Fracture inversion optimization method based on fiber strain monitoring after fracturing pump stop

Abstract

The application discloses a fracture inversion optimization method based on fiber strain monitoring after fracturing pump stopping, relates to the oil and gas field exploitation technical field, and comprises the following steps: determining the filtration coefficient and the fracture complete closure time based on the fiber strain data after fracturing pump stopping; determining the formation filtration characteristics according to the filtration coefficient; adjusting the fracturing fluid system and the construction discharge capacity according to the determined formation filtration characteristics; and adjusting the pump injection time according to the determined fracture complete closure time.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of oil and gas field development, and particularly relates to a fracture inversion optimization method based on optical fiber strain monitoring after fracturing pump stopping. BACKGROUND

[0002] With the deepening of the development of unconventional oil and gas resources, the distributed acoustic sensing (DAS) technology based on adjacent wells has become a key means for diagnosing the fracture propagation pattern during hydraulic fracturing. Through the optical fiber laid in the monitoring well, the technology can obtain high-resolution, continuous spatial data of the strain field induced by fracturing, and has the significant advantages of high monitoring and positioning accuracy and rich information dimension. In particular, the low-frequency DAS (LF-DAS) monitoring has clear physical correlation with the fracture opening / closing process, and is an effective method for realizing fine cross-well fracture diagnosis under the current fracturing background.

[0003] However, the existing DAS-based technology interpretation methods mainly focus on the dynamic strain response during the fracturing pump injection period, and are used for qualitatively or semi-quantitatively identifying the fracture Hit position and judging the fracture propagation height and sequence. The key stage after the fracturing pump stopping, which contains rich reservoir information, has not been fully explored.

[0004] In terms of reservoir parameter acquisition, the current conventional technology mainly relies on the analysis of the bottom hole pressure drop after the fracturing pump stopping (such as G function derivative analysis) to indirectly explain key parameters such as the filtration coefficient and the fracture closure pressure. However, the inversion and interpretation process of such methods strongly depends on the idealized preset of the fracture geometry (such as the Penny shape, PKN or KGD model), and there is often a significant deviation between these simplified models and the complex non-planar fracture propagation in unconventional reservoirs, resulting in a large model uncertainty in the parameter interpretation results and limited universality.

[0005] Therefore, the existing DAS fracture diagnosis method mainly for the pump injection stage cannot be directly applied to the post-pump data to realize the quantitative interpretation of reservoir parameters, and the traditional post-pump pressure drop analysis method is limited by the assumption of the fracture geometry, and cannot be applied to various oil and gas reservoirs with complex geometry, so a new method is needed that can be free from the dependence on the fracture geometry model and directly quantitatively explain the reservoir parameters based on the post-pump optical fiber strain data.

[0006] The above content is only used to assist in understanding the technical solutions of the present application and does not represent the acknowledgement of the above content as prior art. SUMMARY

[0007] The main purpose of the present application is to provide a fracture inversion optimization method based on optical fiber strain monitoring after fracturing pump stopping, which aims to solve or partially solve the above problems.

[0008] To achieve the above object, the application provides a fracture inversion optimization method based on optical fiber strain monitoring after fracturing pump stopping, which comprises the following steps:

[0009] Based on the optical fiber strain data after fracturing pump stopping, the filtration coefficient and the fracture complete closure time are determined.

[0010] According to the filtration coefficient, the formation filtration characteristics are determined.

[0011] According to the determined formation filtration characteristics, the fracturing fluid system and the construction discharge capacity are adjusted.

[0012] According to the determined fracture complete closure time, the pump injection time is adjusted.

[0013] Preferably, in the fracture inversion optimization method based on optical fiber strain monitoring after fracturing pump stopping, the step of determining the filtration coefficient and the fracture complete closure time based on the optical fiber strain data after fracturing pump stopping comprises the following steps:

[0014] The axial strain observation data ε obtained by the low-frequency DAS system in the adjacent well monitoring scene is obtained, and a waterfall plot of the axial strain observation data and time is drawn, and the time when the fracture hits the optical fiber is determined according to the waterfall plot.

[0015] According to the optical fiber strain data after fracturing pump stopping, the fracture width at the position where the fracture hits the optical fiber is calculated, wherein the fracturing pump stopping is performed after the time when the fracture hits the optical fiber.

[0016] According to the calculated fracture width and the fact that the filtration volume and the change amount of the fracture volume are equal after pump stopping, a relationship curve of the fracture width after fracturing pump stopping and the pump stopping time is drawn, and the filtration coefficient is obtained by calculating the slope according to the drawn relationship curve and the straight line technique.

[0017] According to the fracture width at the pump stopping time and the filtration coefficient, the fracture complete closure time is calculated.

[0018] Preferably, in the fracture inversion optimization method based on optical fiber strain monitoring after fracturing pump stopping, the step of determining the time when the fracture hits the optical fiber according to the waterfall plot comprises the following steps:

[0019] On the waterfall plot, the starting position of the abnormal signal is found according to the optical fiber depth axis, and the corresponding optical fiber depth point is determined.

