A method for determining the initiation zone and sequence of fractures during multi-layer hydraulic fracturing and filling of loose sandstone reservoirs.

Abstract

This invention belongs to the field of oil and gas development and exploitation engineering technology, specifically relating to a method for determining the initiation zone and sequence of fractures during multi-layer fracturing and filling operations in loose sandstone reservoirs. Based on the basic physical properties and pumping parameters of each sub-layer, this invention establishes the time-varying process of fluid absorption and wellbore pressure at each sub-layer during multi-layer fracturing and filling operations, enabling rapid identification of sub-layers that have initiated fractures and those that have not. Furthermore, based on the fractured sub-layers, this invention iteratively corrects the fluid absorption capacity of each sub-layer and calculates the changes in fluid pressure at the wellbore of the un-fractured sub-layers, thus determining the sub-layers that will subsequently initiate fractures at each stage. This provides the order and timing of fracture initiation for construction design and on-site analysis, offering a basis for construction design and field analysis.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas development and exploitation engineering technology, specifically relating to a method for determining the initiation layer and the order of initiation during multi-layer fracturing and filling construction of loose sandstone reservoirs. Using a small number of simple formation parameters and pumping parameters, the initiation layer and the order of initiation during multi-layer fracturing and filling can be easily and quickly determined, providing technical support for the optimization of layer combination and construction parameters. Background Technology

[0002] Loose sandstone reservoirs are one of the main reservoir types in my country's oil and gas development. These reservoirs are loosely cemented, with weak diagenesis, and are prone to sand production during production. Fracturing and filling technology, which combines sand control and production enhancement, has become a commonly used stimulation technique in the development of loose sandstone reservoirs. To improve reservoir stimulation effectiveness, fracturing design typically divides the target reservoir into multiple fracturing groups, and each group undergoes fracturing and filling operations separately. Each fracturing group contains multiple sub-layers, each with significant differences in pressure, thickness, permeability, and rock strength. This makes it difficult to accurately determine the number and sequence of fractures initiating in each sub-layer during fracturing, affecting well productivity calculations. Therefore, quantitative analysis and calculations are needed, taking into account the formation pressure, fluid absorption capacity, stress conditions, and construction parameters of each sub-layer.

[0003] In existing technologies, determining the initiation zone of fracturing layers during multi-layer fracturing and filling operations mainly relies on numerical simulation and fracture monitoring. Numerical simulation methods require the establishment of relatively complex models; fracture monitoring methods depend on specialized testing processes and field conditions. Therefore, existing technologies still lack a simple method suitable for effectively determining the initiation zone and fracturing sequence during multi-layer fracturing and filling operations in loose sandstone reservoirs. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a method for determining the initiation layer and sequence of fractures during multi-layer fracturing and filling operations in loose sandstone reservoirs. Addressing the difficulty in quickly and efficiently determining the actual initiation layer and sequence of fractures during existing multi-layer fracturing and filling operations in loose sandstone reservoirs, this invention provides a rapid method for determining the initiation layer and sequence of fractures in multi-layer fracturing and filling operations in loose sandstone reservoirs. This method is used to identify sub-layers that have already fractured and those that have not effectively fractured during the operation, and to determine the actual sequence of fracture initiation for each sub-layer, providing a basis for construction design and on-site analysis.

[0005] The technical problem to be solved by this invention is achieved by the following technical solution: a method for determining the initiation layer and the initiation sequence during multi-layer hydraulic fracturing and filling construction of loose sandstone reservoirs, comprising the following steps:

[0006] S1. Based on the fracture pressure and fracture pressure ratio of each sublayer, the ease of fracture initiation in each sublayer is preliminarily determined.

[0007] For each reservoir layer targeted for construction, the fracturing pressure of that layer is estimated based on its geological and mechanical parameters. At the same time, the fracturing pressure ratio of each layer is calculated to make a preliminary comparison of the ease of fracturing initiation in each layer. Based on mechanical conditions, the ease of fracturing initiation and the order of fracturing initiation in each sub-layer are preliminarily identified.

[0008] Step S1 makes a preliminary judgment on the ease of crack initiation of each sub-layer within the layer group; this step calculates the fracture pressure and fracture pressure ratio of each sub-layer by combining parameters such as the minimum horizontal principal stress and rock tensile strength of each sub-layer.

[0009] S2. Based on the physical properties of each sublayer and the construction pumping parameters, determine the layer where cracking first occurs.

[0010] Step S2 is used to determine the first fracturing layer in the formation and its initiation time. In the early stage of fracturing, no fracturing layer has yet started to fracture. After the fracturing fluid enters the formation, it mainly diffuses through matrix seepage. The fluid absorption of each layer is distributed according to its absorption capacity. The distribution of fracturing fluid in each layer before fracturing is solved. In this invention, the absorption capacity of each layer does not change with time in each single stage.

[0011] Based on this, the relationship between fluid pressure at the wellbore of each sub-layer and injection time is established. Combined with the fracture initiation criteria of each sub-layer, the sub-layer that first fractures and its initiation time are solved, providing initial conditions for judging the fracture initiation of other sub-layers.

