Fractured shale gas reservoir gas-water self-absorption prediction model and prediction method

Abstract

The invention discloses a fractured shale gas reservoir gas-water self-absorption prediction model and method, and the method comprises the following steps: S1, obtaining a core sample of a target reservoir, drying the core sample, and obtaining the basic physical property parameters of the core sample; s2, simulating gas-water self-absorption by adopting the rock core sample to obtain sucked water volumes and contact surface areas at different moments; s3, establishing a fractured shale gas reservoir gas-water self-absorption prediction model, and fitting according to a result of the step S2 to obtain a fitting coefficient of the prediction model; and S4, according to the prediction model after the fitting coefficient is determined, predicting the water absorption capacity and the water absorption rate of the target reservoir in the unit contact area under different water absorption time. According to the method, the water absorption capacity and the water absorption rate of the fractured shale gas well in the water injection development process can be accurately predicted and obtained only through the basic physical property parameters of the rock core.

Description

Technical Field

[0001] The present invention relates to the technical field of gas reservoir development, and in particular to a gas-water self-absorption prediction model and a prediction method for fractured shale gas reservoirs. Background Art

[0002] Spontaneous imbibition is of great significance to the water injection development of shale gas reservoirs. Domestic and foreign scholars have proposed many gas-water spontaneous imbibition prediction models, but the key reservoir parameters involved in these models are calibrated based on experimental data. In the actual production and development process, the formation conditions are a variable value and are not equivalent to indoor experiments. Therefore, they are subject to certain limitations in application. Summary of the invention

[0003] In view of the above problems, the present invention aims to provide a gas-water self-imbibition prediction model and prediction method for fractured shale gas reservoirs.

[0004] The technical solution of the present invention is as follows:

[0005] On the one hand, a prediction model for gas-water self-imbibition in a fractured shale gas reservoir is provided, wherein the prediction model is used to predict the amount of water absorbed per unit contact area and the rate of water absorption during gas-water self-imbibition in a fractured shale gas reservoir. When establishing the prediction model, the following assumptions are made:

[0006] (1) The water self-imbibition process is piston-type displacement;

[0007] (2) Ignore the gas phase pressure gradient at the water phase front;

[0008] (3) Ignore the influence of gravity;

[0009] The prediction model is:

[0010]

[0011] Where: V wt is the volume of water absorbed, cm 3 ; A c is the contact surface area, cm 2 ;P 1 , P 2 , P 3 , P 4 are all fitting coefficients; t is the imbibition time, h; λ wt is the water absorption rate per unit contact area.

[0012] Preferably, the volume of water inhaled is calculated by the following formula:

[0013] V wt =0.25πD 2 LΦS w (3)

[0014] Where: D is the core diameter, cm; L is the core length, cm; Φ is the gas-measured porosity, %; S w is the water saturation, %.

[0015] Preferably, the contact surface area is calculated by the following formula:

[0016] A c =0.5πD 2 +πDL (4)

[0017] Where: D is the core diameter, cm; L is the core length, cm.

[0018] On the other hand, a method for predicting gas-water self-imbibition in a fractured shale gas reservoir is also provided, comprising the following steps:

[0019] S1: Obtain a core sample of the target reservoir, and obtain basic physical property parameters of the core sample after drying it;

[0020] S2: using the core sample to simulate gas-water self-imbibition, and obtaining the volume of absorbed water and the contact surface area at different times t;

[0021] S3: Establish any of the above-mentioned fractured shale gas reservoir gas-water self-imbibition prediction models, and obtain the fitting coefficient P of the prediction model according to the result of step S2. 1 , P 2 , P 3 , P 4 ;

[0022] S4: Based on the prediction model after the fitting coefficient is determined, the water absorption amount and water absorption rate per unit contact area of ​​the target reservoir at different water absorption times are predicted.

[0023] Preferably, in step S1, the basic physical property parameters include core length, core diameter, and gas-measured porosity.

[0024] Preferably, when obtaining the gas-measured porosity, the confining pressure in the gas-measured porosity is approximately equal to the overlying formation pressure of the target reservoir.