[0020] Along the time axis on the waterfall plot, the time point when the optical fiber strain signal is first converted from background noise to a clear and continuous bright line or bright band is identified, that is, the time when the fracture hits the optical fiber.

[0021] Preferably, in the crack inversion optimization method based on the post-fracturing pump stopping optical fiber strain monitoring, the step of calculating the crack width at the position where the crack hits the optical fiber according to the optical fiber strain data after the pump stopping of the fracturing comprises:

[0022] calculating the crack width at the position where the crack hits the optical fiber according to the optical fiber strain data after the pump stopping of the fracturing by using an iterative regularization algorithm;

[0023] wherein the regularization inversion model is:

[0024] ; (1)

[0025] wherein w is the crack width at the position where the crack hits the optical fiber to be solved;

[0026] ε is the optical fiber strain data;

[0027] λ is a regularization coefficient;

[0028] L1 is a regularization matrix;

[0029] G is a forward operator of the optical fiber strain, and the size is MxN;

[0030] wherein the calculation formula of the optical fiber strain data is:

[0031] ; (2)

[0032] z is a coordinate in the direction of the optical fiber;

[0033] L g is the gauge length of the optical fiber;

[0034] μ is a displacement;

[0035] u j is the displacement generated by the displacement discontinuity element j in the axial direction of the optical fiber;

[0036] The calculation formula of G is:

[0037] ; (3)

[0038] V k is an orthogonal basis in the parameter space;

[0039] U k+1 is an orthogonal basis in the data space;

[0040] B k is a small double-diagonal matrix;

[0041] The small-scale regularization optimization problem is defined in the subspace as:

[0042] ; (4)

[0043] y k is a subspace coefficient;

[0044] L k = L1V k is a projection of the regularization matrix within the subspace;

[0045] is a 2-norm of the observation data;

[0046] where the final solution of the fracture width at the fiber location where the fracture is to be solved is mapped back to the global parameter space by .

[0047] Preferably, in the fracture inversion optimization method based on the post-fracturing pump-off fiber strain monitoring, the step of drawing a relationship curve of the fracture width after the pump-off of the fracturing and the pump-off time according to the calculated fracture width and the change amount of the filtration volume and the fracture volume after the pump-off, and obtaining the filtration coefficient by the slope according to the drawn relationship curve and the linear technique, comprises:

[0048] assuming that the fracture length L is unchanged after the pump-off of the fracture, and only the fracture width w decays, the filtration volume after the pump-off of the fracture is:

[0049] ; (5)

[0050] V is the filtration volume after the pump-off of the fracture;

[0051] t p is the moment of the pump-off of the fracture;

[0052] t is a moment after the pump-off of the fracture;

[0053] Q L is the filtration rate; wherein, the calculation formula of the filtration rate is as follows:

[0054] ; (6)

[0055] C L is the filtration coefficient;

[0056] H is the fracture height;

[0057] A is the fracture area at a moment after the pump-off;

[0058] t0(x) is the moment of the filtration starting at the x position;

[0059] assuming that the fracture length L changes with the power law time, i.e.

[0060] ; (7)

[0061] a is the fracture expansion coefficient;

[0062] t P (L) is the time when the filtration is lost at the position of x=L;

[0063] By combining equation (6) and equation (7), we get:

[0064] ; (8)

[0065] When a=1,

[0066] ; (9)

[0067] Let , , equation (9) is transformed into:

[0068] ; (10)

[0069] According to the change amount of filtration volume and fracture volume after stopping the pump is equal, we get:

[0070] ; (11)

[0071] Where,

[0072] ;

[0073] △t is the time when the pump is stopped;

[0074] V f is the fracture volume;

[0075] is the average fracture width;

[0076] is the fracture width at the time when the pump is stopped;

[0077] According to , we get:

[0078] ; (12)

[0079] According to , let k1=1, draw the relationship curve between and after the pump is stopped, and calculate the slope k, the calculation formula is as follows:

[0080] ; (13)

[0081] According to the calculated slope k, calculate the filtration coefficient C L , the calculation formula is as follows:

[0082] (14).

[0083] Preferably, in the fracture inversion optimization method based on the post-fracturing pump-off optical fiber strain monitoring, the step of calculating the fracture complete closure time according to the fracture width at the pump-off time and the filtration coefficient comprises:

[0084] The fracture volume at the fracture pump-off time is:

[0085] (15)

[0086] The total filtration volume during the fracture pump-off stage is:

[0087] (16)

[0088] According to the equal change amount of the filtration volume and the fracture volume after the pump-off, V f is equal to V L , the following is obtained:

[0089] (17)

[0090] According to formula (17), the fracture complete closure time is:

[0091] (18)

[0092] V f is the fracture volume at the fracture pump-off time:

[0093] V L is the total filtration volume during the fracture pump-off stage:

[0094] A P is the fracture area at the fracture pump-off time;

[0095] t c is the fracture complete closure time;

[0096] C L is the filtration coefficient.