[0012] S3. Based on the first fracture initiation layer, determine the subsequent fracture initiation layers and the order of fracture initiation.

[0013] Step S3: Given that a small layer has already started cracking, determine the layer location and cracking sequence of subsequent cracking.

[0014] When a fracture is formed in a certain sub-layer, its fluid absorption index increases significantly, while the fluid absorption index of the non-fractured sub-layer remains unchanged, resulting in a change in the fluid absorption of each sub-layer. The fluid absorption capacity of the fractured sub-layer is corrected, and the fluid absorption of each sub-layer is redistributed under the condition of total fluid volume conservation. Based on this, the fluid pressure change at the well wall of the non-fractured sub-layer is calculated, and then the next sub-layer to fracture and the fracture initiation time are determined.

[0015] By repeating the above process until all target sublayers crack or the construction end time is reached, the actual cracking locations, cracking sequence, and corresponding cracking times of the layer group can be obtained.

[0016] Preferably, step S1 of this invention specifically includes the following steps:

[0017] The fracturing of small layers can be treated as tensile failure conditions in the near-wellbore zone; the minimum horizontal principal stress is related to factors such as the burial depth of the small layer, the stress state, the elastic characteristics of the rock, and the pore pressure, while the tensile strength of the rock is related to factors such as lithology, degree of cementation, and strength of diagenesis. Therefore, the fracturing pressure of different small layers usually varies.

[0018] For the i-th sub-layer, when the fluid pressure at the wellbore rises to a level sufficient to overcome the minimum horizontal principal stress of the formation and the tensile strength of the rock, the sub-layer is considered to have fractured. The formula for the fracture pressure of the sub-layer is:

[0019] (1)

[0020] In the formula, The tensile strength of the i-th smallest layer is given in MPa. The minimum horizontal principal stress at the i-th smallest layer is expressed in MPa. Let be the fracture pressure of the i-th sublayer, in MPa; i is the sublayer number. and Estimated using well logging and rock test data;

[0021] The rupture pressure ratio is defined as the ratio of the rupture pressure of a given sub-layer to the minimum rupture pressure among all candidate layers. The specific formula is as follows:

[0022] (2)

[0023] In the formula, This is the burst pressure ratio, dimensionless; The minimum rupture pressure of all sub-layers within the layer group, in MPa;

[0024] The difficulty of fracturing in a sublayer is positively correlated with the fracturing pressure ratio. The larger the fracturing pressure ratio, the higher the pressure required for the layer to reach the fracturing initiation condition, and the less likely it is to form cracks; conversely, the smaller the fracturing pressure ratio, and the closer it is to 1, the easier it is for the layer to fracture.

[0025] As can be seen from the above relationships, the fracturing pressure is related to the burial depth of the sublayer, rock strength parameters, and differences in pore pressure within the sublayer, ultimately resulting in significant differences in interlayer fracturing pressure; the fracturing pressure obtained in step S1 is greater than R. i It mainly reflects the ease or difficulty of fracturing in a sub-layer under mechanical conditions. It can preliminarily determine the ease or difficulty of fracturing in each sub-layer and the approximate order. However, in actual construction, the fracturing situation of a sub-layer is also affected by the fluid absorption capacity of the sub-layer and the difference in fluid pressure at the well wall over time.

[0026] Preferably, step S2 of this invention specifically includes the following steps:

[0027] During multi-layer fracturing and packing operations, fracturing fluid is distributed among the sub-layers. Due to differences in physical properties such as permeability, thickness, and porosity among the sub-layers, the actual absorption of fracturing fluid varies significantly, resulting in differences in the fluid pressure at the wellbore wall of each sub-layer as the pumping time progresses. This step first determines the distribution of fracturing fluid among the sub-layers before fracturing initiation, calculates the change in fluid pressure at the wellbore wall of each sub-layer as the pumping time progresses, and further identifies the sub-layer that first meets the fracturing initiation conditions.

[0028] S2.1 Calculate the liquid absorption of each sub-layer before crack initiation.

[0029] In the early stage of multi-layer fracturing and filling construction, before any small layers have been fractured, the fracturing fluid enters the formation through the perforation in the wellbore and mainly diffuses outward in the form of matrix pore flow. In the pre-fracture stage, the flow of fracturing fluid after entering the formation can be approximated as the radial flow process of a single-phase slightly compressible fluid in a porous medium. The analysis method is similar to the radial flow treatment in conventional injection problems. Its flow is jointly controlled by factors such as reservoir permeability, thickness, fluid viscosity, and pressure gradient.

