Computing method for wettability reversal degree of tight sandstone

Abstract

The invention relates to the technical field of oil and gas field development, in particular to a method for calculating the wettability reversal degree of tight sandstone, which comprises the following steps: acquiring the before-soaking spontaneous imbibition time, before-soaking spontaneous imbibition amount, soaking precursor pressure difference and soaking precursor amount of a tight sandstone core sample before being soaked in a silicon dioxide nanofluid; obtaining the spontaneous imbibition time after soaking, the spontaneous imbibition amount after soaking, the displacement pressure difference after soaking and the displacement amount after soaking after the tight sandstone core sample is soaked in the silicon dioxide nanofluid; and the wettability inversion degree is obtained on the basis of obtaining the spontaneous imbibition weight and the displacement weight. According to the method for calculating the wettability reversal degree of the tight sandstone, quantitative evaluation of wettability reversal can be achieved only by relying on results obtained by a spontaneous imbibition experiment and a displacement experiment without damaging a rock core, theoretical reference is provided for improving the recovery ratio of a tight sandstone gas reservoir, and the method has important value for achieving efficient development.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas field development, and particularly relates to a method for calculating the wettability reversal degree of tight sandstone. Background Art

[0002] The wettability of rock represents the spreading ability of liquid on the core surface, usually expressed by the size of the contact angle. The wettability strength is inversely proportional to the size of the contact angle. The smaller the contact angle, the stronger the wettability; the larger the contact angle, the weaker the wettability. Tight sandstone gas reservoirs rely on elastic energy for production. Natural gas and formation water present gas-water two-phase flow in the pore throats of the rock. Due to the usually strong wettability of the rock surface, the natural recovery rate of tight sandstone gas reservoirs is generally low. Acidification treatment and carbon dioxide injection can effectively improve the seepage conditions of the gas reservoir and thus increase the recovery rate. However, the extraction mechanisms of acidification treatment and carbon dioxide injection not only include changing the wettability of the rock, but also include extraction mechanisms such as plugging removal and energy supplementation. There are relatively few studies specifically on the wettability of tight sandstone gas reservoir rocks.

[0003] Existing studies have shown that soaking the core with silica nanofluid can significantly reduce the wettability. The methods for evaluating the change degree of wettability before and after soaking include static wettability measurement experiment, spontaneous imbibition experiment and displacement experiment. Among them, the advantage of the static wettability measurement experiment is that the result is the most intuitive, but core slices are required for the experiment; the spontaneous imbibition experiment and displacement experiment directly use the core for the experiment, and the advantage is that the core is not damaged. At the same time, the spontaneous imbibition phenomenon and displacement phenomenon coexist in the pore throats of the rock. Therefore, if a method for calculating the wettability reversal degree of tight sandstone can be established based on the experimental results of the spontaneous imbibition experiment and displacement experiment, it is of great significance for formulating technical countermeasures for improving the recovery rate of tight sandstone gas reservoirs. Summary of the Invention

[0004] The present invention aims at the above problems and proposes a method for calculating the wettability reversal degree of tight sandstone.

[0005] The principle of the present invention is as follows: According to the experimental principle of the spontaneous imbibition experiment, as the imbibition time increases, the imbibition amount increases accordingly. However, when the imbibition time increases to a certain time, the imbibition amount tends to be stable and no longer changes. Record the change data of the imbibition amount with the imbibition time, and take the imbibition time at this time as the spontaneous imbibition time, and take the imbibition amount corresponding to the spontaneous imbibition time as the spontaneous imbibition amount.

[0006] Therefore, a spontaneous imbibition experiment can be carried out on the silica nanofluid before soaking to obtain the pre-soaking spontaneous imbibition time of the silica nanofluid t (0) , and the pre-soaking spontaneous imbibition amount of the silica nanofluid q imb(0) Similarly, for the spontaneous imbibition experiment of silica nanofluid after soaking, the spontaneous imbibition time of silica nanofluid after soaking can be obtained. t (1) And the spontaneous imbibition volume of silica nanofluid after soaking. q imb (1) .