[0097] Preferably, in the fracture inversion optimization method based on the post-fracturing pump-off optical fiber strain monitoring, L1 is set as:

[0098] .

[0099] Preferably, in the fracture inversion optimization method based on the post-fracturing pump-off optical fiber strain monitoring, the step of determining the formation filtration characteristics according to the filtration coefficient comprises:

[0100] The formation filtration characteristics are divided into extra-low filtration formation, low filtration formation, medium filtration formation, high filtration formation and extra-high filtration formation;

[0101] When the filtration coefficient C L <4×10 -5 m·min 0.5 , the formation filtration characteristics are determined as extra-low filtration formation.

[0102] When the filtration coefficient 4×10 -5 m·min 0.5 <C L <8×10 -5 m·min 0.5 , the formation filtration characteristics are determined as low filtration formation.

[0103] When the filtration coefficient 8×10 -5 m·min 0.5 <C L <2×10 -4 m·min 0.5 , the formation filtration characteristics are determined as medium filtration formation.

[0104] When the filtration coefficient 2×10 -4 m·min 0.5 <C L <6×10 -4 m·min 0.5 , the formation filtration characteristics are determined as high filtration formation.

[0105] When the filtration coefficient C L ≥6×10 -4 m·min 0.5 , the formation filtration characteristics are determined as extra-high filtration formation.

[0106] The present application has at least the following beneficial effects:

[0107] The crack inversion optimization method based on the optical fiber strain monitoring after the fracturing pump is stopped, the filtration coefficient and the crack complete closure time are determined based on the optical fiber strain data after the fracturing pump is stopped; the formation filtration characteristics are determined according to the filtration coefficient; the fracturing fluid system and the construction displacement are adjusted according to the determined formation filtration characteristics; the pump injection time is adjusted according to the determined crack complete closure time, so that the fracturing design and construction optimization have key guiding significance.

[0108] Further, the reservoir parameters are quantitatively explained based on the optical fiber strain data after the pump is stopped, so as to optimize the fracturing design.

[0109] Further, by acquiring axial strain observation data ε acquired by a low-frequency DAS system in a neighboring well monitoring scene, a waterfall plot of the axial strain observation data and time is drawn, and the time when the crack hits the optical fiber is determined according to the waterfall plot; the crack width at the position where the crack hits the optical fiber is calculated according to the optical fiber strain data after the fracturing pump is stopped, wherein the fracturing pump is stopped after the time when the crack hits the optical fiber; the relationship curve of the crack width after the fracturing pump is stopped and the pump stopping time is drawn according to the calculated crack width and the fact that the filter loss volume and the crack volume change amount are equal after the pump is stopped; the filter loss coefficient is obtained by calculating the slope according to the relationship curve drawn and the straight line technique; the crack complete closure time is calculated according to the crack width at the pump stopping moment and the filter loss coefficient, so that the reservoir parameter quantitative interpretation based on the distributed optical fiber strain data is realized.

[0110] Further, the present application fully utilizes the optical fiber strain response in the crack closure process after the pump is stopped, the monitoring data has the advantages of relatively low noise background and clear signal physical mechanism (mainly reflecting the crack width attenuation and filter loss behavior), and high-quality data basis is provided for parameter inversion.

[0111] Further, compared with the traditional pressure drop analysis method (such as G function) which depends on strong assumptions of crack geometry, the present application only depends on the local physical response of the interaction between the crack and the optical fiber, the inversion process does not need to preset the PKN, KGD and other crack models, has stronger universality, and can be applied to reservoir parameter evaluation of various oil and gas reservoirs under complex geological conditions.

[0112] Further, the present application introduces regularization constraint in the inversion, effectively suppresses the ill-posedness caused by the single component of the strain data and the high noise characteristics, guarantees the numerical stability of the crack width sequence solution, and provides a reliable foundation for the accurate calculation of the filter loss coefficient and the closure time. BRIEF DESCRIPTION OF DRAWINGS

[0113] Figure 1 The flowchart of the crack inversion optimization method based on the optical fiber strain monitoring after the fracturing pump is stopped according to an embodiment of the present application is provided.

[0114] Figure 2 The flowchart of some embodiments in the middle part is provided. Figure 1

[0115] Figure 3 The relationship curve graph of the crack width and the pump stopping time is provided.

[0116] Figure 4 The relationship diagram of the filter loss coefficient and the crack closure time is provided.

[0117] ​​​The objectives, functional features and advantages of the present application will be further described with reference to the embodiments and the accompanying drawings. DETAILED DESCRIPTION

[0118] In the embodiments of the present application, the term "and / or" describes the association relationship of the associated objects, which means that there can be three relationships, for example, A and / or B can represent the following three cases: A exists alone, A and B exist simultaneously, and B exists alone. The character " / " generally represents an "or" relationship between the associated objects before and after it.

