Method for identifying carbon dioxide leading edge position of carbon dioxide preposed fracturing through high-frequency pressure

Abstract

The invention discloses a method for identifying a carbon dioxide leading edge position of carbon dioxide pre-fracturing through high-frequency pressure, and the method comprises the following steps: collecting carbon dioxide pre-fracturing high-frequency pressure data at a well bottom, and carrying out the nondimensionalization of the pressure data; determining inner and outer areas of the composite oil reservoir according to a well testing theory, and calculating mobility and mobility ratio; determining a dimensionless pressure equation and a definite solution condition according to stratum conditions, obtaining a bottom hole pressure expression, and performing inversion to obtain a numerical relationship with dimensionless combination time to make a typical oil line; the curve form is analyzed, and the stratum parameter influence is determined; and determining a carbon dioxide leading edge position. On the basis of a well testing theory and accurate pressure equation derivation, the characteristics of the composite oil reservoir are fully considered, including factors such as fluidity and fluidity ratio of an inner area and an outer area, shaft storage and skin factors and the like, pressure distribution in the oil reservoir and the flowing state of carbon dioxide can be more accurately described through mathematical modeling and analysis, and the analysis accuracy is improved. Therefore, the front edge position of the carbon dioxide can be determined more accurately.

Description

Technical Field

[0001] This invention relates to the field of oil reservoir development, specifically to a method for identifying the location of the carbon dioxide front in carbon dioxide pre-fracturing using high-frequency pressure. Background Technology

[0002] In the field of oil extraction, carbon dioxide front fracturing technology is an important method to enhance oil recovery. Accurately identifying the location of carbon dioxide at the reservoir front is crucial for optimizing the fracturing process, improving extraction efficiency, and evaluating extraction effectiveness.

[0003] Traditional methods for identifying the location of the carbon dioxide front have certain limitations. For example, while the tracer method can track the flow path of carbon dioxide, the process of tracer deployment and detection is relatively complex, requiring additional equipment and procedures. Furthermore, the diffusion and distribution of the tracer can be affected by various factors, leading to less accurate results.

[0004] In addition, some indirect estimation methods based on physical models often rely on a large number of assumptions and simplifications, which deviate from the complex geological structure and fluid flow characteristics of actual reservoirs, making it difficult to accurately reflect the true location of the carbon dioxide front. Summary of the Invention

[0005] The purpose of this invention is to provide a method for identifying the carbon dioxide leading edge location in carbon dioxide pre-fracturing using high-frequency pressure, so as to solve the problems mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A method for identifying the carbon dioxide leading edge location in carbon dioxide pre-fracturing using high-frequency pressure, comprising the following steps:

[0008] Step S1: After collecting high-frequency pressure data for pre-fracturing carbon dioxide at the bottom of the well, filter and normalize the data. Filtering removes high-frequency noise, and normalization uses the initial formation pressure as a reference to make the pressure data dimensionless.

[0009] Step S2: Determine the inner and outer zones of the composite reservoir based on well test theory, calculate the mobility and mobility ratio, and determine relevant parameters based on permeability and fluid viscosity;

[0010] Step S3: Determine the dimensionless pressure equation and boundary conditions according to the formation conditions. Through Laplace transformation, solve the general solution, and derive the bottom hole pressure expression based on the boundary conditions. Then, invert the numerical relationship with the dimensionless combined time to make a typical oil line.

[0011] Step S4: Calculate the pressure derivative curve from the bottom hole pressure and dimensionless combined time relationship. Analyze the curve shape and determine the influence of formation parameters based on the characteristics of the composite reservoir curve, the mobility characteristics of carbon dioxide and water, and the development of natural fractures.

[0012] Step S5: Compare the characteristics of the actual and theoretical pressure derivative curves with the changes in wellhead pressure to determine the location of the carbon dioxide front.

[0013] In this invention, step S1, during data acquisition, includes using a high-precision pressure sensor to collect high-frequency pressure data during the carbon dioxide pre-fracturing process at the bottom of the well.

[0014] In this invention, during step S1, data processing includes using a low-pass filter to filter the data to remove high-frequency noise. The filter cutoff frequency is higher than the pressure fluctuation frequency and lower than the noise frequency.

[0015] In this invention, when determining the parameters of the composite reservoir in step S2, the permeability acquisition method includes core analysis and well logging data interpretation, and the fluid viscosity comprehensively considers the properties of carbon dioxide and the original fluids in the formation.