[0007] q imb (0) / t (0) represents the spontaneous imbibition rate before soaking, q imb (1) / t (1) represents the spontaneous imbibition rate after soaking. For the spontaneous imbibition experiment, the wettability strength is proportional to the spontaneous imbibition rate. The stronger the wettability, the faster the spontaneous imbibition rate; the weaker the wettability, the slower the spontaneous imbibition rate. Since soaking weakens the wettability of the tight sandstone, so q imb (1) / t (1) < q imb (0) / t (0) , ( q imb (1) / t (1) ) / ( q imb (0) / t (0) ) The smaller it is, the more obvious the weakening degree of soaking on wettability.

[0008] According to the experimental principle of the core flooding experiment, as the injection volume continuously increases, the experimental pressure difference increases accordingly. However, when the injection volume increases to a certain extent, the experimental pressure difference tends to be stable and no longer changes. Record the change data of the experimental pressure difference with the injection volume, and take the experimental pressure difference at this time as the displacement pressure difference, and take the injection volume corresponding to the displacement pressure difference as the displacement volume.

[0009] Therefore, for the core flooding experiment of silica nanofluid before soaking, the displacement pressure difference Δ p (0) of silica nanofluid before soaking can be obtained, q dis (0)Similarly, after soaking the silica nanofluid, a core flooding experiment can be carried out to obtain the flooding pressure difference Δ p (1) after soaking the silica nanofluid, as well as the flooding volume of the silica nanofluid after soaking q dis (1) .

[0010] q dis (0) / Δ p (0) represents the flooding volume per unit pressure difference before soaking, q dis (1) / Δ p (1) represents the flooding volume per unit pressure difference after soaking. For the core flooding experiment, the wettability strength is inversely proportional to the flooding volume per unit pressure difference. The stronger the wettability, the smaller the flooding volume per unit pressure difference, and the weaker the wettability, the larger the flooding volume per unit pressure difference. Since soaking weakens the wettability of the tight sandstone, so q dis (1) / Δ p (1) > q dis (0) / Δ p (0) ,( q dis (1) / Δ p (1) ) / ( q dis (0) / Δ p (0) ) The larger it is, the more obvious the weakening degree of wettability by soaking.

[0011] The ( q imb (1) / t (1) ) / ( q imb (0) / t (0) ) of the spontaneous imbibition experiment and the ( q dis (1) / Δ p (1) ) / ( q dis (0) / Δ p(0) ) both reflect the degree of weakening of wettability by soaking. In the spontaneous imbibition experiment, the ( q imb (1) / t (1) ) / ( q imb (0) / t (0) ) The smaller it is, the more obvious the weakening of wettability. And in the core displacement experiment, the ( q dis (1) / Δ p (1) ) / ( q dis (0) / Δ p (0) ) The larger it is, the more obvious the weakening of wettability.

[0012] During the development of tight sandstone gas reservoirs, spontaneous imbibition and displacement phenomena coexist. If the pore throats of tight sandstone gas reservoir cores are treated with silica nanofluids, the wettability weakens, resulting in a decrease in the spontaneous imbibition rate and an increase in the displacement volume per unit pressure difference. Since the spontaneous imbibition rate is inversely proportional to the recovery factor and the displacement volume per unit pressure difference is directly proportional to the recovery factor, the combined effect of the two leads to an increase in the recovery factor of tight sandstone gas reservoirs.