[0119] It should be noted that the terms "first", "second", and the like in the specification and claims of the present application and the above-described drawings are used to distinguish similar objects, and do not necessarily have to describe a specific order or sequence.

[0120] In the embodiments of the present application, the term "a plurality of" means two or more, and other quantifiers are similar.

[0121] In order to make the objectives, technical solutions and advantages of the embodiments of the present application more clear, the embodiments of the present application will be described in detail below with reference to the drawings. However, those skilled in the art can understand that in the embodiments of the present application, many technical details are proposed in order to make the readers better understand the present application. However, even without these technical details and various changes and modifications based on the following embodiments, the technical solutions claimed by the present application can be implemented. The division of the following embodiments is for the convenience of description, and should not constitute any limitation on the specific implementation of the present application, and the embodiments can be combined and referred to each other without contradiction.

[0122] The present application provides a fracture inversion optimization method based on optical fiber strain monitoring after fracturing pump stop.

[0123] At step S100, the filtration coefficient and the fracture complete closure time are determined based on the optical fiber strain data after the fracturing pump stop. The filtration coefficient directly affects the fracturing fluid efficiency and the proppant placement effect, and the fracture complete closure time can guide the determination of the construction pump stop time, the determination of the pump stop measurement time of DEIT, etc.

[0124] Step S100 includes steps S110 to S140.

[0125] The axial strain observation data ε obtained by the low-frequency DAS system in the adjacent well monitoring scene is acquired at step S110, and a waterfall plot of the axial strain observation data versus time is drawn, and the time when the crack hits the optical fiber is determined according to the waterfall plot. The step of determining the time when the crack hits the optical fiber according to the waterfall plot comprises: finding the starting position of the abnormal signal and determining the corresponding optical fiber depth point according to the optical fiber depth axis (y-axis) on the waterfall plot; identifying the time point when the optical fiber strain signal is first transformed from background noise into a clear continuous bright line or bright band along the time axis (x-axis) on the waterfall plot, that is, the time when the crack hits the optical fiber.

[0126] At step S120, the crack width at the position where the crack hits the optical fiber is calculated according to the optical fiber strain data after the fracturing pump is stopped, wherein the fracturing pump is stopped after the time when the crack hits the optical fiber.

[0127] Specifically, step S120 comprises:

[0128] According to the optical fiber strain data after the fracturing pump is stopped (based on the optical fiber strain data after the time when the crack hits the optical fiber), the crack width at the position where the crack hits the optical fiber is calculated by using an iterative regularization algorithm;

[0129] The regularization inversion model is:

[0130] ; (1)

[0131] Wherein w is the crack width at the position where the crack hits the optical fiber to be solved; according to the optical fiber strain data after the fracturing pump is stopped, the crack width at the position where the crack hits the optical fiber is inverted by using L1 projection iterative regularization method.

[0132] ε is the optical fiber strain data;

[0133] λ is the regularization coefficient;

[0134] L1 is the regularization matrix; wherein considering the boundary constraint that the crack tip width is 0, L1 is:

[0135]

[0136] G is the forward operator of the optical fiber strain, and the size is M×N;

[0137] The formula for calculating the optical fiber strain data is:

[0138] ; (2)

[0139] z is the optical fiber direction coordinate;

[0140] L g is the optical fiber gauge length;

[0141] μ is the displacement;

[0142] u j is the displacement generated by the discontinuity element j in the fiber axial direction;

[0143] The calculation formula of G is:

[0144] ; (3)

[0145] V k is the orthogonal basis in the parameter space;

[0146] U k+1 is the orthogonal basis in the data space;

[0147] B k is a small double-diagonal matrix;

[0148] A small-scale regularization optimization problem is defined in the subspace as:

[0149] ; (4)

[0150] y k is the subspace coefficient;

[0151] L k =L1V k is the projection of the regularization matrix in the subspace;

[0152] is the 2-norm of the observation data;

[0153] Where the final solution of the fracture width where the fracture to be solved meets the fiber position is mapped back to the global parameter space through .

[0154] At step S130, according to the calculated fracture width, and the change amount of the filtration volume and the fracture volume after the pump is stopped, a relationship curve of the fracture width after the fracturing pump is stopped and the pump stopping time is drawn; according to the drawn relationship curve, and the straight line technique, the slope is obtained to obtain the filtration coefficient. Specifically, it includes:

[0155] Assuming that the crack length L is unchanged after the crack is stopped, only the crack width w decays, and the filtration volume after the crack is stopped is:

[0156] ; (5)

[0157] V is the filtration volume after the crack is stopped;

[0158] t p is the moment when the crack is stopped;

[0159] t is a moment after the crack is stopped;

[0160] Q Lis the filtration rate, and the formula of filtration rate is as follows:

[0161] ; (6)

[0162] C L is the filtration coefficient;

[0163] H is the fracture height;

[0164] A is the fracture area at a certain time after the pump is stopped;

[0165] t0(x) is the time when the filtration starts at the position x;

[0166] It is assumed that the fracture length L changes with time in a power law, i.e.