[0030] For the i-th smallest layer, its liquid absorption volume is expressed as:

[0031] (3)

[0032] In the formula, Let mD be the permeability of the i-th smallest layer. ); Let m be the thickness of the i-th smallest layer; This refers to the viscosity of the fracturing fluid. ; The equivalent radius of the effective formation control range that can be utilized during wellbore injection is taken as... (A represents the area controlled by a single well) ), m; Let be the radius of the wellbore, in meters (m). Let be the original formation pressure (original pore pressure) of the i-th sublayer, in MPa; The bottom hole flowing pressure is in MPa. This represents the amount of liquid absorbed by the i-th smallest layer. ;

[0033] Within the same layer group, when the bottom-hole flowing pressure and the formation pressure difference between each sub-layer are significantly greater than the original formation pressure difference between the sub-layers, and the completion conditions of each layer within the same construction layer group are basically the same, the influence of the original formation pressure difference between the layers on the fluid absorption can be approximately ignored; under this condition, the fluid absorption of each sub-layer within the stage can be allocated according to the fluid absorption index ratio.

[0034] For the i-th sublayer, the first stage is defined as the period before any sublayers begin to crack. Within this first stage, the liquid absorption is expressed as:

[0035] (4)

[0036] (5)

[0037] In the formula, This represents the amount of fracturing fluid absorbed by sublayer i under a unit pressure differential during stage 1, and indicates the conductivity of this layer for the injected fluid. ; This represents the amount of liquid absorbed by the i-th smallest layer during the first stage. Formula (4) is an equivalent transformation of Formula (3), defining the coefficient term unrelated to the pressure difference as the liquid absorption index J. 1，i This is to facilitate subsequent phased updates of the liquid absorption capacity of each layer; Formula (3) is the Darcy steady-state expression for radial seepage, which is to explain the initial source and applicable conditions of the model;

[0038] Under multi-layer simultaneous fracturing conditions, the sub-layers are connected through the wellbore, forming a typical parallel seepage system. According to the principle of mass conservation, the total injection volume satisfies:

[0039] (6)

[0040] In the formula, Q is the fracturing fluid injection rate during construction. , where n represents the number of sub-layers.

[0041] Combining formulas (4), (5), and (6), we can obtain the liquid absorption situation of each sublayer in the first stage:

[0042] (7)

[0043] In the first stage of the calculation, the total pumping discharge rate and the liquid absorption capacity parameters of each sub-layer are assumed to remain constant. Therefore, the liquid absorption rate of each sub-layer stage is assumed to be constant. It can be approximated as a constant and can be calculated according to formula (7). J in formula (7) 1,i Then, by substituting the k and h values ​​of each sublayer into formula (5), we can obtain the result.

[0044] S2.2 Calculate the fluid pressure at the wellbore wall of each sub-layer as a function of pumping time to determine the sub-layer that initiates fracturing first.

[0045] In the first stage, according to the radial seepage diffusion theory, the liquid absorption of each sublayer is... Substituting into the transient radial diffusion model, we solve for the dynamic variation of fluid pressure at each wellbore wall layer over time (i.e., solving for the relationship between fluid pressure at the wellbore wall and time, i.e., solving for P). 1，i(t), specific function: used in formula (11) to determine whether a small layer has fractured and to solve for the time of fracture initiation of a small layer), the fluid pressure of the i-th small layer at the well wall in the first stage is expressed as:

[0046] (8)

[0047] (9)

[0048] (10)

[0049] In the formula, The original formation pressure (original pore pressure) of the i-th sublayer can be estimated using well logging and rock test data. Let be the fluid pressure at the wellbore wall of the i-th sublayer at time t in the first stage, in MPa; Let be the pressure change at the wellbore wall of the i-th sublayer during the first stage, in MPa; Let be the pressure diffusion coefficient of the i-th smallest layer. The diffusion coefficient represents the speed at which pressure propagates in the formation; the larger the diffusion coefficient, the faster the pressure propagates. Let be Euler's constant, taken as 0.577; Let be the porosity of the i-th smallest layer, which is dimensionless; Let be the total compressibility coefficient of the i-th smallest layer, which relates to the compressibility of rock and fluid. The transient radial diffusion model is a classic existing model widely used in current reservoir engineering technology.

[0050] As the sub-layer absorbs fluid, the fluid pressure at the wellbore gradually increases. When the fluid pressure at the wellbore reaches the fracture pressure P of that sub-layer... b，i At that time, cracking occurs in the sublayer, that is:

[0051] (11)

[0052] Solve for the candidate crack initiation time of each sublayer in stage 1. :

[0053] (12)

[0054] In the formula, Let s be the candidate crack initiation time of the i-th sublayer in the first stage.

[0055] For the candidate crack initiation time τ of the i-th small layer in stage 1 1，i The initial cracking time (the absolute time from the start of construction) t1 = min{τ 1，i The corresponding stratum is the first layer to reach the fracturing pressure, i.e., the first-fracturing stratum;

[0056] If the following condition is met within the fracturing pump injection time T: t1≤T;

[0057] If no cracks are found, it is considered that the sublayer has started to crack; otherwise, it is considered that no sublayer has started to crack before the construction pumping is completed, and the judgment ends.