[0016] In this invention, in step S2, it is determined whether the reservoir is a composite reservoir based on well testing theory. If it is a composite reservoir, the inner and outer zones are determined, and the mobility k / u and mobility ratio M = (k / u)1 / (k / u)2 of the inner and outer zones are calculated. The permeability k is obtained through core analysis or interpretation of logging data, and the fluid viscosity u is determined based on the injected carbon dioxide and the original fluid properties in the formation.

[0017] In this invention, in step S2, the peripheral fluidity is further judged based on the calculated flow ratio M. When M>1, it indicates that the peripheral fluidity is poor (k / u)2 is small, and when M<1, it indicates that the peripheral fluidity is good (k / u)2 is large. Furthermore, the characteristics of carbon dioxide with low viscosity and high flow ratio MCO2, and water with high viscosity and low flow ratio MWater are used to assist in subsequent analysis.

[0018] In this invention, in step S4, the determination of the shape of the pressure derivative curve is based on a comprehensive assessment of the mobility characteristics of carbon dioxide and water, the development of natural fractures, and the theoretical characteristics of the double logarithmic pressure derivative curve of a composite reservoir.

[0019] In this invention, the determination of the carbon dioxide front position in step S5 is based on a comprehensive judgment of the characteristics of the pressure derivative curve and the wellhead pressure change, with the two mutually corroborating each other to improve the accuracy of the judgment.

[0020] Compared with the prior art, the beneficial effects of the present invention are:

[0021] 1. This invention is based on well testing theory and precise pressure equation derivation. It fully considers the characteristics of complex reservoirs, including the mobility and mobility ratio of the inner and outer zones, as well as wellbore storage and skin factor. Through mathematical modeling and analysis, it can more accurately describe the pressure distribution and carbon dioxide flow state inside the reservoir, thereby more accurately determining the leading edge position of carbon dioxide. Moreover, by using high-frequency pressure data, it can capture small changes and rapid fluctuations in pressure. These changes are closely related to the leading edge propagation of carbon dioxide. Compared with traditional methods, it can more meticulously reflect the dynamic change process of carbon dioxide in the reservoir and improve the accuracy of identifying the leading edge position.

[0022] 2. In this invention, the high-frequency pressure data is acquired in real time during the fracturing process. The position of the carbon dioxide leading edge can be analyzed and determined in a timely manner based on the real-time pressure data. This allows operators to adjust fracturing parameters such as injection speed and pressure in a timely manner to optimize the fracturing process. If it is found that the advance speed of the carbon dioxide leading edge is too fast or too slow and does not meet expectations, the flow rate of injected carbon dioxide can be adjusted immediately to ensure the maximization of fracturing effect. Attached Figure Description

[0023] Figure 1 This is a schematic diagram showing the distribution of the inner and outer zones of the composite reservoir according to the present invention;

[0024] Figure 2 This is a schematic diagram of a typical double logarithmic curve for the composite reservoir of the present invention;

[0025] Figure 3 This is a schematic diagram of the semi-logarithmic curve of the composite reservoir of the present invention;

[0026] Figure 4 This is a schematic diagram of a typical composite reservoir curve of the present invention;

[0027] Figure 5 This is a schematic diagram of a typical curve of a circular composite reservoir according to the present invention;

[0028] Figure 6 This is a schematic diagram of the process of the present invention. Detailed Implementation

[0029] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0030] like Figure 6 As shown, the present invention provides a technical solution:

[0031] A method for identifying the carbon dioxide leading edge location in carbon dioxide pre-fracturing using high-frequency pressure, comprising the following steps:

[0032] Step S1: After collecting high-frequency pressure data for pre-fracturing carbon dioxide at the bottom of the well, filter and normalize the data. Filtering removes high-frequency noise, and normalization uses the initial formation pressure as a reference to make the pressure data dimensionless.

[0033] In this step, data acquisition includes using a high-precision pressure sensor to collect high-frequency pressure data at the bottom of the well during the carbon dioxide pre-fracturing process. Data processing includes using a low-pass filter to filter the data to remove high-frequency noise. The filter cutoff frequency is higher than the pressure fluctuation frequency but lower than the noise frequency.

[0034] Step S2: Determine the inner and outer zones of the composite reservoir based on well test theory, calculate the mobility and mobility ratio, and determine relevant parameters based on permeability and fluid viscosity;

[0035] like Figure 1 As shown, when determining the parameters of a composite reservoir, the permeability acquisition methods include core analysis and well logging data interpretation. Fluid viscosity comprehensively considers the properties of carbon dioxide and the original fluids in the formation. Based on well testing theory, it is determined whether the reservoir is a composite reservoir. If it is a composite reservoir, the inner and outer zones are determined, and the mobility k / u and mobility ratio M = (k / u)1 / (k / u)2 of the inner and outer zones are calculated. The permeability k is obtained through core analysis or well logging data interpretation, and the fluid viscosity u is determined based on the injected carbon dioxide and the original fluid properties in the formation. Furthermore, the peripheral fluidity is judged based on the calculated mobility ratio M. When M>1, it indicates poor peripheral fluidity (k / u)2, and when M<1, it indicates good peripheral fluidity (k / u)2. The characteristics of low viscosity carbon dioxide and high mobility MCO2, and high viscosity water and low mobility MWater are used to assist subsequent analysis.