[0013] The change from strong to weak wettability represents a wettability reversal. The greater the degree of wettability reversal, the smaller the spontaneous imbibition rate and the larger the displacement volume per unit pressure difference, and the greater the increase in the recovery factor. Therefore, the degree of wettability reversal determines the level of increase in the recovery factor. Since the degree of wettability reversal is jointly determined by the spontaneous imbibition rate and the displacement volume per unit pressure difference, a calculation formula for the degree of wettability reversal can be established: DWR = ω dis ×( q dis (1) ×Δ p (0) ) / ( q dis (0) ×Δ p (1) )- ω imb ×( q imb (1) × t (0) ) / ( q imb (0) ×t (1) ) (1) wherein: DWR is the degree of wettability reversal, dimensionless; ω dis is the displacement weight, dimensionless; ω imb is the spontaneous imbibition weight, dimensionless; q dis (0) is the pre - immersion displacement volume, PV; q dis (1) is the post - immersion displacement volume, PV; Δ p (0) is the pre - immersion displacement pressure difference, MPa; Δ p (1) is the post - immersion displacement pressure difference, MPa; t (0) is the pre - immersion spontaneous imbibition time, min; t (1) is the post - immersion spontaneous imbibition time, min; q imb (0) is the pre - immersion spontaneous imbibition volume, PV; q imb (1) is the post - immersion spontaneous imbibition volume, PV.

[0014] It can be seen from equation (1) that the degree of wettability reversal DWR is comprehensively determined by spontaneous imbibition and displacement. At this time q dis (1) , Δ p (0) , q dis (0) , Δ p (1) , q imb (1) , t (0) , q imb (0) and t (1) are known parameters, ω dis and ω imbis an unknown parameter. According to the spontaneous imbibition experiment, the wettability weakens after soaking, and the amount of spontaneous imbibition decreases after soaking. Therefore, the degree of reduction in the amount of spontaneous imbibition before and after soaking ( q imb (0) - q imb (1) ) / q imb (0) represents the contribution of spontaneous imbibition to the degree of wettability reversal DWR According to the core displacement experiment, the wettability weakens after soaking, and the displacement pressure difference decreases after soaking. Therefore, the degree of reduction in the displacement pressure difference before and after soaking (Δ p (0) -Δ p (1) ) / Δ p (0) represents the contribution of displacement to the degree of wettability reversal DWR The degree of reduction in the amount of spontaneous imbibition before and after soaking (

[0015] q imb (0) imb - q (1) ) / q imb (0) (0) and the degree of reduction in the displacement pressure difference (Δ p (1) -Δ p (0) ) / Δ p imb both represent the contribution. Since the degree of wettability reversal DWR is comprehensively determined by spontaneous imbibition and displacement, the degree of reduction in the amount of spontaneous imbibition and the degree of reduction in the displacement pressure difference can be normalized to establish the calculation formula for the spontaneous imbibition weight ω dis and the calculation formula for the displacement weight ω imb as follows: ω imb =(( q (0) imb - q (1) ) / q imb (0) ) / (( q imb (0) - q ​imb (1) ) / q imb (0) +(Δ p (0) -Δ p (1) ) / Δ p (0) )(2) ω dis =((Δ p (0) -Δ p (1) ) / Δ p (0) ) / (( q imb (0) - q imb (1) ) / q imb (0) +(Δ p (0) -Δ p (1) ) / Δ p (0) )(3) According to the experimental results of the spontaneous imbibition experiment and the core displacement experiment, substituting them into formula (2), formula (3) and formula (1) respectively can calculate the degree of wettability reversal. DWR , the greater the degree of wettability reversal DWR , the greater the degree of improvement in oil recovery.

[0016] The technical effect of the present invention is as follows: The present invention establishes a calculation method for the degree of wettability reversal of tight sandstone, which only relies on the results obtained from the spontaneous imbibition experiment and the displacement experiment, and can quantitatively evaluate the wettability reversal without damaging the core, providing a theoretical reference for improving the oil recovery of tight sandstone gas reservoirs and having important value for realizing efficient development. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is the oil recovery distribution diagram of tight sandstone core sample 1 before and after being soaked in silica nanofluid.