[0058] Preferably, step S3 of this invention specifically includes the following steps:

[0059] After fractures form in the fractured sub-layers, the equivalent conductivity is significantly enhanced, resulting in a marked increase in fluid absorption capacity. The fluid absorption of the fractured layer increases, while the fluid absorption of the remaining unfractured sub-layers decreases accordingly, thus affecting whether subsequent sub-layers can meet the fracture initiation conditions and their fracture initiation sequence. Therefore, the equivalent fluid absorption index of the fractured sub-layers is corrected, and the fluid absorption of each sub-layer is redistributed. At the same time, the fluid absorption of each sub-layer in this stage is treated as a constant, and the variation of fluid pressure at the wellbore of the unfractured sub-layers with time is calculated using a segmented cumulative method. Based on this, the subsequent fractured sub-layers and their fracture initiation sequence can be determined.

[0060] S3.1 Calculate the liquid absorption of each sub-layer after the first sub-layer fracturing.

[0061] The enhanced fluid absorption capacity after fracturing in a small layer is influenced by factors such as fracture length, fracture width, intra-fracture conductivity, fracture filtration loss, and the degree of near-wellbore damage improvement. To avoid complex fracture parameter calculations, this invention equates the aforementioned combined effects to a correction for the post-fracture fluid absorption capacity, using a correction coefficient C. frac，j Indicates; C frac，j The determination can be obtained using existing engineering evaluation methods, such as calibration based on statistical data of fracturing operations of adjacent wells in the same block, fracturing interpretation results, or test well data, or by inversion determination using methods such as on-site short-time stepped displacement / pressure response comparison; this invention does not limit the specific means of obtaining it.

[0062] Once a crack begins to appear in a certain sublayer, the next calculation stage begins. The second stage is defined as the period from the beginning of the crack in the first sublayer to the beginning of the crack in the second sublayer.

[0063] Let the j-th sublayer be the one that has already cracked. For the j-th sublayer that has already cracked, the liquid absorption index is corrected in the second stage as follows:

[0064] (13)

[0065] In the formula, This indicates the amount of fracturing fluid absorbed by the fractured sub-layer j under a unit pressure differential during stage 2. ; This represents the correction factor for the liquid absorption index of the cracked sublayer j, which is dimensionless. This indicates the amount of fracturing fluid absorbed by the fractured sub-layer j under a unit pressure differential within the first stage. ;

[0066] For the un-cracked sublayer k, it is assumed that its liquid absorption index has not changed:

[0067] (14)

[0068] In the formula, This represents the amount of fracturing fluid absorbed by the un-fractured sub-layer under a pressure differential of k units during stage 2. , This represents the amount of fracturing fluid absorbed by the un-fractured sub-layer under a pressure differential of k units within the first stage. ;

[0069] In stage 2, the amount of liquid absorbed by each sublayer is expressed as follows:

[0070] (15)

[0071] In the formula, This represents the amount of liquid absorbed by the i-th smallest layer during stage 2. , This represents the amount of fracturing fluid absorbed by sublayer i under a unit pressure differential in stage 2. .

[0072] S3.2, Fluid pressure correction at the wellbore wall of each sub-layer after the first sub-layer fractures.

[0073] To improve computational efficiency, this invention assumes that the fluid absorption of the small layer remains constant between two adjacent fracturing events, and solves the fluid pressure at the wellbore wall of the unfracturing small layer by accumulating the stages (using the fluid pressure at the wellbore wall at the end of the previous stage as the initial value). This approach is an engineering simplification aimed at rapid prediction before construction, rather than a strict solution for continuously variable flow rates.

[0074] During stage 2, for any unfractured sublayer k, the fluid pressure at the wellbore wall is expressed as:

[0075] (16)

[0076] In the formula, Let K be the fluid pressure at the wellbore wall of sublayer k after the end of stage 1, in MPa; The fluid pressure increment at the wellbore wall in stage 2, in MPa; Let be the fluid pressure at the wellbore wall of the kth sublayer at time t in the second stage, in MPa.

[0077] In the second stage, the pressure increment at any unfractured sub-layer k on the wellbore wall is:

[0078] (17)

[0079] In the formula, k kLet mD be the permeability of the kth smallest layer; h be the permeability of the kth smallest layer. k Let m be the thickness of the kth smallest layer. Let be the pressure diffusion coefficient of the kth smallest layer. ;

[0080] For a non-fractured sublayer k, the criterion for fracture initiation is the fluid pressure at the wellbore. The rupture pressure P of this layer is reached b，k :

[0081] (18)

[0082] Similarly, the candidate initiation time for each uncracked sublayer is obtained. The initiation time of the second initiation layer The corresponding sublayer is the second sublayer where cracking occurred;

[0083] If there is If the cracking occurs in a second sublayer, it is considered that a second sublayer has started to crack; otherwise, it is considered that only one sublayer has started to crack before the end of construction.

[0084] When a second fracture initiation layer exists, the liquid absorption capacity of the two fractured layers is corrected according to the same method in step S3, and the liquid absorption of each layer and the relationship between the fluid pressure at the well wall of the unfractured layer and the pumping time are updated. Based on the fracture initiation criteria of the layer, it is determined whether there is a next fracture initiation layer.

[0085] The crack initiation judgment is then completed by iteratively judging the cracks in the same way as described above until the preset termination condition is met.

