Online nuclear magnetic imaging experimental device and method for evaluating shale hydration secondary cracks

Abstract

The invention relates to the technical field of oil production engineering, in particular to an online nuclear magnetic imaging experimental device and method for evaluating shale hydration secondary cracks. The experimental device comprises a rock core, a rock core holder and an online nuclear magnetic device, a rock core is arranged in the rock core holder, and the rock core holder is arranged in the online nuclear magnetic device; one side of the core holder is sequentially connected with a pressure gauge, a constant-flux pump and a liquid inlet tank through a pipeline, and fracturing fluid is placed in the liquid inlet tank; the other side of the core holder is connected with a waste liquid tank; and the other side of the core holder is connected with a hand pump for pressurizing. According to the online nuclear magnetic imaging experimental device and method for evaluating the shale hydration secondary fractures, continuous online nuclear magnetic imaging experiments can be conducted on the shale hydration secondary fractures, hydration secondary fracture distribution evaluation can be conducted on different types of shale cores, the shale reservoir fracturing transformation effect and the after-fracturing production capacity are optimized, and the shale hydration secondary fractures can be evaluated. And the full life cycle single well productivity of the shale oil-gas well is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of oil production engineering, and in particular to an on-line nuclear magnetic imaging experimental device and method for evaluating shale hydration secondary fractures. Background Art

[0002] Hydraulic fracturing is the most effective stimulation method for shale reservoirs. By injecting a large amount of fracturing fluid into the shale reservoir to form hydraulic fractures, establishing more oil and gas seepage channels is an important guarantee for the efficient exploitation of shale oil and gas. After hydraulic fracturing of shale oil reservoirs, the shut-in time and flowback mechanism have not been effectively verified. During the shut-in process after fracturing, due to the extremely developed bedding in shale reservoirs, a large amount of fracturing fluid remains in the pores or fractures of the reservoir, and may undergo hydration with shale, changing the pore structure of the shale reservoir, forming secondary fractures, and thus generating more oil and gas seepage channels. The distribution law of shale hydration secondary fractures has not been clarified.

[0003] Due to the extremely low permeability of shale, conventional displacement and shut-in experiments cannot be carried out, and the distribution of secondary fractures caused by hydration and the optimal hydration time are not clear. This makes it difficult to select a more suitable fracturing fluid type and determine the optimal shut-in time after fracturing when formulating a fracturing plan. There is an urgent need to design an experimental method and device for evaluating the distribution of shale hydration secondary fractures to solve the current problems. Summary of the Invention

[0004] (I) Technical Problems to be Solved The present invention provides an on-line nuclear magnetic imaging experimental device and method for evaluating shale hydration secondary fractures to overcome the defect that existing indoor core experiments cannot evaluate the distribution of shale hydration secondary fractures and determine the optimal hydration time.

[0005] (II) Technical Solutions To solve the above problems, on the one hand, the present invention provides an on-line nuclear magnetic imaging experimental device and method for evaluating shale hydration secondary fractures, including: a core, a core holder, and an on-line nuclear magnetic device; the core is arranged in the core holder, and the core holder is arranged in the on-line nuclear magnetic device; on one side of the core holder, a pressure gauge, a metering pump, and a liquid inlet tank are sequentially connected through pipelines, and fracturing fluid is placed in the liquid inlet tank; the other side of the core holder is connected to a waste liquid tank; still another side of the core holder is connected to a hand pump for pressurization.

[0006] Preferably, the core holder is made of stainless steel material.

[0007] Preferably, a first switch is provided on the pipeline path between the core holder and the waste liquid tank.

[0008] Preferably, a second switch is provided in the pipeline path between the pressure gauge and the metering pump.

[0009] Preferably, the hand pump is connected to the core holder through a confining pressure pipeline, and a third switch is provided on the confining pressure pipeline.

[0010] Preferably, the fracturing fluid is a slickwater breaker fluid, a guar gum breaker fluid or a mixed breaker fluid, and the ratio of slickwater to guar gum in the mixed breaker fluid is 2:8 or 5:5.

