A method for quickly evaluating the fracturing transformation effect of multi-stage fractured horizontal wells in shale gas

Abstract

The present invention discloses a method for quickly evaluating the fracturing transformation effect of a multi-stage fractured horizontal well in shale gas, comprising the following steps: S1: Taking the multi-stage fractured horizontal well in shale gas to be evaluated as the target well, collecting the production and casing pressure data after the fracturing transformation of the target well; S2: According to the production and casing pressure data, performing historical fitting by using the equivalent well index equation; S3: Determining the equivalent well index in the equivalent well index equation according to the result of the historical fitting; S4: Evaluating the fracturing transformation effect of the target well according to the magnitude of the equivalent well index. The present invention can determine the equivalent well index of the multi-stage fractured horizontal well in shale gas only through the post-fracture production and casing pressure data, and then quantitatively evaluate the fracturing transformation effect of the multi-stage fractured horizontal well in shale gas through the equivalent well index, providing technical support for reducing the fracturing cost and improving the shale gas recovery rate.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas reservoir development, and particularly relates to a method for quickly evaluating the fracturing transformation effect of a multi-stage fractured horizontal well in shale gas. Background Art

[0002] Shale is extremely dense, and it is difficult to achieve effective transformation by conventional fracturing methods. Generally, horizontal well + multi-stage fracturing technology is required to achieve economic development. At present, there are still many challenges in how to scientifically and efficiently develop shale gas. Therefore, an objective and accurate method is needed to evaluate the fracturing transformation effect, so as to lay a foundation for optimizing the fracturing plan, thereby reducing the fracturing cost and increasing the shale gas recovery rate.

[0003] At present, there are mainly two methods for evaluating the fracturing transformation effect. One is the direct method represented by microseismic. However, the cost of microseismic is high, and its interpretation results are easily affected by various factors, such as other noise sources during the fracturing operation, the density and distribution of geophones, etc. In addition, microseismic can only distinguish the events of rock fracture during fracturing and cannot distinguish the effective fractures filled with proppant. The other is the indirect method, including transient flow analysis methods based on analytical / semi-analytical solutions, post-fracture pump shut-in pressure drop test methods, etc. The transient flow analysis method based on analytical / semi-analytical solutions needs to simplify the shape of the hydraulic fracture, such as a flat fracture. However, there are significant differences between these simplified fractures and the actual fractures. The post-fracture pump shut-in pressure drop test method is time-consuming, and the test time for a single stage is about several hours, which has a great impact on the fracturing construction progress. Therefore, the on-site test time is short, generally about 15 minutes, and the recorded data points are insufficient, making it difficult to accurately evaluate the fracturing transformation effect. Summary of the Invention

[0004] Aiming at the above problems, the present invention aims to provide a method for quickly evaluating the fracturing transformation effect of a multi-stage fractured horizontal well in shale gas.

[0005] The technical solution of the present invention is as follows:

[0006] A method for quickly evaluating the fracturing transformation effect of a multi-stage fractured horizontal well in shale gas, comprising the following steps:

[0007] S1: Taking the multi-stage fractured horizontal well in shale gas to be evaluated as the target well, and collecting the production and casing pressure data after the fracturing transformation of the target well;

[0008] S2: According to the production and casing pressure data, performing historical fitting by using the equivalent well index equation;

[0009] S3: Determining the equivalent well index in the equivalent well index equation according to the result of historical fitting;

[0010] S4: Evaluate the fracturing effect of the target well according to the magnitude of the equivalent well index.

[0011] Preferably, in step S2, the equivalent well index equation is:

[0012]

[0013]

[0014] In the formula: q(t n+1 ) is the production at time t n+1 , m 3 / d; t is time, d, and the subscripts n + 1 and i represent the (n + 1)-th day and the i-th day respectively; p ca (t i ) and p ca (t i+1 ) are the casing pressures at times t i and t i+1 respectively, Pa; WI is the equivalent well index, m 3 ; μ 0 is the viscosity of the gas under standard conditions, Pa·s; E α,1 is the Mittag-Leffler equation; λ and α are both parameters of the Mittag-Leffler equation, dimensionless; Г is the Gamma function.

[0015] Preferably, in step S2, when performing history matching, only the data after the production peak is used for history matching.

[0016] Preferably, in step S4, the evaluation criterion for evaluating the fracturing effect of the target well according to the magnitude of the equivalent well index is:

[0017] The greater the equivalent well index, the better the fracturing effect of the target well;

[0018] The smaller the equivalent well index, the worse the fracturing effect of the target well.

