, displacing most of the movable fluid, and the water saturation is 74% at this time. Continuing to use 0.3 Mpa for driving, the water content of the whole core is decreases, indicating that natural gas can still carry fluid away from the core. The cumulative driving time is 50 minutes, and the water saturation is 71%.

Figure 5. sample of A25 (4324.9m in Well D-1) middle-permeability GWD-MRI experiment results.

Figure 5. sample of A25 (4324.9m in Well D-1) middle-permeability GWD-MRI experiment results.

The name of the photo represents “Sample Number-Driving Pressure-Cumulative Driving Time-Water Saturation”.

(3) Sample A45 experiment result

The sample of A45 (4106.86 m from Well F-1) was displaced with a driven pressure of up to 3 Mpa. Using 0.3 Mpa to driving for 1 minute, the residual water content of the core was evenly distributed, the water saturation decreased to 86%, reflecting that natural gas quickly passed through the core and displacing the movable water. Continue to use 0.3 Mpa pressure for 1 min, 2 min, 11 min, that is, the cumulative driving time is 15 min, the core water saturation is 76%, and the water saturation is equally and decreases slowly, reflecting the carry effect as the main displacement method. Subsequently, the driving pressure was gradually increased to 3 Mpa, simple calculation, 0.3 Mpa driving for 10 min, 0.5 Mpa driving for 5 min, 0.8 Mpa driving for 10 min, 1 Mpa driving for 5 min, 2 Mpa driving for 10 min, 3 Mpa driving for 5 min, the water saturation decreases of 4%, 1%, 4%, 2%, 7%, and 2% respectively, indicating that the decrease of water saturation caused by the carry effect is related to the gas flow rate. At same time, high driving pressure also can displace smaller pore water, but drive effect not directly observable.

Figure 6. sample of A45 (4106.86 m in Well F-1) high-permeability GWD-MRI experiment results.

Figure 6. sample of A45 (4106.86 m in Well F-1) high-permeability GWD-MRI experiment results.

The name of the photo represents "Sample Number-Driving Pressure-Cumulative Driving Time-Water Saturation".

After completing the MRI of the three samples with different driving times, the NMR T2 spectrum test was still carried out. It can be seen from the T2 spectrum that the T2 relaxation time of the movable fluid in the core is mainly 10~500ms, and the bound water is mainly <10 ms. The movable fluid gradually decreases during the driving process, but does not completely displaced. There is no change in bound water. Under the condition of low-pressure driving for 60 to 90 minutes, the water content of low-permeability cores is generally decrease less than 25%, the water content of medium-permeability cores is decrease about 30%, and the water content of high-permeability cores is decrease 40%. The NMR T2 spectrum also shows that a small part of the movable fluid is still stay in the core. According to the equal distribution of water content in the MRI analysis, the remaining movable fluid will decrease by carry effect of flooding gas.

According to the relationship between water saturation and driving time (Figure 7d), the A25 sample is the carry effect during 20–6 min, and the water saturation slowly decreases by about 10%. The A10 sample 70–95 min is the carry effect, water saturation decrease slowly by 5%. According to trend line analysis, carry effect can make the water saturation drop by 10–15%.

3.2. Trap slowly charge result

The main feature of trap charging is that natural gas will not go away, only water can discharge, which is most fit with the Semi-Permeable Baffle (SPB) charging modeling. This paper has completed the SPB charging experiment of 6 core samples, and the charging pressure is 0~1.4 MPa, the balance time of each pressure is 4~6 h.

After analyzing of 6 samples, we deem that the water saturation change is almost not affected by the rock physical properties. The water saturation of different cores decreases is basically the same, under the increase of charging pressure, but the rock physical properties affect the final water saturation. Sample of A1 (3449 m, Well A-1) has a permeability of 0.1 mD, a porosity of 7.6%, and a water saturation of 55% at a charging pressure of 1MPa. If the charging pressure is increased again, the water saturation will not decrease and remains at 50% stably. Samples of A20 (4008 m, Well B-2) and A26 (5106.9 m, Well D-1), the permeability is 0.29 mD and 0.37 mD, the porosity is 7.2% and 8.5%, respectively. Charging pressure higher than 1.2 MPa, the water saturation is basically unchanged and remains at 45%. Samples A4 (3823.1 m, Well A-1), A7 (3614.9 m, Well A-2) and A17 (3752.7 m, Well B-2), the permeability is 1.9 mD, 8.1 mD, and 2.6 mD, respectively, and water saturation after 1.1MPa is basically unchanged, stable at about 40%. The final water saturation of high porosity and permeability cores will remain at 40%, and low permeability cores will basically remain around 45%–50%.

Figure 8. Relationship between the charging pressure and the water saturation of SPB experiment. a: Sw change of SPB charging modeling of ultra-low permeability (<0.1 mD); b: Sw change of SPB charging modeling of lower permeability (0.1–0.5 mD); c: Sw change of SPB charging modeling of middle and higher permeability (>0.5 mD).