[0018] Figure 2 It is the oil recovery distribution diagram of tight sandstone core sample 2 before and after being soaked in silica nanofluid. DETAILED DESCRIPTION OF THE INVENTION

[0019] A calculation method for the degree of wettability reversal of tight sandstone is as follows: Step 1: Before the tight sandstone core sample is soaked in the silica nanofluid, a pre-soaking spontaneous imbibition experiment is carried out to obtain the data of the imbibition amount varying with the imbibition time. When the imbibition amount no longer changes with the imbibition time, the imbibition time at this moment is taken as the pre-soaking spontaneous imbibition time t (0) , the pre-soaking spontaneous imbibition time t (0) The corresponding imbibition amount is the pre-soaking spontaneous imbibition amount q imb (0) ;

[0020] Step 2: After the tight sandstone core sample is soaked in the silica nanofluid, a post-soaking spontaneous imbibition experiment is carried out to obtain the data of the imbibition amount varying with the imbibition time. When the imbibition amount no longer changes with the imbibition time, the imbibition time at this moment is taken as the post-soaking spontaneous imbibition time t (1) , the post-soaking spontaneous imbibition time t (1) The corresponding imbibition amount is the post-soaking spontaneous imbibition amount q imb (1) ;

[0021] Step 3: Before the tight sandstone core sample is soaked in the silica nanofluid, a pre-soaking core displacement experiment is carried out to obtain the data of the experimental pressure difference varying with the injection volume. When the experimental pressure difference no longer changes with the injection volume, the experimental pressure difference at this moment is taken as the pre-soaking displacement pressure difference Δ p (0) , the pre-soaking displacement pressure difference Δ p (0) The corresponding injection volume is the pre-soaking displacement volume q dis (0) ;

[0022] Step 4: After the tight sandstone core sample is soaked in the silica nanofluid, a post-soaking core displacement experiment is carried out to obtain the data of the experimental pressure difference varying with the injection volume. When the experimental pressure difference no longer changes with the injection volume, the experimental pressure difference at this moment is taken as the post-soaking displacement pressure difference Δ p (1) , the post-soaking displacement pressure difference Δ p (1) The corresponding injection volume is the post-soaking displacement volume q dis (1) ;

[0023] Step 5: Calculate the spontaneous imbibition weight according to Equation (2) ω imb , calculate the displacement weight according to Equation (3)ω dis ; Furthermore, calculate the wettability reversal degree according to Equation (1). DWR .

[0024] Specific experimental cases Select 2 tight sandstone core samples. According to "GB / T 28912-2012 Determination method for relative permeability of two-phase fluids in rocks" and "SY / T 5153-2017 Determination method for wettability of reservoir rocks", carry out the spontaneous imbibition experiment before soaking and the core displacement experiment before soaking; after soaking with 0.4% silica nanofluid for 24 hours, carry out the spontaneous imbibition experiment after soaking and the core displacement experiment after soaking.

[0025] (1) Carry out the spontaneous imbibition experiment before soaking on tight sandstone core sample 1, and carry out the spontaneous imbibition experiment after soaking after soaking with silica nanofluid. The experimental results are shown in Table 1; Table 1 Spontaneous imbibition experiment results of tight sandstone core sample 1 ; It can be seen from Table 1 that as the imbibition time increases, the imbibition volume increases accordingly. In the spontaneous imbibition experiment before soaking, when the imbibition time increases to 70 min, the imbibition volume is 0.62 PV. As the imbibition time continues to increase, the imbibition volume no longer changes, that is, the spontaneous imbibition time before soaking of tight sandstone core sample 1 t (0) = 70 min, and the corresponding spontaneous imbibition volume before soaking q imb (0) = 0.62 PV; In the spontaneous imbibition experiment after soaking, when the imbibition time increases to 30 min, the imbibition volume is 0.07 PV. As the imbibition time continues to increase, the imbibition volume no longer changes, that is, the spontaneous imbibition time after soaking of tight sandstone core sample 1 t (1) = 30 min, and the corresponding spontaneous imbibition volume after soaking q imb (1) = 0.07 PV.