[0019] The beneficial effects of the present invention are:

[0020] The production and casing pressure data required by the present invention are easy to obtain and have low costs; there is no need to assume the fracture shape and no complex numerical calculations are involved, and rapid quantitative evaluation of the fracturing effect of shale gas multi-stage fractured horizontal wells can be achieved; it will not affect the fracturing construction progress. Description of the Drawings

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0022] Figure 1 Schematic diagram of the casing pressure data result of horizontal well 1 in a specific embodiment;

[0023] Figure 2 Schematic diagram of the casing pressure data result of horizontal well 2 in a specific embodiment;

[0024] Figure 3 Schematic diagram of the production data and historical fitting result of horizontal well 1 in a specific embodiment;

[0025] Figure 4 Schematic diagram of the production data and historical fitting result of horizontal well 2 in a specific embodiment. Detailed implementation manners

[0026] The following further describes the present invention in conjunction with the drawings and embodiments. It should be noted that, without conflict, the embodiments in this application and the technical features in the embodiments can be combined with each other. It should be pointed out that unless otherwise specified, all technical and scientific terms used in this application have the same meaning as commonly understood by those of ordinary skill in the technical field to which this application belongs. The terms such as "including" or "comprising" used in the disclosure of the present invention mean that the elements or objects appearing before this word cover the elements or objects listed after this word and their equivalents, without excluding other elements or objects.

[0027] The present invention provides a method for quickly evaluating the fracturing effect of shale gas multi-stage fractured horizontal wells, including the following steps:

[0028] S1: Taking the shale gas multi-stage fractured horizontal well to be evaluated as the target well, and collecting the production and casing pressure data after the fracturing of the target well;

[0029] S2: According to the production and casing pressure data, perform historical fitting using the equivalent well index equation; the equivalent well index equation is:

[0030]

[0031]

[0032] In the formula: q(t n+1 ) is the production at time t n+1 , m 3 / d; t is time, and the subscripts n + 1 and i of d represent the (n + 1)-th day and the i-th day respectively; p ca (t i ) and p ca (t i+1 ) are the casing pressures at t i and t i+1 respectively, in Pa; WI is the equivalent well index, in m 3 ; μ 0 is the viscosity of the gas under standard conditions (25 °C, 0.1 MPa), in Pa·s; E α,1 is the Mittag-Leffler equation; λ and α are both parameters of the Mittag-Leffler equation, dimensionless; Г is the Gamma function.

[0033] The derivation process of the equivalent well index equation described in the present invention is as follows:

[0034] Based on the Duhamel principle, the production rate can be expressed as the convolution of the derivative of the pressure drop and the production rate impulse response:

[0035]

[0036] Δp(t) = p i - p wf (t) ≈ p m - p ca (t) (4)

[0037] where q(t) is the production rate at t, in m 3 / d; Δp(τ) is the pressure drop at τ, in Pa; τ is the integration variable; Δp'(τ) is the derivative of Δp(τ), in Pa; q u (t - τ) is the production rate impulse response equation, in m 3 / d / Pa; p i is the initial reservoir pressure, in Pa; p wf is the bottom-hole flowing pressure, in Pa; p m is the maximum casing pressure after production startup, in Pa; p ca (t) is the casing pressure at t, in Pa.

[0038] Based on the rectangular integration method and the definition of the derivative, the discrete form of the equation can be expressed as:

[0039]

[0040] Furthermore, the production rate impulse response function is constructed using the fractional-order equation, that is:

[0041]

[0042] In the formula: q iniis the production at the initial moment, m 3 / d; Δp ini is the pressure drop at the initial moment, Pa;

[0043] Combining formulas (5) and (6) gives:

[0044]

[0045] Finally, transforming formula (7) gives:

[0046]

[0047] Using the equivalent well index equation of the present invention, it is possible to obtain the equivalent well index through history matching only by using the production and casing pressure data after fracturing transformation, so as to quantitatively evaluate the fracturing transformation effect of the target well.

[0048] In a specific embodiment, during history matching, only the data after the production peak is used for history matching. This can avoid the influence of fracturing fluid backflow and improve the accuracy of the results.

[0049] S3: Determine the equivalent well index in the equivalent well index equation according to the results of history matching;

[0050] S4: Evaluate the fracturing transformation effect of the target well according to the magnitude of the equivalent well index, and the evaluation criteria are:

[0051] The larger the equivalent well index, the better the fracturing transformation effect of the target well;

[0052] The smaller the equivalent well index, the worse the fracturing transformation effect of the target well.