Figure 8. Relationship between the charging pressure and the water saturation of SPB experiment. a: Sw change of SPB charging modeling of ultra-low permeability (<0.1 mD); b: Sw change of SPB charging modeling of lower permeability (0.1–0.5 mD); c: Sw change of SPB charging modeling of middle and higher permeability (>0.5 mD).

3.3. Trap rapidly charge result

The main characteristic of short-term high-intensity charge is that the charging pressure is extremely high and the time is relatively short, which is most fit with the centrifugal experiment. Select 7 samples to complete the centrifugal experiments at 1500, 3000, 4000, 5000, 6000, 6500, and 7000 rpm. After each centrifugation is completed, the NMR T2 spectrum test is performed. According to the speed, the length of the centrifugal arm, and the length of the core, the centrifugal speed can be converted into the pressure at the bottom of the core.

$$P_c=1.097\times {10}^{-9}\mathsf{\Delta }\mathsf{\rho }\mathrm{L}(R_e-\frac{L}{2})n^2$$

(1)

Where, Pc is the centrifugal charge pressure, MPa; Δρ is the density difference between the two fluids, g/cm³; L is the core length, cm; Re is the max radius of the core, cm; n is the centrifugal rotation speed, rpm.

Converting the centrifugal speed to the pressure at the bottom of the core, it can be seen that, A7, A25, and A45 have the permeability of >2.5 mD, when the centrifugal pressure less than 0.6 MPa, water saturation rapid decrease, if the pressure is above 0.6MPa, the water saturation decrease slowly, and the water saturation is considered to be Stable at 15%~30%. Samples of A5, A10, and A26 have permeability of 0.3~1 mD, when the pressure bigger than 1 MPa, the water saturation decrease slowly. According to curve extension, it is considered that the water saturation is stable at about 30%. The permeability of the A1 sample is 0.1 mD. Under the experimental conditions, there is no inflection point for the decrease of water saturation. The residual water saturation at the same pressure is higher than other samples. This reflects that, the inflection point of water decrease influent by core permeability, physical properties is better, inflection point is lower.

Figure 9. Cen-NMR T2 spectrum and water saturation relationship. a; T2 spectrum of A26 under different rotating speed; b: T2 spectrum of A5 under different rotating speed; c: T2 spectrum of A45 under different rotating speed; d: water saturation change of different centrifugal force.

Figure 9. Cen-NMR T2 spectrum and water saturation relationship. a; T2 spectrum of A26 under different rotating speed; b: T2 spectrum of A5 under different rotating speed; c: T2 spectrum of A45 under different rotating speed; d: water saturation change of different centrifugal force.

4. Discussion

4.1. Final water saturation of different physical modeling

The final water saturation of cores with different physical properties is quite different on several experiments. For cores with permeability <0.1mD, there is only one sample of A1. The Semi-Permeable Baffle (SPB) and the Cen-NMR experiment have been completed, but the MRI has not been completed. As the same driving pressure for the ultra-low permeability samples, the water saturation of the SPB modeling is low, and the Cen-NMR is higher.

The sample with a permeability of 0.1–0.5 mD represents that there are 3 cores A10, A20, and A26 in total. Among them, the A10 sample completed the GWD-MRI and Cen-NMR experiments, and the A20 sample, with similar properties, completed the SPB experiment. The final water saturation of the SPB experiment of low permeability samples was the lowest, Cen-NMR was medium, and the GWD-MRI was the highest.

The samples with permeability 0.5–2 mD represent cores with A4 and A25. A25 completes GWD-NMR and Cen-NMR experiments, and A4 samples with similar permeability complete SPB experiments. The water saturation of Semi-Permeable Baffle, Centrifugal-NMR, and Gas Drive Water Displacement - Nuclear Magnetic Imaging of medium permeability samples are relatively similar.

The samples with permeability> 2 mD are A7, A17, and A45. Among them, the A45 sample has completed the GWD-MRI and Cen-NMR experiment, A7 sample with the similar permeability has completed the SPB. Displacement Imaging has the highest water saturation, and the final water saturation of SPB and Cen-NMR is relatively close.

Figure 10. Comparison of water saturation of different modeling experiments. a: comparison of ultra-low permeability(<0.1 mD); b, comparison of lower permeability (0.1–0.5 mD); c: comparison of middle permeability (0.5–2 mD); d: comparison of higher permeability (>2 mD).

Figure 10. Comparison of water saturation of different modeling experiments. a: comparison of ultra-low permeability(<0.1 mD); b, comparison of lower permeability (0.1–0.5 mD); c: comparison of middle permeability (0.5–2 mD); d: comparison of higher permeability (>2 mD).

The water saturation of displacement imaging is generally 10% higher than that of Semi-Permeable Baffle and Centrifugal-NMR modeling under the same driving pressure (Figure 10b,d), indicating that the gas charging of the migration layer is wicker than that of the reservoir within the trap. Permeability >0.1 mD, Semi-Permeable Baffle and Centrifugal Nuclear Magnetic water saturation are relatively similar (Figure 10b–d), indicating that trap charging have similar water saturation when drive pressure is low( not consider the charging time).