[0036] Step S3: Determine the dimensionless pressure equation and boundary conditions according to the formation conditions. Through Laplace transformation, solve the general solution, and derive the bottom hole pressure expression based on the boundary conditions. Then, invert the numerical relationship with the dimensionless combined time to make a typical oil line.

[0037] In this embodiment, if the formation is infinitely large or circular, considering wellbore storage C D After considering the epidermal factor S, the equation and boundary conditions satisfied by the dimensionless pressure distribution can be written as follows:

[0038]

[0039] P 1D (r D ,t D =0)=0 (4-110b)

[0040] P 2D (r D ,t D =0)=0 (4-110c)

[0041] P 1D (r fD ,t D ) = P 2D (r fD ,t D (4-110d)

[0042]

[0043] P 2D (r D →∞,t D ) = 0 (infinite boundary) (4-110f-1)

[0044] P 2D (R D ,t D ) = 0 (Isobaric boundary) (4-110f-2)

[0045]

[0046] By performing a Laplace transformation on the above equation (4-110), we can write out the equations and boundary conditions satisfied by the pressures in the two regions of Laplace space.

[0047]

[0048] The general solution to the above equation (4-111) is:

[0049]

[0050] According to the outer boundary condition (4-111d), we can obtain

[0051] D = Ra·C (4-113)

[0052] Ra = 0 (infinite outer boundary)

[0053] (Isobaric pressure at the outer boundary)

[0054] (Outer boundary closed)

[0055] N=ω 12 / M 12

[0056] Based on the continuity of pressure at the interface, i.e., equation (4-111b), the following equation can be obtained.

[0057]

[0058] Based on the fact that the velocities at the interface are equal, i.e., equation (4-111c), we can obtain the following equation.

[0059]

[0060] From equations (4-114) and (4-115), we obtain

[0061] B = F·A (6-116)

[0062] In the formula

[0063]

[0064] Substituting equation (4-116) into the inner boundary conditions (4-111e) and (4-111f), and after simplification, the bottom hole pressure in the Laplace space is finally obtained.

[0065]

[0066] In the formula

[0067]

[0068] By performing a Laplace numerical inversion on equation (4-117), the bottom hole pressure P can be obtained. WD Combined with dimensionless time t D / C D The numerical relationships were then determined. Finally, a typical oil line for a composite reservoir was constructed.

[0069] Step S4: Calculate the pressure derivative curve from the bottom hole pressure and dimensionless combined time relationship. Analyze the curve shape and determine the influence of formation parameters based on the characteristics of the composite reservoir curve, the mobility characteristics of carbon dioxide and water, and the development of natural fractures. The judgment of the pressure derivative curve shape is based on a comprehensive assessment of the mobility characteristics of carbon dioxide and water, the development of natural fractures, and the theoretical characteristics of the double logarithmic pressure derivative curve of the composite reservoir.

[0070] In this step, the pressure derivative curve is calculated based on the numerical relationship between bottomhole pressure and dimensionless combined time. Analysis is performed based on the characteristics of the two horizontal lines of the double logarithmic pressure derivative curve for composite reservoirs. The first horizontal derivative line corresponds to a derivative value of 0.5, indicating that the flow in the inner ring has reached radial flow. The derivative value corresponding to the second horizontal derivative line is related to the flow in the outer ring reaching radial flow. The shape of the pressure derivative curve is determined based on the mobility characteristics of carbon dioxide and water, as well as the development of natural fractures. When carbon dioxide is in the outer region (corresponding to a curve with M < 1), the derivative drops; when carbon dioxide is in the inner region (corresponding to a curve with M > 1), the derivative rises. When natural fractures are developed in the formation, viscous fingering occurs. In severe cases, carbon dioxide drops in the outer zone derivative and flows along natural fractures carrying water. During fracturing, the pressure at the wellhead decreases rapidly during depressurization and production. When natural fractures are not well-developed in the formation, the viscous fingering phenomenon is weak or absent, and carbon dioxide is trapped in the inner zone derivative curve, which curves upward and mixes with the oil in the inner zone, resulting in a slower rate of pressure drop at the wellhead. The fitting of the pressure derivative curve with typical curves is observed to determine the influence of formation characteristic parameters such as mobility ratio, reservoir ratio, and dimensionless inner ring radius on the curve. When the outer ring reaches radial flow, the pressure derivative is at a specific value. The larger the reservoir ratio, the higher the transition curve of the derivative approaches. The larger the dimensionless inner ring radius, the later the pressure derivative deviates from the horizontal line with a value of 0.5.