[0011] On the other hand, the present invention also provides an on-line nuclear magnetic imaging experimental method for evaluating shale hydration secondary fractures, including: Step S1, fabricating an artificial fracture core or a core with a central hole; Step S2, clamping the fabricated artificial fracture core or core with a central hole in the core holder, and the core holder is made of stainless steel; Step S3, placing the core holder in an on-line nuclear magnetic device, and connecting a liquid tank, a hand pump, a metering pump, the core holder, a pressure gauge and a switch with pipelines; Step S4, turning on the on-line nuclear magnetic device, performing nuclear magnetic resonance scanning before hydration, and recording the state of the core before hydration; Step S5, injecting fracturing fluid into the inlet liquid tank; Step S6, opening the third switch, and applying confining pressure to the core holder through the hand pump; Step S7, opening the first switch and the second switch, injecting fracturing fluid into the core through the metering pump, and when it is observed that fracturing fluid flows out from the liquid outlet pipeline connected to the waste liquid tank, closing the first switch and starting to build pressure; Step S8, after observing the pressure gauge reading to the target pressure value, first closing the metering pump, and then closing the second switch; Step S9, turning on the on-line nuclear magnetic device again, performing real-time nuclear magnetic monitoring during hydration and recording the pressure change according to the pressure gauge, and performing on-line nuclear magnetic resonance and pressure analysis; Step S10, replacing the fracturing fluid, and repeating the above steps S4 - S9.

[0012] Preferably, it further includes: after the experiment, removing the confining pressure and injection pressure, opening the first switch, discharging the waste liquid into the waste liquid bucket, removing the connection pipelines and then removing the core holder, and taking out the core.

[0013] Preferably, the fracturing fluid is a slickwater breaker fluid, a guar gum breaker fluid or a mixed breaker fluid, and the ratio of slickwater to guar gum in the mixed breaker fluid is 2:8 or 5:5.

[0014] Preferably, the online nuclear magnetic resonance and pressure analysis method specifically includes: Evaluation of online nuclear magnetic resonance imaging results: By comparing the online nuclear magnetic resonance signals and signal distribution states after different liquids are hydrated, those with obvious and uniform signals represent a large degree of fracturing fluid penetration and relatively significant hydration effects; One-dimensional nuclear magnetic resonance evaluation: Using the nuclear magnetic resonance signal relaxation time as the abscissa and the signal intensity as the ordinate, draw the signal curves of different fracturing fluid-hydrated cores. A large peak area enclosed by the curve indicates an obvious hydration effect and is prone to form hydration fractures; Pressure monitoring evaluation: Compare and analyze the pressure changes in the core holder. A large pressure reduction amplitude during the hydration process indicates a more obvious hydration effect and is prone to form hydration fractures; When the pressure stabilizes after a period of decrease, it indicates that the expansion of the secondary hydration fractures is complete, and this time is the optimal hydration time.

[0015] (III) Beneficial effects The present invention provides an online nuclear magnetic resonance imaging experimental device and method for evaluating shale secondary hydration fractures to solve the problem that existing indoor core experiments cannot evaluate the distribution of shale secondary hydration fractures and determine the optimal hydration time. It can conduct continuous online nuclear magnetic resonance imaging experiments on shale secondary hydration fractures, evaluate the distribution of secondary hydration fractures for different types of shale cores, provide theoretical support for selecting the type of fracturing fluid and determining the shut-in time after fracturing in the formulation of fracturing plans, optimize the fracturing transformation effect and post-fracture production capacity of shale reservoirs, and improve the single-well production capacity of shale oil and gas wells throughout the entire life cycle. Description of the drawings

[0016] Figure 1 Structural diagram of the online nuclear magnetic resonance imaging experimental device for evaluating shale secondary hydration fractures; Figure 2 Online nuclear magnetic resonance results of three artificially fractured cores; Figure 3 Curve of the relationship between pressure and hydration time.