[0053] In a specific embodiment, taking two multi-stage fractured horizontal wells (Horizontal Well 1 and Horizontal Well 2) on a shale gas platform as an example, the method for rapidly evaluating the fracturing transformation effect of multi-stage fractured horizontal wells of shale gas of the present invention is used to evaluate their fracturing transformation effects.

[0054] The casing pressure data of the Horizontal Well 1 and the Horizontal Well 2 are respectively as Figure 1 and Figure 2 shown, and the production data of the Horizontal Well 1 and the Horizontal Well 2 are respectively as Figure 3 and Figure 4 shown.

[0055] Using the equivalent well index equation shown in formulas (1)-(2) to perform history matching on the data after the production peak of the two target wells, the fitting results are respectively as Figure 3 and Figure 4 shown. Among them, the fitting parameter corresponding to the history matching result of the Horizontal Well 1 is: WI = 3.21m3 , λ = 44.84.2, α = 0.34; the fitting parameters corresponding to the historical fitting result of the horizontal well 2 are: WI = 21.58m 3 , λ = 324.18, α = 0.31. According to the equivalent well index WI results of the two target wells, the equivalent well index of the horizontal well 2 is greater than that of the horizontal well 1. Therefore, the fracturing effect of the horizontal well 2 is better than that of the horizontal well 1.

[0056] In addition, the current casing pressure of the horizontal well 1 is 3.43 MPa, and the daily gas production is 3.42×10 4 m 3 / d, and the cumulative gas production in 10 months of production is 1532×10 4 m 3 . The current casing pressure of the horizontal well 2 is 3.7 MPa, and the daily gas production is 3.64×10 4 m 3 / d, and the cumulative gas production in 8 months of production is 1609.6×10 4 m 3 . Compared with the horizontal well 1, the production time of the horizontal well 2 is short (8 months), but the cumulative gas production is greater than that of the horizontal well 1. In addition, the current casing pressure and daily gas production of the horizontal well 2 are also higher than those of the horizontal well 1. Only wells with good fracturing effects can have high daily gas production and large cumulative gas production. Therefore, this also indirectly shows that the fracturing effect of the horizontal well 2 is better than that of the horizontal well 1, thus verifying the reliability of the present invention.

[0057] In summary, the present invention can quickly, accurately, and quantitatively evaluate the fracturing effect of shale gas multi-stage fractured horizontal wells through easily obtainable post-fracture production and casing pressure data; compared with the prior art, it has significant progress.

[0058] The above is only a preferred embodiment of the present invention, and it is not intended to limit the present invention in any form. Although the present invention has been disclosed above with a preferred embodiment, it is not intended to limit the present invention. Any person skilled in the art can make some changes or modifications to the above-disclosed technical content to form equivalent embodiments with equivalent changes, but as long as the content does not depart from the technical solution of the present invention, any simple modification, equivalent change, and modification made to the above embodiments based on the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A method for quickly evaluating the fracturing effect of multi-stage fractured horizontal wells in shale gas, characterized in that, it includes the following steps: S1: Taking the multi-stage fractured horizontal well in shale gas to be evaluated as the target well, and collecting the production and casing pressure data after the fracturing of the target well; S2: According to the production and casing pressure data, historical fitting is carried out using the equivalent well index equation; the equivalent well index equation is: where: q(t n+1 ) is the production at time t n+1 , m 3 / d; t is time, d, and the subscripts n+1 and i represent the (n+1)-th day and the i-th day respectively; p ca (t i ), p ca (t i+1 ) are the casing pressures at times t i and t i+1 respectively, Pa; WI is the equivalent well index, m 3 ; μ 0 is the viscosity of the gas under standard conditions, Pa·s; E α,1 is the Mittag-Leffler equation; both λ and α are parameters of the Mittag-Leffler equation, dimensionless; Г is the Gamma function; S3: Determine the equivalent well index in the equivalent well index equation according to the result of historical fitting; S4: Evaluate the fracturing effect of the target well according to the magnitude of the equivalent well index.

2. The method for quickly evaluating the fracturing effect of multi-stage fractured horizontal wells in shale gas according to claim 1, characterized in that, in step S2, when carrying out historical fitting, only the data after the production peak is used for historical fitting.

3. The method for quickly evaluating the fracturing effect of multi-stage fractured horizontal wells in shale gas according to claim 1 or 2, characterized in that, in step S4, the evaluation criteria for evaluating the fracturing effect of the target well according to the magnitude of the equivalent well index are: the larger the equivalent well index, the better the fracturing effect of the target well; the smaller the equivalent well index, the worse the fracturing effect of the target well.

Images