Carbonate reservoir fracture-cavity structure characterization method based on tracer technology

Abstract

The invention relates to the technical field of geological exploration, in particular to a carbonate reservoir fracture-cavity structure characterization method based on a tracer technology, which comprises the following steps: introducing a settlement coefficient kdep to characterize the settlement effect of a tracer, introducing an adsorption factor R to characterize the adsorption effect of the tracer, and calculating the fracture-cavity structure of the carbonate reservoir based on the tracer technology. Respectively establishing a tracer seepage model in a crack channel and a tracer seepage model in a karst cave channel; the method comprises the following steps: regarding a fracture-cave reservoir as a form of series connection of fractures and karst caves, and establishing a tracer seepage model in a fracture-karst cave channel; and establishing a tracer concentration curve fitting mathematical model, fitting an actually measured tracer output concentration curve on the basis of the tracer seepage mathematical model, and performing inversion to obtain to-be-solved parameters in the tracer seepage mathematical model so as to realize quantitative characterization of the fracture-cavity structure of the carbonate reservoir. By means of the characterization method, quantitative characterization of the fracture-cavity structure of the carbonate reservoir can be achieved.

Description

Technical Field

[0001] The invention relates to the technical field of geological exploration, and in particular to a method for characterizing fracture-cavity structure of carbonate reservoirs based on tracer technology. Background Art

[0002] Typical fracture-cavity carbonate reservoirs (such as the Ordovician in Xinjiang Tahe Oilfield) are deeply buried, with a very dense matrix, and basically no ability to store and seep oil and gas. However, their caves are large in volume and have developed fractures, which are the main storage space and seepage channels. The distribution of fractures and caves is highly random, and the connectivity between the two is complex. Technical means such as logging and seismic can only obtain the location of the caves, but cannot clearly determine the size of the caves and the connectivity between fractures and caves. Tracers, as an important monitoring method, can make up for the shortcomings of these technologies, and at the same time can understand the flow direction of formation fluids, the heterogeneity between injection and production wells in the well group, and the connectivity between wells.

[0003] In the prior art, a Chinese invention patent document with a publication number of CN110348154A and a publication date of October 18, 2019 is proposed. The technical solution disclosed in the patent document is as follows: a method for gas tracer interpretation and cave identification in a fracture-cavity oil reservoir well group, comprising the following steps: S1, establishing a simplified physical model of fracture-cavity connectivity between wells; S2, establishing a flow model of gas tracers in caves; S3, establishing a physical model of gas tracers in fractures, and establishing a flow model of gas tracers in fractures based on the physical model; S4, establishing a gas tracer interpretation model for the well group; S5, establishing a mathematical model for fitting a tracer concentration curve, fitting the theoretical produced tracer concentration calculated by the model with the field measured produced tracer concentration curve, so as to interpret parameters such as cave volume and fracture sweep volume in the formation; S6, judging whether there are caves on the tracer flow path based on the tracer concentration curve and the concentration derivative curve.

[0004] During the actual use of the above technical solution, the following problems may arise: the invention constructs a gas tracer convection-diffusion model for the target area, and does not consider the adsorption of the tracer by the formation rock. At the same time, carbonate gas reservoirs (such as the Dengying Formation gas reservoir in the Sichuan Basin) generally contain hydrogen sulfide, and the sampling of gas tracers is risky and costly. During the oil test, the particulate tracer is injected into the reservoir along with the completion fluid and other working fluids. By analyzing the changes in the tracer concentration in the return fluid, the characteristics of the carbonate reservoir can be characterized. This technology is simple to operate and has low testing costs. It has been promoted and applied in the dynamic monitoring of sulfur-containing gas reservoirs. Conventional particulate tracers generally do not consider the effects of sedimentation and adsorption on tracer seepage. Therefore, the constructed model cannot accurately characterize the migration law of the tracer in porous media. Summary of the invention

[0005] In order to solve the above technical problems, the present invention proposes a method for characterizing the fracture-cavity structure of carbonate reservoirs based on tracer technology, establishes a tracer seepage mathematical model and interpretation method considering convection, sedimentation and adsorption, clarifies the seepage law and main influencing factors of tracers in different fracture-cavity structures, and combines the measured tracer data to realize the quantitative characterization of the fracture-cavity structure of carbonate reservoirs.