[0086] Through the above steps, we can finally obtain the sub-layers initiating cracks, the crack initiation sequence, and the crack initiation time during the multi-layer fracturing and filling operation.

[0087] Preferably, the preset termination condition of this invention is the construction termination condition: when t≥T, the determination of the crack initiation layer is stopped;

[0088] If the minimum candidate fracturing time for all undrilled sublayers exceeds the pumping time T, it is considered that no new sublayers can be fracturing during the construction period. This condition means that further fracturing cannot be carried out, and the entire multi-layer fracturing operation ends.

[0089] Preferably, the preset termination condition of this invention is a layer termination condition: when all sublayers in the layer group have been determined to be cracked, the iteration stops;

[0090] At this point, through the aforementioned calculation steps, all layers have initiated cracks, successfully initiating cracks in the target layers of the multi-layer fracturing operation. All predetermined target layers have completed crack propagation, meeting the design objectives.

[0091] Key problems with existing technologies:

[0092] During multi-layer fracturing and filling operations, differences exist in formation pressure, permeability, thickness, and strength among the sub-layers within the same fracturing layer group. Consequently, the fluid absorption capacity and pressure changes over time vary among different sub-layers. Therefore, the fracturing initiation patterns of different sub-layers often differ, leading to the following main challenges in actual field operations:

[0093] (1) The initiation layer is difficult to determine accurately and quickly:

[0094] After construction is completed, it is usually difficult to quickly and accurately identify which layers have actually cracked and which layers have failed to crack effectively, and there is a lack of a simple and quick method for judging the crack initiation layers.

[0095] (2) The order of crack initiation at different strata is difficult to identify accurately:

[0096] Within the same construction layer group, the crack initiation time of different sub-layers varies greatly. In actual construction, there are often sub-layers that crack first, crack later, or fail to crack effectively. Therefore, it is difficult to determine the actual crack initiation sequence of each layer.

[0097] The core technological innovation of this invention:

[0098] (1) Without relying on complex numerical simulation models and difficult-to-obtain geological and mechanical parameters, quantitative calculation and analysis can be performed using a small amount of simple strata and construction data, which can easily and quickly determine the actual fracturing initiation layer in multi-layer fracturing and filling construction.

[0099] (2) The present invention divides the crack initiation judgment process into multiple stages. By calculating the liquid absorption of each small layer and the fluid pressure at the well wall in each stage, and using simple cyclic iterative analysis, the order of crack initiation of each small layer and the corresponding crack initiation time can be determined.

[0100] Compared with the prior art, the beneficial effects of the present invention are:

[0101] (1) It can easily and quickly determine the actual crack initiation layer in multi-layer fracturing and filling construction.

[0102] Based on the basic physical properties and construction pumping parameters of each sub-layer, this invention establishes the process of fluid absorption and fluid pressure at the wellbore during multi-layer fracturing and filling construction, which can quickly identify sub-layers that have started fracturing and those that have not yet started fracturing.

[0103] (2) Able to determine the order of crack initiation and the time of crack initiation in each sublayer.

[0104] This invention is based on the fractured sub-layers. Through iterative cycles, the fluid absorption capacity of the sub-layers is continuously modified. The change in fluid pressure at the well wall of the unfractured sub-layers is solved to determine the sub-layers that will fracture in subsequent stages, and the order and time of fracture initiation of each sub-layer are obtained.

[0105] (3) It has strong applicability on site and can easily and quickly obtain judgment results.

[0106] Most of the basic parameters required for this invention can be obtained from conventional well logging interpretation, formation testing data, and construction parameters. It does not require complex numerical models or rely on specialized fracture monitoring techniques and field equipment. The calculation process employs a phased iterative analysis to determine the fracture initiation zone and sequence, facilitating rapid evaluation and decision-making by engineers under field conditions.

[0107] (4) It overcomes the large error caused by simply judging the order of fracture based on the fracture pressure in the past. By comprehensively considering the formation conditions and construction parameters, quantitative calculation and analysis are carried out, which improves the rigor of the judgment process and the accuracy of the results. Attached Figure Description

[0108] Figure 1 This is a schematic diagram simulating the crack initiation process and results of each sublayer. Detailed Implementation

[0109] The technical solutions in the embodiments of the present invention will now be clearly and completely described in conjunction with the accompanying drawings.

[0110] Taking a multi-layered well in a loose sandstone reservoir as an example, after well completion, it was determined that ten sandstone sub-layers needed to be simultaneously fracturing and filling. The wellbore radius r w The depth is 0.1m. Data from well logging and rock testing were used to estimate various parameters for the sub-layer, as shown in Table 1 below. According to the established fracturing plan, the pump injection rate Q is... The total pumping time during construction is T, which is 90 minutes, and the viscosity of the fracturing fluid is μ. Based on statistical analysis of fracturing data from adjacent wells and wells with similar fracturing techniques in the same block, the fluid absorption capacity after fracturing in a small layer can typically increase by about 1.2-3 times compared to before fracturing. The correction factor for fracture fluid absorption capacity in this embodiment is... A value of 1.5 is used for calculation. The method according to this invention analyzes the small layers that initiate fracturing after the fracturing operation and determines the order of fracturing initiation:

[0111] Table 1 Basic Data Table for Each Sub-layer

[0112]

[0113] The parameters in Table 1 are derived from well logging interpretation data, well test / pressure tests, core and laboratory test data. These are commonly used data in oilfield field operations.