[0017] Wherein: 1 - Online nuclear magnetic resonance device, 2 - Core holder, 3 - Core, 4 - Pressure gauge, 5 - Switch No. 1, 6 - Switch No. 2, 7 - Switch No. 3, 8 - Waste liquid pipeline, 9 - Liquid inlet pipeline No. 1, 10 - Confining pressure pipeline, 11 - Liquid inlet pipeline No. 2, 12 - Constant flow pump, 13 - Hand pump, 14 - Liquid inlet tank, 15 - Waste liquid tank. Embodiment

[0018] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0019] As Figure 1 shown, the present invention provides an on-line nuclear magnetic imaging experimental device for evaluating shale hydration secondary fractures, including: a core 3, a core holder 2, and an on-line nuclear magnetic device 1; the core is arranged in the core holder, and the core holder is arranged in the on-line nuclear magnetic device 1; Among them, the on-line nuclear magnetic device 1 can continuously perform nuclear magnetic scans for on-line nuclear magnetic imaging and generation of one-dimensional nuclear magnetic data; the core holder 2 is used to hold the test core 3 and is made of special materials for nuclear magnetic resonance analysis; specifically, the core holder is made of stainless steel material. By using the stainless steel core holder to hold the core 3, the on-line nuclear magnetic resonance technology can penetrate the core holder and directly scan the core to obtain on-line nuclear magnetic resonance images and one-dimensional nuclear magnetic data to evaluate the distribution of hydration secondary fractures.

[0020] One side of the core holder 2 is sequentially connected to a pressure gauge 4, a positive displacement pump 12, and a liquid inlet tank 14 through a liquid inlet pipeline No. 9 and a liquid inlet pipeline No. 11. The liquid inlet tank 14 contains fracturing fluid with corrosion resistance and high temperature resistance characteristics.

[0021] Specifically, the fracturing fluid is slickwater breaker fluid, guar gum breaker fluid, or a mixed breaker fluid. The ratio of slickwater to guar gum in the mixed breaker fluid is 2:8 or 5:5.

[0022] The slickwater breaker fluid and guar gum breaker fluid in this embodiment are existing mature products and can be directly purchased. The mixed liquid can be mixed according to the ratio to enhance the sand-carrying performance.

[0023] The pressure gauge 4 is used to monitor the pressure inside the core holder, and its range meets the experimental requirements; The positive displacement pump 12 is used to inject liquid into the core in the core holder. The positive displacement pump is operated to open and close and set the injection pressure through a digital interface. A switch No. 2 6 is provided in the pipeline between the pressure gauge 4 and the positive displacement pump 12.

[0024] On the other side of the core holder 2, it is connected to a waste liquid tank 15 through a waste liquid pipeline 8, and a switch No. 1 5 is provided on the waste liquid pipeline 8.

[0025] On the other side of the core holder 2, a hand pump for pressurization is connected through a confining pressure pipeline 10, and a third switch 7 is provided on the confining pressure pipeline 10. The hand pump 13 is connected to the core holder 2 through the confining pressure pipeline 10.

[0026] The hand pump 13 is used to apply confining pressure to the core in the core holder, and an internal pressure gauge indicates the pressure value. The first switch and the second switch are used to open or close the inlet liquid and outlet liquid, and the third switch is used to open or close the confining pressure pipeline. The above switches have the characteristics of high temperature resistance and high pressure resistance.

[0027] The above waste liquid pipeline 8, confining pressure pipeline 10, first inlet pipeline 9 and second inlet pipeline 11 have the characteristics of high temperature resistance and high pressure resistance.

[0028] The on-line nuclear magnetic imaging experimental device for evaluating shale hydration secondary fractures provided by the present invention solves the problem that the existing indoor core experiments cannot evaluate the distribution of shale hydration secondary fractures and determine the optimal hydration time. It can conduct continuous on-line nuclear magnetic imaging experiments on shale hydration secondary fractures, and can evaluate the distribution of hydration secondary fractures for different types of shale cores, providing theoretical support for selecting the type of fracturing fluid and determining the shut-in time after fracturing in the formulation of fracturing programs, optimizing the fracturing transformation effect and post-fracture production capacity of shale reservoirs, and improving the single-well production capacity of shale oil and gas wells throughout the life cycle.