[0006] The present invention is achieved by adopting the following technical solutions:

[0007] A method for characterizing fracture-cavity structure of carbonate reservoirs based on tracer technology comprises the following steps:

[0008] Step S 1 .The mathematical models of tracer seepage with different fracture-cavity characteristics are established respectively, including:

[0009] Introducing sedimentation coefficient k dep Characterize the sedimentation effect of the tracer, introduce the adsorption factor R to characterize the adsorption effect of the tracer, and then establish the tracer seepage model in the fracture channel and the tracer seepage model in the cave channel respectively; regard the fracture-cavity reservoir as a form of fracture and cave in series, and establish the tracer seepage model in the fracture-cavity channel;

[0010] Step S 2 .A mathematical model for fitting the tracer concentration curve is established. On the basis of the above-mentioned mathematical model for tracer seepage, the measured tracer output concentration curve is fitted, and the parameters to be determined in the mathematical model for tracer seepage are inverted to achieve quantitative characterization of the fracture-cavity structure of carbonate reservoirs.

[0011] The sedimentation coefficient k dep for:

[0012]

[0013] In the formula, φ s is the porosity of the porous medium, v is the average seepage velocity, d c is the particle diameter, and η is the unit collector efficiency of the porous medium.

[0014] The adsorption factor R is:

[0015]

[0016] In the formula, φ s is the porosity of the porous medium, K d is the adsorption distribution coefficient, ρ s is the density of the solid phase, φ f is the fluid porosity.

[0017] The tracer seepage model in the fracture channel is:

[0018]

[0019] Where C is the tracer concentration at the outlet, C 0 is the initial concentration of the tracer at the inlet, v is the average seepage velocity, x is the tracer migration distance, D is the tracer diffusion coefficient, t is the time from tracer injection to production, k dep is the sedimentation coefficient and R is the adsorption factor.

[0020] The tracer seepage model in the cave channel is:

[0021]

[0022] Where C is the tracer concentration at the outlet, D is the tracer diffusion coefficient, R is the adsorption factor, Q is the flow rate per unit thickness, and k dep is the sedimentation coefficient, t is the time from tracer injection to extraction, r is the cave radius, and φ is the porosity.

[0023] The tracer seepage model in the fracture-cavern channel is:

[0024]

[0025] Where, CF 1 is the first crack on the channel, x 1 is the displacement distance when the tracer passes through the first crack; N is the total time step, k is the kth time step in the total time step N, Δt is the time step, Δr is the radius step, C 0 is the initial concentration of the tracer at the inlet, D is the tracer diffusion coefficient, R is the adsorption factor, k dep is the sedimentation coefficient, v is the average seepage velocity, Q is the flow rate per unit thickness, φ is the porosity, CF j is the jth crack on the channel, x j is the displacement distance of the tracer when it passes through the jth crack, CV j is the jth cave on the channel, is the tracer concentration in the i-1th grid, r i is the radial radius in the i-th grid, is the tracer concentration in the ith grid, is the tracer concentration in the i+1th grid, Tracer concentration at the ith grid cell in the previous time step.

[0026] The step S 2 The objective function of the mathematical model for the tracer concentration curve fitting is:

[0027]

[0028] Where, t is the time from tracer injection to extraction, t n is the total time of tracer monitoring; C cal (t) is the theoretical calculated value of the tracer concentration at the extraction end at time t; C obs (t) is the experimental measured value of the tracer concentration at the production end at time t.

[0029] Also includes step S 3 , calculation is used to determine step S 2 The determination coefficient of the fitting effect is R i The calculation method is:

[0030]

[0031] In the formula, C cal (n) is the output concentration predicted by the tracer seepage model corresponding to the nth tracer sample point, C obs (n) is the measured tracer output concentration corresponding to the nth tracer sample point, is the average value of the measured tracer output concentration corresponding to the nth tracer sample point.