[0025] The beneficial effects of the present invention are:

[0026] On the one hand, the present invention has high processing efficiency. Only through the basic physical property data of the core, the water absorption and water absorption rate of the fractured shale gas well during the water injection development process can be simulated and calculated; on the other hand, the present invention meets the engineering requirements, and the error between the predicted water absorption per unit contact area and the actual test data is less than 3%. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0028] Figure 1 It is a schematic diagram comparing the predicted and measured results of the water absorption per unit contact area of ​​a specific embodiment. DETAILED DESCRIPTION

[0029] The present invention is further described below in conjunction with the accompanying drawings and embodiments. It should be noted that, in the absence of conflict, the embodiments in this application and the technical features in the embodiments can be combined with each other. It should be noted that, unless otherwise specified, all technical and scientific terms used in this application have the same meanings as those generally understood by those of ordinary skill in the art to which this application belongs. The words "including" or "comprising" and the like used in the disclosure of the present invention mean that the elements or objects appearing before the word cover the elements or objects listed after the word and their equivalents, without excluding other elements or objects.

[0030] On the one hand, the present invention provides a prediction model for gas-water self-imbibition in fractured shale gas reservoirs, wherein the prediction model is used to predict the water absorption amount and water absorption rate per unit contact area during gas-water self-imbibition in fractured shale gas reservoirs; when establishing the prediction model, the following assumptions are made:

[0031] (1) The water self-imbibition process is piston-type displacement;

[0032] (2) Ignore the gas phase pressure gradient at the water phase front;

[0033] (3) Ignore the influence of gravity;

[0034] The prediction model is:

[0035]

[0036] Where: V wt is the volume of water absorbed, cm 3 ; A c is the contact surface area, cm 2 ;P 1 , P 2 , P 3 , P 4 are all fitting coefficients; t is the imbibition time, h; λ wt is the water absorption rate per unit contact area.

[0037] In the above embodiment, the water absorption rate per unit contact area is obtained by deducing the following formula:

[0038]

[0039] In the above embodiment, the present invention uses lg(t 0.5 ) is used as the basis to establish the prediction model, which enables the present invention to divide the entire imbibition process into four stages: imbibition activation stage, rapid imbibition stage, transition stage and final equilibrium stage, which conforms to the law that "in the initial stage of imbibition, the imbibition rate is low and the water absorption is small; when the water saturation reaches a certain level, the imbibition rate increases continuously and approaches a constant, and the water absorption increases rapidly; as the water absorption time increases, the imbibition rate decreases, and the water absorption also tends to be stable, and finally reaches the maximum water absorption". This enables the present invention to more intuitively observe and analyze the imbibition characteristics of shale gas reservoirs in each imbibition stage.

[0040] In addition, since the quantitative characterization of the relationship between the water absorption amount / water absorption rate per unit contact area and the water absorption time is affected by multiple factors such as rock microstructure, microscopic mineral composition and properties of the imbibing liquid, compared to directly establishing a prediction model based on t, the present invention first processes the data of the water absorption time, which makes it easier to obtain the quantitative parameters of the gas-water self-imbibition model of the target shale gas reservoir.

[0041] In a specific embodiment, the volume of water inhaled is calculated by the following formula:

[0042] V wt =0.25πD 2 LΦS w (3)

[0043] Where: D is the core diameter, cm; L is the core length, cm; Φ is the gas-measured porosity, %; S w is the water saturation, %.

[0044] It should be noted that the method for calculating the inhaled water volume in the above embodiment is only a preferred method for obtaining the inhaled water volume in the present invention. In addition to the method in the above embodiment, other methods for obtaining this parameter in the prior art (for example, the initial water volume minus the remaining water volume, etc.) can also be applied to the present invention.

[0045] In a specific embodiment, the contact surface area is calculated by the following formula:

[0046] A c =0.5πD 2 +πDL (4)

[0047] Where: D is the core diameter, cm; L is the core length, cm.

[0048] It should be noted that the contact surface area calculation method in the above embodiment is only a preferred contact surface area acquisition method of the present invention. In addition to the method in the above embodiment, other methods for acquiring the parameter in the prior art may also be applicable to the present invention.

[0049] On the other hand, the present invention also provides a method for predicting gas-water self-imbibition in fractured shale gas reservoirs, comprising the following steps:

[0050] S1: Obtain a core sample of the target reservoir, and obtain basic physical property parameters of the core sample after drying it.

[0051] In a specific embodiment, the basic physical property parameters include core length, core diameter, and gas-measured porosity. Optionally, when obtaining the gas-measured porosity, the confining pressure in the gas-measured porosity is approximately equal to the overlying formation pressure of the target reservoir.

[0052] S2: The core sample is used to simulate gas-water self-imbibition to obtain the volume of absorbed water and the contact surface area at different times t.

[0053] S3: Establish any of the above-mentioned fractured shale gas reservoir gas-water self-imbibition prediction models, and obtain the fitting coefficient P of the prediction model according to the result of step S2. 1 , P 2 , P 3 , P 4 .