4.3. Characters of trap charging

Trap accumulation is mainly an episodic charging process, even one pulse of episodic, it mainly 0.01–0.1Ma. Therefore, at the beginning of episodic charging, it is a rapid charging process, the charging pressure is high, charge power is mainly controlled by source rock, and the charging time is short, which is suitable for modeling of Centrifugal—Nuclear Magnetic Resonance experiment. When the charging pulse is mid-late period, the charging pressure is small, power is mainly buoyancy, and the charging time is longer, it is suitable to use the Semi-Permeable Baffle experiment to simulate.

(1) Rapidly trap charging

The centrifugal modeling is most suitable for the short-term gas charging process at the initial stage of episodic charging. The mid-late episodic charging is suitable for Semi-Permeable Baffle charging model. The NMR curves of different centrifugal forces (Figure 9) show that, cores with permeability <0.1 mD have almost no large pores, cores with permeability around 0.5 mD are relatively close on water content of large pores and small pores, cores with permeability >1 mD has a very high water content of large pores. After centrifugation, the water saturation and permeability have a good relationship, and the permeability is higher, the water saturation is lower after centrifugation (Figure 12). The results of Cen-NMR experiments prove that the short-term rapid charging is completely affected by the physical properties of the rock (Figure 12).

The water saturation decrease quickly when the centrifugal force is lower than 0.8 MPa, and decline slowly if it is above 0.8 MPa (Figure 9d). When the centrifugal force is above 2 MPa, it is difficult to judge the change trend of water saturation due to experimental limitations. However, according to the curve extension of A7 and A45 samples, it can be seen that the water saturation of samples with higher permeability will not change as the pressure increases. For samples with lower permeability, the water saturation will still decrease as the centrifugal force increases. This shows that the rapid charging of the trap can completely fill at a about 2 MPa, if the reservoir is good physical properties.

(2) Slowly trap charging

• Time factor

The main permeability of the reservoirs in the study area is about 0.1–10 mD, there are exist obviously Gas Water Contact. The charging pressure of the gas trap is calculated according to the gas-water density difference.

$$\mathrm{P}=({\rho }_w-{\rho }_g)g\Delta h$$

(2)

There, P—trap charging power, MPa; ${\rho }_w$—formation water density, g/cm3; ${\rho }_g$—natural gas density under formation conditions, g/cm3; g—gravitational acceleration, N/m; Δh—depth between gas-water contact and measuring point, km.

According to the formation conditions of the study area, the natural gas density is generally 0.2 g/cm³. The height of the gas column of mainly gas reservoirs is 150–200 m. The maximum charging pressure of the reservoir top is about 1.3–1.6MPa, and the max SPB charging pressure is the 1.4 MPa, which is similar with the maximum charging power of the reservoir in the study area. Even the equilibrium time for the semi-permeable charging modeling is very long, there is still a huge gap compared with the historical period of charging geology.

The gas water contact drilled in Well A-4 is about 3550 m, the gas water contact is obvious, and the water saturation at about 3510 m is 40%–55% by sealed cores measure (Table 2). According to the charging pressure cause by the difference in gas/water density, the charging pressure in a small area above the gas water contact is very small, only about 0.2 MPa. The water saturation of SPB experimental modeling is 80%–90%, which is huge gap between that of sealed core test. This indicates that the time factor is the most important factor to control the water saturation of the reservoir. On the contrary, the pressure factor in the modeling experiment has almost negligible influence on the water saturation.

Considering the time factor, even the charging pressure is low, the reservoir can still reach a high gas saturation. Therefore, for traps with relatively good physical properties, the water saturation of the reservoir above the gas water contact reaches the minimum. The conclusion that the charging pressure affects the water saturation in experimental modeling is not suitable for the slow charging of traps in the geological accumulation. From the experimental modeling and sealed cores analysis, the slow trap accumulation is only affected by the time factor.

2.

Final water saturation

Even though the SPB charging experiment modeling cannot truly describe the trap accumulation process, the minimum water saturation obtained by the SPB is very matched with the sealed core data.

According to the SPB charging modeling experiment, when the charging pressure is about 1.2 MPa, the water saturation of the reservoir is basically stable, the increase of charging pressure will no longer cause the water saturation to decrease (Figure 8a–c), and the water saturation is basically maintained at about 40%-50%, it shows that the water saturation of 40%–50% is the lowest water saturation that the reservoir can reach, that is, the maximum gas saturation of the study area is 50%–60%. The water saturation of the sealed cores measurement in well A-4 is basically 40%–55% (Table 2), which matches the final water saturation of 40%–50% obtained by the SPB experimental modeling (Figure 12).

The water saturation data of the sealed cores is negatively correlated with the rock physical properties, but the correlation is poor, which also shows that the physical properties have little influence on the water saturation of the reservoir, and are greatly affected by the charging time.

Figure 13. Comparison of final water saturation between sealed cores and SPB modeling.

Figure 13. Comparison of final water saturation between sealed cores and SPB modeling.

5. Conclusion

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