[0032] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0033] 1. The present invention takes carbonate reservoir as the research object, and establishes mathematical models of tracer seepage in fracture channels, cave channels and fracture-cave channels respectively; by defining the sedimentation coefficient k dep , the adsorption factor R characterizes the influence of sedimentation and adsorption on the migration of tracers during the migration of porous media; a tracer output concentration interpretation method is established, and based on the mathematical model of tracer seepage, the measured tracer output concentration is fitted to calculate parameters such as fracture length and cave radius. The present invention clarifies the migration mechanism and law of tracers in different types of porous media, realizes the quantitative characterization of fracture-cavity structure of carbonate reservoirs, and provides theoretical support for the establishment of a dynamic analysis method for carbonate gas reservoirs based on tracer technology.

[0034] 2. During the seepage process of the fracture-cavity reservoir, the tracer will be retained due to adsorption, and the tracer itself has a certain particle sedimentation property. Therefore, the present invention introduces the sedimentation coefficient k into the tracer seepage model in the fracture channel. dep and the adsorption factor R, which can more accurately characterize the flow law of the tracer in the fracture.

[0035] 3. The present invention assumes that the tracer fluid flows stably in the cave, taking into account the diffusion and convection of the fluid, as well as the sedimentation and adsorption effects. The tracer seepage model in the cave channel obtained in this way is also more accurate.

[0036] 4. The present invention simplifies the fracture-cavity reservoir, regards the fracture-cavity reservoir as a form of fractures and caves in series, integrates the mathematical model of tracer seepage in fractures and caves, and establishes the mathematical model of tracer seepage in the case of multiple fractures and caves based on the simplified model of fracture-cavity series reservoir.

[0037] 5. The present invention also includes calculating the coefficient of determination to further determine the fitting effect and ensure the accuracy of the final tracer seepage mathematical model. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, wherein:

[0039] Figure 1 It is a schematic diagram of the process of the present invention;

[0040] Figure 2 This is a schematic diagram of the fracture-hole series model in the present invention;

[0041] Figure 3 is a schematic diagram of a karst cave in the present invention;

[0042] Figure 4 This is a schematic diagram of the tracer flowback concentration fitting results of Well A1 in the present invention;

[0043] Figure 5 This is the fitting result of the tracer flowback concentration of Well A2 in the present invention. DETAILED DESCRIPTION

[0044] Example 1

[0045] As a basic embodiment of the present invention, the present invention includes a method for characterizing fracture-cavity structure of carbonate reservoirs based on tracer technology, comprising the following steps:

[0046] Step S 1 .The mathematical models of tracer seepage with different fracture-cavity characteristics are established respectively, including:

[0047] Introducing sedimentation coefficient k dep Characterize the sedimentation of the tracer, introduce the adsorption factor R to characterize the adsorption of the tracer, and then establish the tracer seepage model in the fracture channel and the tracer seepage model in the cave channel. When establishing the tracer seepage model in the cave channel, it is assumed that the tracer fluid flows in the cave in a stable radial direction, and fluid diffusion and convection are also considered.

[0048] The fracture-cavity reservoir is regarded as a series connection between fractures and caves, and a tracer seepage model in the fracture-cavity channel is established.

[0049] Step S 2.A mathematical model for fitting the tracer concentration curve is established. On the basis of the above-mentioned mathematical model for tracer seepage, the measured tracer output concentration curve is fitted, and the parameters to be determined in the mathematical model for tracer seepage are inverted. The parameters to be determined include fracture length, cave radius and other parameters, so as to achieve quantitative characterization of the fracture-cave structure of carbonate reservoirs.

[0050] Example 2

[0051] As a preferred embodiment of the present invention, the present invention includes a method for characterizing fracture-cavity structure of carbonate reservoirs based on tracer technology, comprising the following steps:

[0052] Step S 1 .The mathematical models of tracer seepage with different fracture-cavity characteristics are established respectively, including:

[0053] Introducing sedimentation coefficient k dep The sedimentation effect of the tracer is characterized, and the adsorption factor R is introduced to characterize the adsorption effect of the tracer. Then, the tracer seepage model in the fracture channel and the tracer seepage model in the cave channel are established respectively.

[0054] Wherein, the sedimentation coefficient k dep for:

[0055]

[0056] In the formula, φ s is the porosity of the porous medium, v is the average seepage velocity, d c is the particle diameter, and η is the unit collector efficiency of the porous medium.