[0167] ; (7)

[0168] a is the fracture propagation coefficient;

[0169] t P (L) is the time when the filtration starts at the position x=L;

[0170] By combining equation (6) and equation (7), we obtain:

[0171] ; (8)

[0172] When a=1,

[0173] ; (9)

[0174] Let , , then equation (9) is transformed into:

[0175] ; (10)

[0176] According to the fact that the filtration volume after the pump is stopped is equal to the change in fracture volume, we obtain:

[0177] ; (11)

[0178] wherein

[0179] ;

[0180] △t is the time when the pump is stopped;

[0181] V f is the fracture volume;

[0182] is the average fracture width;

[0183] The fracture width at the fracture pump stopping moment;

[0184] According to Get:

[0185] ; (12)

[0186] According to , let k1=1, draw the fracture after stopping pumping and the relationship curve of k, the calculation formula is as follows:

[0187] ; (13)

[0188] According to the calculated slope k, calculate the filtration coefficient C L , the calculation formula is as follows:

[0189] ; (14).

[0190] At step S140, the fracture completely closing time is calculated according to the fracture width at the pump stopping moment and the filtration coefficient.

[0191] Specifically, step S140 includes:

[0192] The fracture volume at the fracture pump stopping moment is:

[0193] ; (15)

[0194] The total filtration volume during the fracture stopping pumping stage is:

[0195] ; (16)

[0196] Since the fracture width is zero when the fracture is completely closed, according to the change amount of the filtration volume and the fracture volume after stopping pumping, V f is equal to V L , it is obtained that:

[0197] ; (17)

[0198] According to formula (17), the fracture completely closing time is obtained as:

[0199] ; (18)

[0200] V f is the fracture volume at the fracture pump stopping moment:

[0201] V L is the total filtration volume during the fracture stopping pumping stage, that is, the filtration volume from the fracture stopping pumping to the fracture closing: ​

[0202] A P is the fracture area at the fracture pump-off time;

[0203] t c is the fracture closure time;

[0204] C L is the filtration coefficient.

[0205] At step S200, the formation filtration property is determined according to the filtration coefficient.

[0206] The formation filtration property is divided into extra-low filtration formation, low filtration formation, medium filtration formation, high filtration formation, and extra-high filtration formation. Of course, in other embodiments, the formation filtration property can also be divided according to other rules.

[0207] Taking the case of dividing the formation filtration property into extra-low filtration formation, low filtration formation, medium filtration formation, high filtration formation, and extra-high filtration formation.

[0208] When the filtration coefficient C L <4×10 -5 m·min 0.5 , it is determined that the formation filtration property is divided into extra-low filtration formation. Among them, the extra-low filtration formation is dense, the natural fracture is extremely underdeveloped, the fracturing fluid filtration is difficult, the fracture closure is extremely slow, and the closure time is far beyond normal.

[0209] When the filtration coefficient 4×10 -5 m·min 0.5 <C L <8×10 -5 m·min 0.5 , it is determined that the formation filtration property is divided into low filtration formation. Among them, the low filtration formation is relatively dense, the natural fracture is underdeveloped, the fracturing fluid filtration is slow, and the fracture completely closure time is long.

[0210] When the filtration coefficient 8×10 -5 m·min 0.5 <C L <2×10 -4 m·min 0.5 , it is determined that the formation filtration property is divided into medium filtration formation. Among them, the medium filtration formation has medium formation property, the natural fracture is moderately developed, the fracturing fluid filtration rate is normal, and the fracture completely closure time is in line with normal.

[0211] When the filtration coefficient 2×10 -4 m·min 0.5 <C L <6×10 -4 m·min 0.5When the filtration coefficient C

[0212] When the filtration coefficient C L ≥6×10 -4 m·min 0.5 , the formation filtration characteristics are determined to be high filtration formation. Among them, the high filtration formation is relatively loose, the natural fracture is relatively developed, the fracturing fluid filtration is fast, and the fracture closure time is short

[0213] For example, the filtration coefficient is 3.16×10 -5 m·min 0.5 , which belongs to a very low filtration formation.

[0214] Step S300 adjusts the fracturing fluid system and construction discharge according to the determined formation filtration characteristics.

[0215] Among them, step S300 includes adjusting the fracturing fluid system according to the determined formation filtration characteristics. For a very low filtration formation, the fracturing fluid system needs to reduce liquid resistance and strengthen flowback: reduce viscosity, reduce or cancel the use of filtration reducers, strengthen gel breaking system, reduce the proportion of preflush, and appropriately increase the low viscosity fluid stage.