[0026] Carry out the core displacement experiment before soaking on tight sandstone core sample 1, and carry out the core displacement experiment after soaking after soaking with silica nanofluid. The experimental results are shown in Table 2; Table 2 Core displacement experiment results of tight sandstone core sample 1 ; As can be seen from Table 2, as the injection volume increases continuously, the experimental pressure difference increases accordingly. In the core displacement experiment before soaking, when the injection volume increases to 1.3 PV, the experimental pressure difference is 0.252 MPa. As the injection volume continues to increase, the experimental pressure difference no longer changes. Therefore, the pre-soaking displacement pressure difference Δ p (0) = 0.252 MPa for the dense sandstone core sample 1, and the corresponding pre-soaking displacement volume q dis (0) = 1.3 PV; In the core displacement experiment after soaking, when the injection volume increases to 0.9 PV, the experimental pressure difference is 0.075 MPa. As the injection volume continues to increase, the experimental pressure difference no longer changes. Therefore, the post-soaking displacement pressure difference Δ p (1) = 0.075 MPa for the dense sandstone core sample 1, and the corresponding post-soaking displacement volume q dis (1) = 0.9 PV.

[0027] According to Equation (2), the spontaneous imbibition weight of the dense sandstone core sample 1 is calculated to be ω imb = 55.82%. According to Equation (3), the displacement weight of the dense sandstone core sample 1 is calculated to be ω dis = 44.18%. Furthermore, according to Equation (1), the wettability reversal degree of the dense sandstone core sample 1 is calculated to be DWR = 0.88.

[0028] (2) Conduct a pre-soaking spontaneous imbibition experiment on the dense sandstone core sample 2, and conduct a post-soaking spontaneous imbibition experiment after soaking with silica nanofluid. The experimental results are shown in Table 3; Table 3 Spontaneous Imbibition Experiment Results of Dense Sandstone Core Sample 2 ; The pre-soaking spontaneous imbibition time of the dense sandstone core sample 2 is obtained as t (0) = 50 min, and the corresponding pre-soaking spontaneous imbibition volume q imb (0) = 0.68 PV. The post-soaking spontaneous imbibition time t (1) = 40 min, and the corresponding post-soaking spontaneous imbibition volume q imb (1) = 0.17 PV.

[0029] Core displacement experiments were carried out on the tight sandstone core sample 2 before soaking. After soaking with silica nanofluid, core displacement experiments were carried out after soaking. The experimental results are shown in Table 4; Table 4 Core displacement experimental results of tight sandstone core sample 2 ; The pre-soaking displacement pressure difference Δ p (0) = 0.296 MPa of the tight sandstone core sample 2 was obtained, and the corresponding pre-soaking displacement volume q dis (0) = 1.0 PV. The post-soaking displacement pressure difference Δ p (1) = 0.174 MPa, and the corresponding pre-soaking displacement volume q dis (0) = 0.9 PV.

[0030] According to Equation (2), the spontaneous imbibition weight of the tight sandstone core sample 2 was calculated to be ω imb = 64.66%. According to Equation (3), the displacement weight of the tight sandstone core sample 2 was calculated to be ω dis = 35.34%. Furthermore, according to Equation (1), the wettability reversal degree of the tight sandstone core sample 2 was calculated to be DWR = 0.34.

[0031] The recovery factor distributions of the tight sandstone core sample 1 before and after soaking with silica nanofluid are as Figure 1 shown. The recovery factor distributions of the tight sandstone core sample 2 before and after soaking with silica nanofluid are as Figure 2 shown. It can be seen that the recovery factors of both the tight sandstone core sample 1 and the tight sandstone core sample 2 after soaking have increased significantly. Since the wettability reversal degree of the tight sandstone core sample 1 is greater than that of the tight sandstone core sample 2, and at the same time, the increase amplitude of the recovery factor of the tight sandstone core sample 1 is significantly higher than that of the tight sandstone core sample 2, it is confirmed that the greater the wettability reversal degree DWR the greater the degree of increase in the recovery factor.