[0057] The adsorption factor R is:

[0058]

[0059] In the formula, φ s is the porosity of the porous medium, K d is the adsorption distribution coefficient, ρ s is the density of the solid phase, φ f is the fluid porosity.

[0060] The fracture-cavity reservoir is regarded as a series connection between fractures and caves, and a tracer seepage model in the fracture-cavity channel is established.

[0061] Step S 2 .A mathematical model for fitting the tracer concentration curve is established. On the basis of the above-mentioned mathematical model for tracer seepage, the measured tracer output concentration curve is fitted, and the parameters to be determined in the mathematical model for tracer seepage are inverted to achieve quantitative characterization of the fracture-cavity structure of carbonate reservoirs.

[0062] Example 3

[0063] As another preferred embodiment of the present invention, the present invention includes a method for characterizing fracture-cavity structure of carbonate reservoirs based on tracer technology, comprising the following steps:

[0064] Step S 1 .The mathematical models of tracer seepage with different fracture-cavity characteristics are established respectively, including:

[0065] Introducing sedimentation coefficient k dep The sedimentation effect of the tracer is characterized, and the adsorption factor R is introduced to characterize the adsorption effect of the tracer. Then, the tracer seepage model in the fracture channel and the tracer seepage model in the cave channel are established respectively.

[0066] Wherein, the tracer seepage model in the fracture channel is:

[0067]

[0068] Where C is the tracer concentration at the outlet, C 0 is the initial concentration of the tracer at the inlet, v is the average seepage velocity, x is the tracer migration distance, D is the tracer diffusion coefficient, t is the time from tracer injection to production, k dep is the sedimentation coefficient and R is the adsorption factor.

[0069] The tracer seepage model in the cave channel is:

[0070]

[0071] Where C is the tracer concentration at the outlet, D is the tracer diffusion coefficient, R is the adsorption factor, Q is the flow rate per unit thickness, and k dep is the sedimentation coefficient, t is the time from tracer injection to extraction, r is the cave radius, and φ is the porosity.

[0072] The fracture-cavity reservoir is regarded as a series connection between fractures and caves, and a tracer seepage model in the fracture-cavity channel is established.

[0073] The tracer seepage model in the fracture-cavern channel is:

[0074]

[0075] Where, CF 1 is the first crack on the channel, x 1 is the displacement distance when the tracer passes through the first crack; N is the total time step, k is the kth time step in the total time step N, Δt is the time step, Δr is the radius step, C 0 is the initial concentration of the tracer at the inlet, D is the tracer diffusion coefficient, R is the adsorption factor, k depis the sedimentation coefficient, v is the average seepage velocity, Q is the flow rate per unit thickness, φ is the porosity, CF j is the jth crack on the channel, x j is the displacement distance of the tracer when it passes through the jth crack, CV j is the jth cave on the channel, is the tracer concentration in the i-1th grid, r i is the radial radius in the i-th grid, is the tracer concentration in the ith grid, is the tracer concentration in the i+1th grid, Tracer concentration at the ith grid cell in the previous time step.

[0076] Step S 2 .A mathematical model for fitting the tracer concentration curve is established. On the basis of the above-mentioned mathematical model for tracer seepage, the measured tracer output concentration curve is fitted, and the parameters to be determined in the mathematical model for tracer seepage are inverted to achieve quantitative characterization of the fracture-cavity structure of carbonate reservoirs.

[0077] Example 4

[0078] As another preferred embodiment of the present invention, the present invention includes a method for characterizing fracture-cavity structure of carbonate reservoirs based on tracer technology, comprising the following steps:

[0079] Step S 1 .The mathematical models of tracer seepage with different fracture-cavity characteristics are established respectively, including:

[0080] Introducing sedimentation coefficient k dep The sedimentation effect of the tracer is characterized, and the adsorption factor R is introduced to characterize the adsorption effect of the tracer. Then, the tracer seepage model in the fracture channel and the tracer seepage model in the cave channel are established respectively.

[0081] The fracture-cavity reservoir is regarded as a series connection between fractures and caves, and a tracer seepage model in the fracture-cavity channel is established.