[0071] like Figure 2 As shown, in an infinitely large stratum, C D e 2S =1000, r fD =1000, M 12 =0.5, PC 12 When M = 0.5, the typical curve for a complex reservoir shows two horizontal lines on the double logarithmic pressure derivative curve. The first horizontal derivative line corresponds to a derivative value of 0.5, indicating that the flow in the inner loop reaches radial flow. The second horizontal derivative line corresponds to a derivative value of M. 12 / 2 indicates that the flow in the outer ring reaches radial flow.

[0072] like Figure 3 As shown, this characteristic of the typical double logarithmic curve of a complex reservoir is reflected in the semi-logarithmic plot as follows: The semi-logarithmic plot of pressure versus time shows two straight lines. The slope m1 of the first straight line reflects the formation characteristics of the inner ring. The slope m2 of the second straight line reflects the formation characteristics of the outer ring. The ratio of the slopes of the two lines is m2 / m1 = M. 12 .

[0073] like Figure 4 box Figure 5 As shown, in complex reservoirs, the parameters affecting the typical curve, besides C... D e 2S In addition, there is also the mobility ratio M 12 Storage capacity ratio PC12 Dimensionless inner ring radius r fD M 12 The effect on the typical curve is manifested when the outer ring reaches radial flow, the pressure derivative is M. 12 / 2. PC 12 The effect on the typical curve is PC 12 The larger the value, the closer the derivative approaches M. 12 The higher the transition curve of / 2, the more r fD The effect on the typical curve is manifested in: r fD The larger the value, the later the pressure derivative deviates from the horizontal line with a value of 0.5. In circular reservoirs, when the outer boundary affects the bottomhole pressure, for fully enclosed formations, both the double logarithmic pressure curve and the double logarithmic derivative curve rise. When the flow reaches a pseudo-steady state, the two curves become tangent and form a straight line at 45°. For circular isobaric formations, the double logarithmic pressure curve flattens, and the derivative curve drops. For straight boundaries, a superposition method can be used. The basic solution of the superposition method is...

[0074] Step S5: Compare the characteristics of the actual and theoretical pressure derivative curves with the changes in wellhead pressure to determine the location of the carbon dioxide front. The judgment of the location of the carbon dioxide front is based on a comprehensive judgment of the characteristics of the pressure derivative curve and the changes in wellhead pressure. The two are mutually verified to improve the accuracy of the judgment.

[0075] In this step, the position of the carbon dioxide front is determined by comparing the actual pressure derivative curve with the pressure derivative curve characteristics corresponding to different theoretical carbon dioxide front positions. When the pressure derivative curve shows a downward characteristic corresponding to carbon dioxide entering the outer zone (M<1), it is determined that the carbon dioxide front has reached this area. The position of the carbon dioxide front is further confirmed by combining the wellhead pressure change. When the pressure derivative curve shows the above characteristics and the wellhead pressure drops rapidly, which is consistent with the characteristics of carbon dioxide carrying water along natural fractures, the position of the carbon dioxide front is determined. When the wellhead pressure drops slowly and is consistent with the characteristics of carbon dioxide mixing with oil in the inner zone, it is determined that the carbon dioxide front is still in the inner zone or has just begun to move towards the outer zone.

[0076] Specifically, for complex reservoirs, the analysis of measured bottom-hole pressure data is complex due to the large number of curve parameters. For bottom-hole pressure data with multiple flow rates, multi-flow rate correction of the pressure derivative is generally required, along with the calculation of typical curves for multiple flow rates. Curve fitting yields the time fitting value TM, the pressure fitting value TM, and the mobility ratio M. 12 Storage capacity ratio PC 12 Dimensionless inner and outer ring radii r fD Combination parameter C D e 2S dimensionless outer boundary distance L iDFrom these fitting parameters, the following interpretation results can be obtained.