[0114] S1. Calculate the rupture pressure and rupture pressure ratio.

[0115] The rupture pressure of each sub-layer is calculated according to formula (1). Taking the first sub-layer as an example (the calculation results for other sub-layers are shown in Table 2):

[0116]

[0117] Calculate the rupture pressure ratio of each sub-layer according to formula (2). Taking the first sub-layer as an example (the calculation results for other sub-layers are shown in Table 2):

[0118]

[0119] Based on the fracture pressure ratio, a preliminary assessment suggests that layers 1, 5, 6, and 9 are relatively more prone to fracture initiation.

[0120] S2. Based on the physical properties of each sublayer and the construction pumping parameters, determine the layer where cracking first occurs.

[0121] S2.1 Calculate the liquid absorption of each sub-layer before crack initiation:

[0122] The liquid absorption of each sublayer in the first stage is calculated according to formula (7), taking the first sublayer as an example:

[0123]

[0124] S2.2 Calculate the change in fluid pressure at the wellbore wall of the small layer with pumping time to determine the small layer that first fractures.

[0125] The pressure diffusion coefficient of each sublayer can be solved using formula (10). Taking the first sub-layer as an example:

[0126]

[0127] According to formula (12), the candidate crack initiation time of each sublayer is calculated. Taking the first sub-layer as an example:

[0128]

[0129]

[0130] As can be seen from the calculation results in Table 2, =25.066min, corresponding to the crack initiation in the 5th sublayer.

[0131] And it satisfies: Therefore, it is believed that the fifth sublayer was the first to crack at 25.066 min.

[0132] S3. Based on the first fracture initiation layer, determine the subsequent fracture initiation layers and the order of fracture initiation.

[0133] S3.1, Liquid absorption of each sub-layer after the 5th sub-layer cracks.

[0134] Substitute the time into formulas (8) and (9). 25.066 min = 1503.96 s. Solve for the fluid pressure at the wellbore wall of the un-fractured sub-layer at the end of the first stage, taking the first sub-layer as an example:

[0135]

[0136]

[0137]

[0138] Take dimensionless correction coefficients =1.5, and the liquid absorption capacity of the 5th sublayer is corrected according to formula (13):

[0139]

[0140] Other un-cracked sublayers' liquid absorption capacity:

[0141]

[0142] In the second stage, the liquid absorption of each sublayer is calculated according to formula (15). Taking the first sublayer as an example:

[0143]

[0144] S3.2 Judgment of fluid pressure at the un-fractured wellbore and subsequent fracturing conditions

[0145] Combining equations (16), (17), and (18), we obtain the candidate crack initiation times for each sub-layer in the second stage. Taking the first sub-layer as an example:

[0146]

[0147] Right now:

[0148]

[0149] Solving for:

[0150] As can be seen from the calculation results in Table 2, At 45.102 min, the 9th sublayer cracked.

[0151] And it satisfies: Therefore, it is believed that cracks also occurred in the 9th sublayer.

[0152] Similarly, repeating the operation in S3, performing the calculations in stages 3, 4, and 5 respectively, we can obtain:

[0153] .

[0154] .

[0155] .

[0156] Therefore, based on the above calculation results, it can be concluded that after the sixth sublayer was fractured, no other sublayers were fractured.

[0157] Detailed intermediate data from the calculation process are shown in Table 2 of the calculation results.

[0158] Table 2 Summary of Calculation Results

[0159]

[0160] Continued from Table 2: Summary of Calculation Results

[0161]

[0162] The results of this embodiment show that during the multi-layer fracturing and filling construction process in this case, for the 10 sandstone sub-layers, after the construction pumping was completed, a total of 4 sub-layers experienced fracturing initiation, with the fracturing initiation sequence being sub-layers 5, 9, 1, and 6, and the corresponding fracturing initiation times being 25.066 min, 45.102 min, 62.105 min, and 82.117 min, respectively. The other sub-layers did not experience fracturing initiation. The simulation diagram of the fracturing process and fracturing results of each sub-layer is shown below. Figure 1 As shown.

[0163] Through the above steps, the present invention can ultimately obtain the small layer initiating cracks, the crack initiation sequence, and the crack initiation time during the multi-layer fracturing and filling construction operation.