[0054] S4: Based on the prediction model after the fitting coefficient is determined, the water absorption amount and water absorption rate per unit contact area of ​​the target reservoir at different water absorption times are predicted.

[0055] In a specific embodiment, a target reservoir of the Longmaxi Formation is taken as an example, and the gas-water self-imbibition prediction method of the fractured shale gas reservoir of the present invention is used to predict the gas-water self-imbibition of the target reservoir. In this embodiment, the basic physical property parameters of the core sample of the target reservoir are shown in Table 1:

[0056] Table 1 Basic physical properties of core samples

[0057] Core number Length, cm Diameter, cm Gas porosity, % 4-2-3 6.240 2.518 7.31

[0058] The fitting coefficients obtained from the gas-water self-imbibition simulation are:

[0059] P 1 =-0.0312827129046971 (6)

[0060] P 2 =-0.181743242510779 (7)

[0061] P 3=-6.09403880339779 (8)

[0062] P 4 =0.038867701828799 (9)

[0063] The water absorption amount and water absorption rate per unit contact area under different water absorption times are predicted by the prediction model with determined fitting coefficients, wherein the comparison results of the water absorption amount per unit contact area predicted by the present invention and the water absorption amount per unit contact area actually measured are as follows: Figure 1 As shown. Figure 1 It can be seen that the prediction result of the water absorption of the present invention is basically consistent with the test result of the actual water absorption, with an error within 3%, which meets the engineering requirements.

[0064] In summary, the present invention can quickly and accurately predict the water absorption amount and water absorption rate per unit contact area only by using the basic physical property parameters of the core sample. Compared with the prior art, the present invention is a significant improvement.

[0065] The above description is only a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Although the present invention has been disclosed as a preferred embodiment as above, it is not intended to limit the present invention. Any technician familiar with the profession can make some changes or modifications to equivalent embodiments of equivalent changes using the technical contents disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the scope of the technical solution of the present invention.

Claims

1. A prediction model for gas-water self-absorption in fractured shale gas reservoirs, characterized in that: The prediction model is used to predict the water absorption amount and water absorption rate per unit contact area during gas-water self-imbibition in fractured shale gas reservoirs. When establishing the prediction model, the following assumptions are made: (1) The water self-imbibition process is piston-type displacement; (2) Ignore the gas phase pressure gradient at the water phase front; (3) Ignore the influence of gravity; The prediction model is: Where: V wt is the volume of water absorbed, cm 3 ; A c is the contact surface area, cm 2 ; P1, P2, P3, P4 are all fitting coefficients; t is the imbibition time, h; λ wt is the water absorption rate per unit contact area.

2. The gas-water self-absorption prediction model for fractured shale gas reservoirs according to claim 1, characterized in that: The volume of water inhaled is calculated by the following formula: V wt =0.25πD 2 LΦS w (3) Where: D is the core diameter, cm; L is the core length, cm; Φ is the gas-measured porosity, %; S w is the water saturation, %.

3. The gas-water self-absorption prediction model for fractured shale gas reservoirs according to claim 1, characterized in that: The contact surface area is calculated by the following formula: A c =0.5πD 2 +πDL (4) Where: D is the core diameter, cm; L is the core length, cm.

4. A method for predicting gas-water self-absorption in fractured shale gas reservoirs, characterized in that: The following steps are involved: S1: Obtain a core sample of the target reservoir, and obtain basic physical property parameters of the core sample after drying it; S2: using the core sample to simulate gas-water self-imbibition, and obtaining the volume of absorbed water and the contact surface area at different times t; S3: Establishing the gas-water self-imbibition prediction model for the fractured shale gas reservoir according to any one of claims 1 to 3, and obtaining the fitting coefficients P1, P2, P3, and P4 of the prediction model according to the result of step S2; S4: Based on the prediction model after the fitting coefficient is determined, the water absorption amount and water absorption rate per unit contact area of ​​the target reservoir at different water absorption times are predicted.

5. The method for predicting gas-water self-absorption in fractured shale gas reservoirs according to claim 4, characterized in that: In step S1, the basic physical property parameters include core length, core diameter, and gas measurement porosity.

6. The method for predicting gas-water self-absorption in fractured shale gas reservoirs according to claim 5, characterized in that: When the gas-measured porosity is obtained, the confining pressure in the gas-measured porosity is approximately equal to the overlying formation pressure of the target reservoir.

Images