[0082] Step S 2 .A mathematical model for fitting the tracer concentration curve is established. On the basis of the above-mentioned mathematical model for tracer seepage, the measured tracer output concentration curve is fitted, and the parameters to be determined in the mathematical model for tracer seepage are inverted to achieve quantitative characterization of the fracture-cavity structure of carbonate reservoirs.

[0083] Wherein, the step S 2 The objective function of the mathematical model for the tracer concentration curve fitting is:

[0084]

[0085] Where: t is the time from tracer injection to extraction, t n is the total time of tracer monitoring; C cal (t) is the theoretical calculated value of the tracer concentration at the extraction end at time t; C obs (t) is the experimental measured value of the tracer concentration at the production end at time t.

[0086] Step S 3 . Calculation is used to determine step S 2 The determination coefficient of the fitting effect is R i The calculation method is:

[0087]

[0088] In the formula, C cal (n) is the output concentration predicted by the tracer seepage model corresponding to the nth tracer sample point, C obs (n) is the measured tracer output concentration corresponding to the nth tracer sample point, is the average value of the measured tracer output concentration corresponding to the nth tracer sample point.

[0089] Example 5

[0090] As the best embodiment of the present invention, the present invention includes a method for characterizing fracture-cavity structure of carbonate reservoirs based on tracer technology. Figure 1 , including the following steps:

[0091] Step S 1 .The mathematical models of tracer seepage with different fracture-cavity characteristics are established respectively, including:

[0092] Introducing sedimentation coefficient k dep The sedimentation effect of the tracer is characterized, and the adsorption factor R is introduced to characterize the adsorption effect of the tracer. Then, the tracer seepage model in the fracture channel and the tracer seepage model in the cave channel are established respectively.

[0093] Specifically, about the tracer seepage model in fracture channels.

[0094] During the seepage process of the fracture-cavity reservoir, the tracer will be retained due to adsorption, and the tracer itself has a certain particle sedimentation property. Therefore, in order to accurately characterize the flow law of the tracer in the fracture, the sedimentation coefficient k is introduced. dep Characterize the sedimentation of the tracer, and the adsorption factor R characterizes the adsorption of the tracer. Assuming that the tracer seepage in the homogeneous formation is a one-dimensional stable flow and the sedimentation and adsorption of the tracer particles are irreversible, a convection diffusion-sedimentation-adsorption seepage mathematical model is established:

[0095]

[0096] Where D is the diffusion coefficient of the tracer, cm 2 / s; C is the tracer concentration at the outlet, g / cm 3 ; ρ is the tracer density, g / cm 3 ; v is the average seepage velocity, cm / s; t is the time from tracer injection to extraction, s; x is the tracer migration distance, cm; φ s is the porosity of porous media; σ is the ratio of the volume of deposited particles to the pore volume of the formation.

[0097] In formula (1), the sedimentation coefficient k dep The adsorption factor R can be defined as:

[0098]

[0099]

[0100]

[0101] In the above formula, d c is the particle diameter, cm; η is the unit collector efficiency of the porous medium; K d is the adsorption distribution coefficient; ρ s is the density of the solid phase (rock); φ f is the fluid porosity.

[0102] Combining equation (3) with equation (1) yields the expression for the tracer considering sedimentation and adsorption:

[0103]

[0104] The initial condition of the tracer seepage equation is:

[0105] C(x,0)=0 x>0 (6)

[0106] The boundary conditions are:

[0107]

[0108] In the formula, C 0 is the initial tracer concentration at the inlet, mg / L.

[0109] The tracer seepage model in the fracture channel is:

[0110]

[0111] Specifically, on the tracer seepage model in cave channels.

[0112] Assuming that the tracer fluid flows steadily in the cave, for further simplification, refer to the appendix of the manual. Figure 3 , assuming that the cave is a torus, the inner diameter of the torus, i.e. the radius of the small ring, is r 1 , the outer diameter, i.e. the radius of the large ring, is r 2 , thickness is h, porosity is φ, tracer density is ρ, and the diffusion and convection of the fluid are considered.

[0113] The flow rate of the tracer in the small ring is v 1 , concentration is C 1 ; The flow rate of the tracer flowing through the large ring is v 2 , concentration is C 2 ΔrvC is expressed as the difference between the two, that is, the flow rate and concentration of the tracer flowing through the shaded part.