[0216] For low filtration formation, the fracturing fluid system needs to maintain balance and fine-tune optimization: maintain moderate viscosity, use conventional amount of filtration reducer, optimize gel breaker type and combination, use standard preflush ratio, and maintain appropriate low viscosity fluid ratio.

[0217] For medium filtration formation, the fracturing fluid system adopts a standard scheme and is stable and controllable: use standard viscosity fracturing fluid, strictly implement standard filtration reducer scheme, use conventional gel breaking procedure, and apply classic preflush and proppant carrying fluid ratio.

[0218] For high filtration formation, the fracturing fluid system needs to strengthen plugging and control filtration: increase fracturing fluid viscosity, significantly increase the amount of filtration reducer, appropriately extend the gel breaking time, and increase the preflush ratio.

[0219] For a very high filtration formation, the fracturing fluid system needs extreme plugging to ensure construction: significantly increase the viscosity of the fracturing fluid, significantly increase and compound the use of multiple high-efficiency filtration reducers, accurately control and as much as possible significantly extend the gel breaking time, significantly increase the preflush ratio, and consider using high sand ratio slug and other technologies to quickly plug the near-wellbore seepage channel.

[0220] For example, the fracturing fluid system is reduced from the original formula of 10 mPa s to 6-8 mPa s; the amount of filtration reducer is appropriately reduced to reduce the cost; the amount of oxidation breaker is increased to adjust the release time of the breaker to ensure complete gel breaking within 4-6 hours after pump stop. For low filtration formation, the fracture is not needed to be formed by excessive preflush, and the proportion is adjusted from the conventional 30-40% to 20-25%; the proportion of slickwater is increased to 15-20% of the total liquid amount to improve the complex fracture formation near the wellbore.

[0221] Step S300 further comprises adjusting the construction discharge capacity according to the determined formation filtration characteristics. For example, for the ultra-low filtration formation, the construction discharge capacity adopts a medium or low discharge capacity to control the fracture length, increase the fracture width, and form a complex fracture network. For the low and medium filtration formation, the construction discharge capacity adopts a standard discharge capacity to stabilize the extension and optimize the placement. For the high filtration formation, the construction discharge capacity adopts a medium-high discharge capacity to quickly form a fracture and strongly carry the proppant. For the ultra-high filtration formation, the construction discharge capacity adopts an extremely high discharge capacity to ensure the fracture extension and proppant entering.

[0222] For example, for the ultra-low filtration formation, the current 4.0 m 3 / min discharge capacity is adjusted to 3.0-3.5 m 3 / min to improve the fracture morphology, reduce unnecessary fracture height growth, and be beneficial to the proppant transport and placement.

[0223] Step S400 adjusts the pumping time according to the determined fracture complete closure time.

[0224] When the formation filtration characteristics are the ultra-low filtration formation, the pumping time is moderately prolonged, the minimum monitoring time is 1.2 times the fracture complete closure time t c , the data acquisition focuses on the high-frequency acquisition in the first 120 min, and the safe flowback window is 0.8 times t c .

[0225] When the formation filtration characteristics are the low filtration formation, the pumping time is slightly prolonged, the minimum monitoring time is 1.3-1.5 times t c , the data acquisition focuses on the high-frequency acquisition in the first 60-90 min, and the safe flowback window is 0.7-0.8 times t c .

[0226] When the formation filtration characteristics are the medium filtration formation, the standard pumping time is adopted, the minimum monitoring time is 1.5-2.0 times t c , the data acquisition focuses on the high-frequency acquisition in the first 45-60 min, and the safe flowback window is 0.6-0.7 times t c .

[0227] When the formation has high filtration characteristics, the pumping time needs to be shortened, with a minimum monitoring time of 2.0-2.5 times the t. c Data collection should focus on high-frequency acquisition within the first 30-45 minutes, with a safe return window of 0.5-0.6 times the t. c .

[0228] When the formation has extremely high filtration characteristics, the pumping time is significantly shortened, with the minimum monitoring time being 2.5-3.0 times the time of t. c Data collection should focus on high-frequency acquisition within the first 15-30 minutes, with a safe return window of 0.4-0.5 times the t. c .

[0229] For example, in ultra-low filtration formations, the closure time t c The injection time is set at 337 min: The current 40 min injection time is adjusted to 45-50 min, delaying the injection time to fully utilize the low filtration loss characteristics and improve proppant coverage optimization; the minimum monitoring time is 404 min, and it is recommended to adjust it to 450 min (1.25 times t). c The data collection focus is on the first 120 minutes (0.35 times t). c High-frequency data acquisition; the safe backflow window is 270 minutes, and it is recommended that the backflow start time be 300 minutes (0.9 times t). c The return flow rate is initially low and gradually increases.