Claims

1. A method for determining the initiation zone and sequence of fractures during multi-layer hydraulic fracturing and filling construction in loose sandstone reservoirs, characterized in that: Includes the following steps: S1. Based on the fracture pressure and fracture pressure ratio of each sublayer, the ease of fracture initiation in each sublayer is preliminarily determined. For each reservoir layer targeted for construction, the fracturing pressure of that layer is estimated based on its geological and mechanical parameters. At the same time, the fracturing pressure ratio of each layer is calculated to make a preliminary comparison of the ease of fracturing initiation in each layer. S2. Based on the physical properties of each sublayer and the construction pumping parameters, determine the layer where cracking first occurs. In the early stage of fracturing operation, no fracturing layer has yet started to fracture. After the fracturing fluid enters the formation, it spreads through matrix seepage. The amount of fluid absorbed by each layer is distributed according to its absorption capacity. Solve for the distribution of fracturing fluid in each layer before fracturing starts. Based on this, the relationship between fluid pressure at the wellbore of each sub-layer and injection time is established. Combined with the fracture initiation criteria of each sub-layer, the sub-layer that first fractures and its initiation time are solved, providing initial conditions for judging the fracture initiation of other sub-layers. S3. Based on the first fracture initiation layer, determine the subsequent fracture initiation layers and the order of fracture initiation. When a small layer fracturing and forming a fracture, its fluid absorption capacity increases, while the fluid absorption capacity of the unfracturing small layer remains unchanged. The fluid absorption capacity of the fracturing small layer is corrected, and the fluid absorption of each small layer is redistributed under the condition of total fluid volume conservation. Based on this, the fluid pressure change at the well wall of the unfracturing small layer is calculated, and then the next small layer to fracture and the fracturing time are determined. By repeating the above process until all target sublayers crack or the construction end time is reached, the actual cracking locations, cracking sequence, and corresponding cracking times of the layer group can be obtained.

2. The method for determining the initiation zone and initiation sequence during multi-layer hydraulic fracturing and filling construction of loose sandstone reservoirs according to claim 1, characterized in that, Step S1 specifically includes the following steps: For the i-th sub-layer, when the fluid pressure at the wellbore rises to a level sufficient to overcome the minimum horizontal principal stress of the formation and the tensile strength of the rock, the sub-layer is considered to have fractured. The formula for the fracture pressure of the sub-layer is: （1） In the formula, The tensile strength of the i-th smallest layer is given in MPa. The minimum horizontal principal stress at the i-th smallest layer is expressed in MPa. The fracture pressure of the i-th sublayer is given in MPa; i is the sublayer number. The rupture pressure ratio is defined as the ratio of the rupture pressure of a given sub-layer to the minimum rupture pressure among all candidate layers. The specific formula is as follows: （2） In the formula, The burst pressure ratio is dimensionless. The minimum rupture pressure of all sub-layers within the layer group, in MPa; The ease with which a small layer can initiate cracking is positively correlated with the ratio of fracture pressure.

3. The method for determining the initiation zone and initiation sequence during multi-layer hydraulic fracturing and filling construction of loose sandstone reservoirs according to claim 2, characterized in that, Step S2 specifically includes the following steps: S2.1 Calculate the liquid absorption of each sub-layer before crack initiation. For the i-th smallest layer, its liquid absorption volume is expressed as: （3） In the formula, Let mD be the permeability of the i-th smallest layer. ; Let m be the thickness of the i-th smallest layer; This refers to the viscosity of the fracturing fluid. ; The equivalent radius of the effective formation control range that can be utilized during wellbore injection is taken as... A represents the area controlled by a single well. ,m; Let be the radius of the wellbore, in meters (m). is the original formation pressure of the i-th smallest layer, i.e., the original pore pressure, in MPa; The bottom hole flowing pressure is in MPa. This represents the amount of liquid absorbed by the i-th smallest layer. ; For the i-th sublayer, the first stage is defined as the period before any sublayers begin to crack. Within this first stage, the liquid absorption is expressed as: （4） （5） In the formula, This represents the amount of fracturing fluid absorbed by sublayer i under a unit pressure differential during stage 1, and indicates the conductivity of this layer for the injected fluid. ; This represents the amount of liquid absorbed by the i-th smallest layer during the first stage. ; Under multi-layer simultaneous fracturing conditions, the sub-layers are connected through the wellbore, forming a typical parallel seepage system. According to the principle of mass conservation, the total injection volume satisfies: （6） In the formula, Q is the fracturing fluid injection rate during construction. n represents the number of sub-layers; Combining formulas (4), (5), and (6), we can obtain the liquid absorption situation of each sublayer in the first stage: （7） S2.2 Calculate the fluid pressure at the wellbore wall of each sub-layer as a function of pumping time to determine the sub-layer that first fractures. In the first stage, according to the radial seepage diffusion theory, the liquid absorption of each sublayer is... Substituting into the transient radial diffusion model, the dynamic variation of fluid pressure at the wellbore wall of each sub-layer over time is solved. The fluid pressure at the wellbore wall of the i-th sub-layer in the first stage is expressed as: （8） （9） （10） In the formula, The original formation pressure, i.e., the original pore pressure, is the i-th smallest layer. Let be the fluid pressure at the wellbore wall of the i-th sublayer at time t in the first stage, in MPa; Let be the pressure change at the wellbore wall of the i-th sublayer during the first stage, in MPa; Let be the pressure diffusion coefficient of the i-th smallest layer. The diffusion coefficient represents the speed at which pressure propagates in the formation; the larger the diffusion coefficient, the faster the pressure propagates. Let be Euler's constant, taken as 0.577; Let be the porosity of the i-th smallest layer, which is dimensionless; Let be the total compressibility coefficient of the i-th smallest layer, which relates to the compressibility of rock and fluid. ; As the sub-layer absorbs fluid, the fluid pressure at the wellbore gradually increases. When the fluid pressure at the wellbore reaches the fracture pressure P of that sub-layer... b，i At that time, cracking occurs in the sublayer, that is: （11） Solve for the candidate crack initiation time of each sublayer in stage 1. : （12） In the formula, Let be the candidate crack initiation time of the i-th small layer in the first stage, in seconds; For the candidate crack initiation time τ of the i-th small layer in stage 1 1，i The initial split time t1 = min{τ 1，i The corresponding stratum is the first layer to reach the fracturing pressure, i.e., the first-fracturing stratum; If the following condition is met within the fracturing pump injection time T: t1≤T; If no cracks are found, it is considered that the sublayer has started to crack; otherwise, it is considered that no sublayer has started to crack before the construction pumping is completed, and the judgment ends.