[0114] Then the change of the total amount of tracer fluid per unit time is:

[0115]

[0116] According to the principle of conservation of matter, the following should be true per unit time: change in fluid mass within the unit body = mass of fluid flowing into the unit body - mass of fluid flowing out of the unit body.

[0117] Therefore, we can get:

[0118]

[0119] in:

[0120] ΔrvC=r 1 v 1 C 1 -r 2 v 2 C 2 (11)

[0121] The difference in diffuse flux per unit circle area is Δq d for:

[0122] Δq d =q d1 -q d2 (12)

[0123] In the formula, q d1 Indicates flow through a small ring (r 1 ) diffusion flux; q d2 Indicates flow through a large ring (r 2 ) of the diffusion flux.

[0124] The diffuse flux on the unit circle area is defined as:

[0125]

[0126] If Δr is infinitely close to 0, then taking the limit of equation (10) can be transformed into:

[0127]

[0128] Where r is the radius of the cave. The flow rate Q per unit thickness is defined as:

[0129] Q=2πrv (15)

[0130] Substituting the diffusion flux expression into equation (14) for simplification, we can finally obtain the radial flow model of the tracer:

[0131]

[0132] Similarly, sedimentation and adsorption also need to be considered in caves, so equation (16) can be rewritten as:

[0133]

[0134] After mathematical transformation, we can get:

[0135]

[0136] make:

[0137]

[0138] Then formula (19) can be rewritten as:

[0139]

[0140] It is difficult to solve equation (20) analytically, so a numerical solution method is needed to solve the model. Differentiating equation (20) yields:

[0141]

[0142] In the formula, is the tracer concentration in the i-1th grid; r i is the radial radius in the i-th grid; is the tracer concentration in the i-th grid; is the tracer concentration in the i+1th grid; Tracer concentration at the ith grid cell in the previous time step.

[0143] To simplify the calculation, let:

[0144]

[0145]

[0146]

[0147] P=-(Δr) 2 (25)

[0148] Substituting equations (22) to (25) into equation (21), we can write:

[0149]

[0150] Where Δr is the radius step and Δt is the time step, representing the radius and time that the injected tracer can just fill the formation. Equation (21) can be written in matrix form and solved by programming iteration:

[0151]

[0152] Where n is the total number of radius steps.

[0153] The fracture-cavity reservoir is regarded as a series connection between fractures and caves, and a tracer seepage model in the fracture-cavity channel is established.

[0154] Specifically, the fracture-cavity reservoir is mainly composed of two media, caves and fractures. The actual situation is complicated. The distribution and connectivity of caves and fractures, as well as the size and shape of caves and the length of fractures are all different. In order to obtain the flow law and interpretation model of tracers in fracture-cavity reservoirs, it is necessary to simplify the fracture-cavity reservoir and regard it as a form of fractures and caves in series, as shown in the attached manual. Figure 2 shown.

[0155] The fracture-cavity reservoir model is mainly composed of n caves and n+1 fractures. The tracer is injected from the first fracture CF 1 The entrance is C 0 After the concentration enters, it is then 1 The exit leads to the first cave CV 1 , and then flows into the second crack CF 2 , and continues until the n+1th crack CF n+1 Outflow from the outlet. This process represents that the tracer in the actual reservoir enters the fracture, then enters the cave through the fracture, and then enters the next cave through the next fracture until the tracer is discharged from the wellhead.

[0156] When the tracer enters the first fracture, according to the tracer concentration output equation obtained previously, that is:

[0157]

[0158] Where Δx represents the distance the tracer migrates per unit time.

[0159] Similarly, when the tracer passes through the first cave and enters the second fracture, the concentration at the entrance of the second fracture can be considered as the superposition of instantaneous point sources at each moment. The tracer is considered to be injected into the second fracture by continuous injection, so the concentration at any position in the j+1 fracture (j≥1) can be derived:

[0160]

[0161] Where N is the total time step, k is the kth time step in the total time step N; Δr and Δt are the radius step and time step, respectively; D is the tracer diffusion coefficient, cm 2 / s; v is the average seepage velocity, cm / s; x is the tracer migration distance, cm.