[0230] Example:

[0231] Artificially synthesized fiber optic strain observation data, in this case: interlayer stress profile is 65-60-65 MPa, rock elastic modulus is 30.0 GPa, Poisson's ratio is 0.2, and fracture toughness is 0.5 MPa·m. 0.5 The displacement for single-cluster fracturing is 4.0 m³. 3 The fracturing fluid viscosity was 10 mPa·s, the orifice diameter was 5 mm, the orifice abrasion coefficient was 0.8, and the number of perforations per cluster was 16. The total operation time was 60 min, including 40 min of pumping time and 20 min of pump shutdown time. The monitoring well spacing was 150 m, and the fiber optic gauge length Lg was 1 m. The inversion results are as follows: Figure 3 and Figure 4 .

[0232] Based on fiber optic strain monitoring data after fracturing pump shutdown, the crack width at the hit location after pump shutdown was determined using an iterative regularization algorithm. ,like Figure 2 As shown, the crack width can be obtained at the pump shutdown time of 40 minutes. It is 2.18 10 -3 m.

[0233] Plot With the relationship curve as shown in Figure 4 , it can be obtained that the slope of the straight line is 2 x 10 -5 , and the filtration coefficient C L is 3.16 x 10 -5 m·min 0.5 , which is calculated according to formula (14).

[0234] The fracture width at the pump-off moment is known , and the filtration coefficient C L , the fracture closure time t c is 337 min, which is obtained according to formula (18).

[0235] The filtration coefficient is 3.16 x 10 -5 m·min 0.5 , which belongs to a very low filtration formation. It is necessary to reduce the viscosity of the fracturing fluid, improve the gel breaking performance, and enhance the flowback capacity. For example, the fracturing fluid system is reduced from the original formula of 10 mPa·s to 6-8 mPa·s; the amount of the filtration reducer is appropriately reduced to reduce the cost; the amount of the oxidation gel breaker is increased, and the gel breaking agent release time is adjusted to ensure complete gel breaking within 4-6 hours after the pump is stopped. In the low filtration formation, there is no need for too much preflush to form a fracture, and the conventional 30-40% proportion is adjusted to 20-25%; the slickwater proportion is increased to 15-20% of the total liquid amount to improve the formation of the complex fracture near the wellbore. The discharge requirement is reduced, and the current discharge of 4.0 m 3 / min is adjusted to 3.0-3.5 m 3 / min to improve the fracture morphology, reduce unnecessary fracture height growth, and be beneficial to the transport and placement of the proppant. The closure time t c is 337 min: the pump injection time is adjusted from the current 40 min to 45-50 min to delay the pump injection time, fully utilize the low filtration characteristics, and improve the proppant coverage optimization; the minimum monitoring time is 404 min, which is recommended to be adjusted to 450 min (1.25 times t c ), and the data acquisition focus is the high-frequency acquisition of the first 120 min (0.35 times t c ); the safe flowback window is after 270 min, and the flowback start time is recommended to be 300 min (0.9 times t c ), and the flowback rate is low at the initial stage and gradually increases.

[0236] Obviously, the above-described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, other different forms of changes or variations can be made by those skilled in the art without creative labor, and all should belong to the protection scope of the present application.

Claims

1. A fracture inversion optimization method based on post-fracture pump-off fiber strain monitoring, characterized in that, The method comprises the following steps: Based on the optical fiber strain data after the fracturing pump is stopped, the filtration coefficient and the complete fracture closure time are determined; According to the filtration coefficient, the formation filtration characteristics are determined; According to the determined formation filtration characteristics, the fracturing fluid system and the construction discharge capacity are adjusted; According to the determined complete fracture closure time, the pump injection time is adjusted; The step of determining the filtration coefficient and the complete fracture closure time based on the optical fiber strain data after the fracturing pump is stopped comprises the following steps: obtaining the axial strain observation data ε obtained by the low-frequency DAS system in the adjacent well monitoring scene, and drawing the waterfall plot of the axial strain observation data and time, determining the time when the fracture hits the optical fiber according to the waterfall plot; according to the optical fiber strain data after the fracturing pump is stopped, the fracture width at the position where the fracture hits the optical fiber is calculated, wherein the fracturing pump is stopped after the time when the fracture hits the optical fiber; according to the calculated fracture width and the equal change amount of the filtration volume and the fracture volume after the pump is stopped, the relationship curve of the fracture width after the fracturing pump is stopped and the pump stopping time is drawn; according to the drawn relationship curve and the straight line technology, the slope is obtained to obtain the filtration coefficient; according to the fracture width at the pump stopping time and the filtration coefficient, the complete fracture closure time is calculated; The step of determining the filtration coefficient and the complete fracture closure time based on the optical fiber strain data after the fracturing pump is stopped comprises the following steps: assuming that the fracture length L is unchanged after the fracture pump is stopped, only the fracture width w decays, and the filtration volume after the fracture pump is stopped is: V is the filtration volume after the fracture pump is stopped; ；（5） t is a time after the fracture pump is stopped; t p tfrac = time when fracture is shut-in; H is the fracture height; Q L is the filtration rate; wherein the filtration rate is calculated according to the following formula: ；（6） C L C is the filtration coefficient; A is the fracture area at a certain time after the pump is stopped; t0(x) is the time when the filtration starts at the x position; Assuming that the fracture length L changes with time in a power-law manner, i.e. a is the fracture propagation coefficient; ；（7） By combining formula (6) and formula (7), the following formula is obtained: t P (L) is the time at which the x = L position starts to be filtered out. When a=1, ；（8） According to the equal change amount of the filtration volume and the fracture volume after the pump is stopped, the following formula is obtained: ；（9） Let , , then equation (9) becomes: ；（10） wherein, ；（11） △t is the pump stopping time; ； The step of calculating the complete fracture closure time according to the fracture width at the pump stopping time and the filtration coefficient comprises the following steps: V f is the fracture volume; for average crack width; fracture width at the time of fracture pump shut-off; According to Obtained: ；（12） According to , let k1=1, draw the curve of the relationship between the fracture pressure and the time after the pump is stopped , and , and calculate the slope k. The formula is as follows: ；（13） Based on the calculated slope k, the filter loss coefficient C is calculated L The calculation formula is as follows: ；（14） The fracture volume at the pump stopping time is: The total filtration volume during the fracture pump stopping stage is: ；（15） According to formula (17), the complete fracture closure time is obtained as: ；（16） According to the equal change of the filtration volume after pump-off and the fracture volume, V f is equal to V L , we get: ；（17） The step of determining the time when the fracture hits the optical fiber according to the waterfall plot comprises the following steps: ；（18） V f Fracture volume at fracture pump-off time: V L Total fluid loss volume for fracture shut-in stage: A P FracArea is the fracture area at the time of fracture pump shut-in; t c t is the time for complete crack closure; C L is the filtration coefficient.