4. The method for determining the initiation zone and initiation sequence during multi-layer hydraulic fracturing and filling construction of loose sandstone reservoirs according to claim 3, characterized in that, Step S3 specifically includes the following steps: S3.1 Calculate the liquid absorption of each sub-layer after the first sub-layer fracturing. Once a crack begins to appear in a certain sublayer, the next calculation stage begins. The second stage is defined as the period from the beginning of the crack in the first sublayer to the beginning of the crack in the second sublayer. Let the j-th sublayer be the one that has already cracked. For the j-th sublayer that has already cracked, the liquid absorption index is corrected in the second stage as follows: （13） In the formula, This indicates the amount of fracturing fluid absorbed by the fractured sub-layer j under a unit pressure differential during stage 2. ; This represents the correction factor for the liquid absorption index of the cracked sublayer j, which is dimensionless. This indicates the amount of fracturing fluid absorbed by the fractured sub-layer j under a unit pressure differential within the first stage. ; For the un-cracked sublayer k, it is assumed that its liquid absorption index has not changed: （14） In the formula, This represents the amount of fracturing fluid absorbed by the un-fractured sub-layer under a pressure differential of k units during stage 2. , This represents the amount of fracturing fluid absorbed by the un-fractured sub-layer under a pressure differential of k units within the first stage. ; In stage 2, the amount of liquid absorbed by each sublayer is expressed as follows: （15） In the formula, This represents the amount of liquid absorbed by the i-th smallest layer during stage 2. , This represents the amount of fracturing fluid absorbed by sublayer i under a unit pressure differential in stage 2. ; S3.2, Fluid pressure correction at the wellbore wall of each sub-layer after the first sub-layer fractures. During stage 2, for any unfractured sublayer k, the fluid pressure at the wellbore wall is expressed as: （16） In the formula, Let K be the fluid pressure at the wellbore wall of sublayer k after the end of stage 1, in MPa; The fluid pressure increment at the wellbore wall in stage 2, in MPa; Let be the fluid pressure at the wellbore wall of the kth sublayer at time t in the second stage, in MPa; In the second stage, the pressure increment at any unfractured sub-layer k on the wellbore wall is: （17） In the formula, k k Let mD be the permeability of the kth smallest layer; h be the permeability of the kth smallest layer. k Let m be the thickness of the kth smallest layer. Let be the pressure diffusion coefficient of the kth smallest layer. ; For a non-fractured sublayer k, the criterion for fracture initiation is the fluid pressure at the wellbore. Reaching the rupture pressure of this layer P b，k : （18） Similarly, the candidate initiation time for each uncracked sublayer is obtained. The initiation time of the second initiation layer The corresponding sublayer is the second sublayer where cracking occurred; If there is If the cracking occurs in a second sublayer, it is considered that a second sublayer has started to crack; otherwise, it is considered that only one sublayer has started to crack before the end of construction. When a second fracture initiation layer exists, the liquid absorption capacity of the two fractured layers is corrected according to the same method in step S3, and the liquid absorption of each layer and the relationship between the fluid pressure at the well wall of the unfractured layer and the pumping time are updated. Based on the fracture initiation criteria of the layer, it is determined whether there is a next fracture initiation layer. The crack initiation judgment is then completed by iteratively judging the cracks in the same way as described above until the preset termination condition is met.

5. The method for determining the initiation zone and initiation sequence during multi-layer hydraulic fracturing and filling construction of loose sandstone reservoirs according to claim 4, characterized in that, The preset termination condition is the construction termination condition: when t≥T, the determination of the crack initiation layer is stopped.

6. The method for determining the initiation zone and initiation sequence during multi-layer hydraulic fracturing and filling construction of loose sandstone reservoirs according to claim 4, characterized in that, The preset termination condition is the layer termination condition: when all sub-layers in the layer group have been determined to have cracked, the iteration stops.

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