[0162] Based on the mathematical model of tracer seepage in fractures and caves and the simplified model of fracture-cavity series reservoir, the mathematical model of tracer seepage in the case of multiple fractures and caves can be established:

[0163]

[0164] Where, CF 1 is the first crack on the channel, x 1 is the displacement distance when the tracer passes through the first crack; N is the total time step, k is the kth time step in the total time step N, Δt is the time step, Δr is the radius step, C 0 is the initial concentration of the tracer at the inlet, D is the tracer diffusion coefficient, R is the adsorption factor, k dep is the sedimentation coefficient, v is the average seepage velocity, Q is the flow rate per unit thickness, φ is the porosity, CF j is the jth crack on the channel, x j is the displacement distance of the tracer when it passes through the jth crack, CV j is the jth cave on the channel, is the tracer concentration in the i-1th grid, r i is the radial radius in the i-th grid, is the tracer concentration in the ith grid, is the tracer concentration in the i+1th grid, Tracer concentration at the ith grid cell in the previous time step.

[0165] Step S 2.Establish a mathematical model for fitting the tracer concentration curve. On the basis of the above-mentioned mathematical model for tracer seepage, fit the measured tracer output concentration curve, invert and obtain the parameters to be determined in the mathematical model for tracer seepage, and realize the quantitative characterization of the fracture-cavity structure of carbonate reservoirs. The parameters to be determined include fracture length, fracture diffusion coefficient, fracture adsorption factor, fracture sedimentation coefficient, cave diameter, cave diffusion coefficient, cave adsorption factor and cave porosity.

[0166] The objective function of the mathematical model for tracer concentration curve fitting is as follows:

[0167]

[0168] Where t is the time from tracer injection to extraction, t n is the total time of tracer monitoring; C cal (t) is the theoretical calculated value of the tracer concentration at the extraction end at time t; C obs (t) is the experimental measured value of the tracer concentration at the extraction end at time t. The objective function is a nonlinear equation, and the parameters to be determined are variable values.

[0169] Step S 3 . Calculation is used to determine step S 2 The determination coefficient of the fitting effect is an indicator that reflects the degree of approximation between two sets of data, that is, the determination coefficient can reflect the degree of approximation between the theoretical calculated value of the tracer concentration (the tracer concentration predicted by the tracer seepage model) and the experimental measured value of the tracer concentration. The closer it is to 1, the stronger the correlation between the two sets of data, and the better the algorithm fitting effect. If the fitting result is poor, it is necessary to adjust the model parameters and refit the result.

[0170] Among them, the coefficient of determination R i The calculation method is:

[0171]

[0172] In the formula, C cal (n) is the output concentration predicted by the tracer seepage model corresponding to the nth tracer sample point, mg / mL; C obs (n) is the measured tracer output concentration corresponding to the nth tracer sample point, mg / mL; is the average value of the measured tracer output concentration corresponding to the nth tracer sample point, mg / mL.

[0173] Example 6

[0174] This example analyzes the tracer monitoring data of two typical wells, uses the tracer seepage mathematical model and the tracer concentration curve fitting mathematical model in the above Example 5, and develops a tracer output concentration for fitting. The tracer backflow concentration fitting results of Well A1 in the fractured reservoir are shown in the attached manual. Figure 4 As shown in the figure, the tracer flowback concentration fitting results of Well A2 in the fracture-cavity reservoir are shown in the attached manual. Figure 5 shown.

[0175] The analysis shows that there are obvious differences in the tracer flowback curves between fracture-type reservoirs and fracture-cavity reservoirs. The tracer breakthrough time in the fracture-type reservoir is earlier than that in the fracture-cavity reservoir, and the tracer output concentration is higher and has an obvious peak response.

[0176] On this basis, the measured tracer output concentration curve was fitted, and the quantitative interpretation results of relevant parameters are shown in Table 1 below.

[0177] Table 1 Tracer concentration curve fitting results

[0178]

[0179] In summary, after reading the present invention document, ordinary technicians in this field can make various other corresponding transformation schemes based on the technical scheme and technical concept of the present invention without creative mental labor, which all fall within the scope of protection of the present invention.