[0077] The pressure fitting value PM can be obtained

[0078]

[0079] Obtained from the time-fitted value TM

[0080]

[0081] By C D e 2S get

[0082]

[0083] ΔP S =S / PM(4-121b)

[0084] Based on the mobility ratio M 12 get

[0085]

[0086] The ratio of inner and outer ring storage capacity was obtained

[0087]

[0088] By r fD e S get

[0089] r f =r w e -S [r fD e S (4-124)

[0090] By L iD e S get

[0091] L i =r w e -S [L iD e S (4-124a)

[0092] Among them, L i This represents the distance from the well to the i-th side. i=1 indicates that the reservoir is circular or has a straight boundary. i=2 indicates that the reservoir is angular or channel-shaped. i=3 indicates that the reservoir is U-shaped. i=4 indicates that the reservoir is rectangular.

[0093] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0094] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for identifying the carbon dioxide leading edge location in carbon dioxide pre-fracturing using high-frequency pressure, characterized in that, Includes the following steps: Step S1: After collecting high-frequency pressure data for pre-fracturing carbon dioxide at the bottom of the well, filter and normalize the data. Filtering removes high-frequency noise, and normalization uses the initial formation pressure as a reference to make the pressure data dimensionless. Step S2: Determine the inner and outer zones of the composite reservoir based on well test theory, calculate the mobility and mobility ratio, and determine relevant parameters based on permeability and fluid viscosity; Step S3: Determine the dimensionless pressure equation and boundary conditions according to the formation conditions. Through Laplace transformation, solve the general solution, and derive the bottom hole pressure expression based on the boundary conditions. Then, invert the numerical relationship with the dimensionless combined time to make a typical oil line. Step S4: Calculate the pressure derivative curve from the bottom hole pressure and dimensionless combined time relationship. Analyze the curve shape and determine the influence of formation parameters based on the characteristics of the composite reservoir curve, the mobility characteristics of carbon dioxide and water, and the development of natural fractures. Step S5: Compare the characteristics of the actual and theoretical pressure derivative curves with the changes in wellhead pressure to determine the location of the carbon dioxide front.

2. The method for identifying the carbon dioxide leading edge position in carbon dioxide pre-fracturing using high-frequency pressure according to claim 1, characterized in that: In step S1, data acquisition includes using a high-precision pressure sensor to collect high-frequency pressure data during the carbon dioxide pre-fracturing process at the bottom of the well.

3. The method for identifying the carbon dioxide leading edge position in carbon dioxide pre-fracturing using high-frequency pressure according to claim 1, characterized in that: In step S1, data processing includes using a low-pass filter to filter the data to remove high-frequency noise. The filter cutoff frequency is higher than the pressure fluctuation frequency but lower than the noise frequency.

4. The method for identifying the carbon dioxide leading edge position in carbon dioxide pre-fracturing using high-frequency pressure according to claim 1, characterized in that: In step S2, when determining the parameters of the composite reservoir, the permeability acquisition method includes core analysis and well logging data interpretation, and the fluid viscosity comprehensively considers the carbon dioxide and the original fluid properties of the formation.

5. The method for identifying the carbon dioxide leading edge position in carbon dioxide pre-fracturing using high-frequency pressure according to claim 1, characterized in that: In step S2, it is determined whether the reservoir is a composite reservoir based on well test theory. If it is a composite reservoir, the inner and outer zones are determined, and the mobility k / u and mobility ratio M = (k / u)1 / (k / u)2 of the inner and outer zones are calculated. The permeability k is obtained through core analysis or interpretation of logging data, and the fluid viscosity u is determined based on the injected carbon dioxide and the original fluid properties in the formation.

6. The method for identifying the carbon dioxide leading edge position in carbon dioxide pre-fracturing using high-frequency pressure according to claim 1, characterized in that: In step S2, the peripheral fluidity is further judged based on the calculated flow ratio M. When M>1, it indicates that the peripheral fluidity is poor (k / u)2 is small. When M<1, it indicates that the peripheral fluidity is good (k / u)2 is large. The characteristics of carbon dioxide with low viscosity and high flow ratio MCO2 and water with high viscosity and low flow ratio MWater are used to assist in subsequent analysis.

7. The method for identifying the carbon dioxide leading edge position in carbon dioxide pre-fracturing using high-frequency pressure according to claim 1, characterized in that: In step S4, the determination of the shape of the pressure derivative curve is based on a comprehensive assessment of the mobility characteristics of carbon dioxide and water, the development of natural fractures, and the theoretical characteristics of the double logarithmic pressure derivative curve of the composite reservoir.

8. The method for identifying the carbon dioxide leading edge position in carbon dioxide pre-fracturing using high-frequency pressure according to claim 1, characterized in that: In step S5, the determination of the carbon dioxide front position is based on a comprehensive judgment of the characteristics of the pressure derivative curve and the wellhead pressure change, with the two mutually corroborating each other to improve the accuracy of the judgment.

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