2. The fracture inversion optimization method based on post-fracture pump-off optical fiber strain monitoring of claim 1, wherein, On the waterfall plot, the starting position of the abnormal signal is found according to the optical fiber depth axis, and the corresponding optical fiber depth point is determined; Along the time axis on the waterfall plot, the time point when the optical fiber strain signal is first changed from background noise to a clear and continuous bright line or bright band is identified, which is the time when the fracture hits the optical fiber. The step of calculating the fracture width at the position where the fracture hits the optical fiber according to the optical fiber strain data after the fracturing pump is stopped comprises the following steps:

3. The fracture inversion optimization method based on post-fracture pump-off optical fiber strain monitoring of claim 1, wherein, According to the optical fiber strain data after the fracturing pump is stopped, the fracture width at the position where the fracture hits the optical fiber is calculated by using the iterative regularization algorithm; wherein, the regularization inversion model is: wherein, w is the fracture width at the position where the fracture hits the optical fiber to be solved; ；（1） ε is the optical fiber strain data; λ is the regularization coefficient; L1 is the regularization matrix; ​ G is a forward operator of fiber strain, and has a size of M*N; Wherein, the calculation formula of fiber strain data is: ；（2） Z is a fiber direction coordinate; L g L is the fiber span; μ is displacement; u j δj is the displacement generated in the fiber axial direction for the displacement discontinuity element j; The calculation formula of G is: ；（3） V k are orthogonal bases in the parameter space; U k+1 are orthonormal bases in the data space; B k is a small bicausal matrix; A small-scale regularization optimization problem is defined in the subspace as: ；（4） y k are subspace coefficients; L k = L1V k is the projection of the regularization matrix within the subspace; is the 2-norm of the observation data; where the final solution for the crack width at the location where the crack to be solved hits the optical fiber is mapped back to the global parameter space. global parameter space.

4. The fracture inversion optimization method based on post-fracture pump-off optical fiber strain monitoring of claim 3, wherein, According to the boundary constraint that the width of the seam tip is 0, L1 is set as: 。 5. The fracture inversion optimization method based on post-fracture pump-off fiber strain monitoring of claim 1, wherein, The step of determining the formation filtration property according to the filtration coefficient comprises: The formation filtration property is divided into a very low filtration formation, a low filtration formation, a medium filtration formation, a high filtration formation and a very high filtration formation. When the filtration coefficient C L <4 x 10 -5 m·min 0.5 The formation filtration characteristics are determined to be ultra-low filtration formation. When the filtration coefficient is 4 x 10 -5 m·min 0.5 < C L < 8 x 10 -5 m·min 0.5 , the formation filtration characteristics are determined to be low filtration formation; When the filtration coefficient 8 x 10 -5 m·min 0.5 < C L < 2 x 10 -4 m·min 0.5 When the filtration coefficient is 8 x 10 When the filtration coefficient 2 x 10 -4 m·min 0.5 < C L < 6 x 10 -4 m·min 0.5 , the formation filtration characteristics are determined to be high filtration formation; When the filtration coefficient C L ≥ 6 × 10 -4 m·min 0.5 , the formation filtration characteristics are determined to be extra-high filtration formation.

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