Claims

1. A calculation method for the degree of wettability reversal of tight sandstone, characterized in that, The method is as follows: Obtain the pre - immersion spontaneous imbibition time of the tight sandstone core sample before being immersed in silica nanofluid t (0) , the pre - immersion spontaneous imbibition volume q imb (0) , the pre - immersion displacement pressure difference Δ p (0) and the pre - immersion displacement volume q dis (0) ; Obtain the spontaneous imbibition time after soaking of the tight sandstone core samples in silica nanofluid t (1) and the spontaneous imbibition volume after soaking q imb (1) and the displacement pressure difference Δ after soaking p (1) and the displacement volume after soaking q dis (1) ; Calculate the spontaneous imbibition weight according to the following formula (2) ω imb , calculate the displacement weight according to the following formula (3) ω dis : ω imb =(( q imb (0) - q imb (1) ) / q imb (0) ) / (( q imb (0) - q imb (1) ) / q imb (0) +(Δ p (0) -Δ p (1) ) / Δ p (0) )(2) ω dis =((Δ p (0) -Δ p (1) ) / Δ p (0) ) / (( q imb (0) - q imb (1) ) / q imb (0) +(Δ p (0) -Δ p (1) ) / Δ p (0) )(3) Calculate the degree of wettability reversal according to the following formula (1) DWR :[[]]END]] DWR = ω dis ×( q dis (1) ×Δ p (0) ) / ( q dis (0) ×Δ p (1) )- ω imb ×( q imb (1) × t (0) ) / ( q imb (0) × t (1) )(1)。 2. The calculation method of the wettability reversal degree of tight sandstone according to claim 1, characterized in that The spontaneous imbibition time before soaking t (0) The specific obtaining process is as follows: Before the tight sandstone core sample is soaked in the silica nanofluid, a spontaneous imbibition experiment before soaking is carried out. The imbibition time corresponding to the time when the imbibition amount does not change with the imbibition time is the spontaneous imbibition time before soaking t (0) .

3. The calculation method for the degree of wettability reversal of tight sandstone according to claim 1, characterized in that The spontaneous imbibition time after soaking t (1) The specific obtaining process is as follows: After the tight sandstone core sample is soaked in the silica nanofluid, a spontaneous imbibition experiment after soaking is carried out, and the imbibition time corresponding to the situation where the imbibition volume does not change with the imbibition time is the spontaneous imbibition time after soaking t (1) .

4. The calculation method for the degree of wettability reversal of tight sandstone according to claim 1, characterized in that, The spontaneous imbibition amount before soaking q imb (0) is the spontaneous imbibition time before soaking t (0) and the corresponding imbibition amount.

5. The calculation method for the degree of wettability reversal of tight sandstone according to claim 1, characterized in that, The spontaneous imbibition volume after soaking q imb (1) is the spontaneous imbibition time after soaking t (1) and the corresponding imbibition volume.

6. The calculation method for the degree of wettability reversal of tight sandstone according to claim 1, characterized in that, The soaking pre-displacement pressure difference Δ p (0) is obtained as follows: Before the dense sandstone core sample is soaked with the silica nanofluid, a pre-soaking core displacement experiment is carried out. The experimental pressure difference corresponding to the situation where the experimental pressure difference does not change with the injection volume is the soaking pre-displacement pressure difference Δ p (0) .

7. The calculation method for the degree of wettability reversal of tight sandstone according to claim 1, characterized in that The displacement pressure difference Δ after soaking p (1) is obtained through the following process: After the tight sandstone core sample is soaked in the silica nanofluid, a core displacement experiment after soaking is carried out. The displacement pressure difference corresponding to the situation where the experimental pressure difference does not change with the injection volume is the displacement pressure difference Δ after soaking p (1) .

8. The calculation method for the degree of wettability reversal of tight sandstone according to claim 1, characterized in that, The soaking pre-displacement volume q dis (0) is the injection volume corresponding to the soaking pre-displacement pressure difference Δ p (0) .

9. The calculation method for the degree of wettability reversal of tight sandstone according to claim 1, wherein, The displacement volume after soaking q dis (1) is the displacement pressure difference Δ p (1) corresponding injection volume.

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