Claims

1. A method for characterizing fracture-cavity structure of carbonate reservoirs based on tracer technology. Features: The following steps are involved: Step S 1 .The mathematical models of tracer seepage with different fracture-cavity characteristics are established respectively, including: Introducing sedimentation coefficient k dep Characterize the sedimentation effect of the tracer, introduce the adsorption factor R to characterize the adsorption effect of the tracer, and then establish the tracer seepage model in the fracture channel and the tracer seepage model in the cave channel respectively; regard the fracture-cavity reservoir as a form of fracture and cave in series, and establish the tracer seepage model in the fracture-cavity channel; Step S 2 .A mathematical model for fitting the tracer concentration curve is established. On the basis of the above-mentioned mathematical model for tracer seepage, the measured tracer output concentration curve is fitted, and the parameters to be determined in the mathematical model for tracer seepage are inverted to achieve quantitative characterization of the fracture-cavity structure of carbonate reservoirs.

2. According to claim 1, a method for characterizing fracture-cavity structure of carbonate reservoirs based on tracer technology, Features: The sedimentation coefficient k dep for: In the formula, φ s is the porosity of the porous medium, v is the average seepage velocity, d c is the particle diameter, and η is the unit collector efficiency of the porous medium.

3. A method for characterizing fracture-cavity structure of carbonate reservoirs based on tracer technology according to claim 2, Features: The adsorption factor R is: In the formula, φ s is the porosity of the porous medium, K d is the adsorption distribution coefficient, ρ s is the density of the solid phase, φ f is the fluid porosity.

4. A method for characterizing fracture-cavity structure of carbonate reservoirs based on tracer technology according to claim 3, Features: The tracer seepage model in the fracture channel is: Where C is the tracer concentration at the outlet, C 0 is the initial concentration of the tracer at the inlet, v is the average seepage velocity, x is the tracer migration distance, D is the tracer diffusion coefficient, t is the time from tracer injection to production, k dep is the sedimentation coefficient and R is the adsorption factor.

5. A method for characterizing fracture-cavity structure of carbonate reservoirs based on tracer technology according to claim 4, Features: The tracer seepage model in the cave channel is: Where C is the tracer concentration at the outlet, D is the tracer diffusion coefficient, R is the adsorption factor, Q is the flow rate per unit thickness, and k dep is the sedimentation coefficient, t is the time from tracer injection to extraction, r is the cave radius, and φ is the porosity.

6. A method for characterizing fracture-cavity structure of carbonate reservoirs based on tracer technology according to claim 5, Features: The tracer seepage model in the fracture-cavern channel is: Where CF 1 is the first crack on the channel, x 1 is the displacement distance when the tracer passes through the first crack; N is the total time step, k is the kth time step in the total time step N, Δt is the time step, Δr is the radius step, C 0 is the initial concentration of the tracer at the inlet, D is the tracer diffusion coefficient, R is the adsorption factor, k dep is the sedimentation coefficient, v is the average seepage velocity, Q is the flow rate per unit thickness, φ is the porosity, CF j is the jth crack on the channel, x j is the displacement distance of the tracer when it passes through the jth crack, CV j is the jth cave on the channel, is the tracer concentration in the i-1th grid, r i is the radial radius in the i-th grid, is the tracer concentration in the ith grid, is the tracer concentration in the i+1th grid, Tracer concentration at the ith grid cell in the previous time step.

7. The method for characterizing fracture-cavity structure of carbonate reservoirs based on tracer technology according to claim 1, Features: The step S 2 The objective function of the mathematical model for the tracer concentration curve fitting is: Where, t is the time from tracer injection to extraction, t n is the total time of tracer monitoring; C cal (t) is the theoretical calculated value of the tracer concentration at the extraction end at time t; C obs (t) is the experimental measured value of the tracer concentration at the production end at time t.

8. A method for characterizing fracture-cavity structure of carbonate reservoirs based on tracer technology according to claim 7, Features: Also includes step S 3 , calculation is used to determine step S 2 The determination coefficient of the fitting effect is R i The calculation method is: In the formula, C cal (n) is the output concentration predicted by the tracer seepage model corresponding to the nth tracer sample point, C obs (n) is the measured tracer output concentration corresponding to the nth tracer sample point, is the average value of the measured tracer output concentration corresponding to the nth tracer sample point.

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