[0029] In addition, the present invention also provides an on-line nuclear magnetic imaging experimental method for evaluating shale hydration secondary fractures, specifically including the following steps: a. Select cores from the same block and make artificial fracture cores with a core specification of 2.5 * 5 cm. b. Clamp the made artificial fracture core or the core with a central hole in a special core holder. c. Place the core holder in the on-line nuclear magnetic device, and connect the liquid tank, hand pump, peristaltic pump, core holder, pressure gauge, and switch with pipelines. After the connection is completed, check that the seals of all parts are intact. d. Turn on the on-line nuclear magnetic device, conduct nuclear magnetic resonance scanning before hydration, and record the state of the core before hydration. e. Pour the fracturing fluid into the injection liquid tank. f. Open the third switch and apply a confining pressure of 25 MPa to the core holder with the hand pump. g. Open the first switch and the second switch, use the peristaltic pump to inject the fracturing fluid into the core, and close the first switch to start pressure buildup when fracturing fluid flows out of the outlet pipeline connected to the waste liquid tank. h. Observe the pressure gauge reading until the target pressure value is reached, then turn off the peristaltic pump first, and then turn off the second switch. In this step, the target pressure value is 20 Mpa.

[0030] i. Turn on the online nuclear magnetic resonance device, conduct real-time nuclear magnetic monitoring during the hydration process, and record the pressure change according to the pressure gauge. The pressure change in the core holder monitored by the pressure gauge is used to evaluate the distribution of secondary shale hydration fractures through the pressure drop amplitude. The distribution of secondary shale hydration fractures and the determination of the optimal hydration time are comprehensively evaluated through online nuclear magnetic resonance and pressure monitoring. j. Replace the fracturing fluid and repeat the above steps. In this embodiment, the online nuclear magnetic experiment is continuously carried out for 5 days. After the experiment, remove the confining pressure and injection pressure, turn on Switch No. 1, drain the waste liquid into the waste liquid bucket, and remove the core holder after removing the connecting pipeline to take out the core.

[0031] The online nuclear magnetic imaging experiments on secondary shale hydration fractures were carried out three times according to the above steps to evaluate the distribution of secondary hydration fractures of different fracturing fluids. The fracturing fluids used were: slickwater, guar gum, and a 5:5 mixture of slickwater:guar gum, as shown in Table 1 for details.

[0032] Table 1 One-dimensional nuclear magnetic peak area of shale hydration Serial number Fracturing fluid Peak area 1 Slickwater 1964.87 2 Guar gum 1833.97 3 Slickwater:Guar gum = 5:5 1898.46 Table 1 shows the peak areas calculated from the one-dimensional nuclear magnetic data of shale hydration. The peak area is the area enclosed by the relationship curve between the nuclear magnetic signal relaxation time and the signal intensity, which is obtained by the integration method and characterizes the hydration effect. The larger the peak area, the easier it is to form hydration fractures. The conclusion is that the hydration effect of slickwater on the artificially fractured shale core is the most obvious.

[0033] Figure 2 For the comparison of the online nuclear magnetic results of three artificially fractured cores, the fracturing fluids used from left to right are slickwater, guar gum, and a 5:5 mixture of slickwater:guar gum. The online nuclear magnetic imaging results show that for the shale selected in this experiment, the hydration effect of slickwater is the most obvious but the distribution is relatively uneven; the hydration effect of guar gum is weaker than that of slickwater, but the distribution of secondary fractures is relatively uniform; the hydration effect and the distribution of secondary fractures of the 5:5 mixture of slickwater:guar gum are both between slickwater and guar gum.

[0034] As Figure 3 shown, based on the one-dimensional nuclear magnetic data calculation and online nuclear magnetic imaging methods, the pressure change in the core holder is also combined. The pressure drop amplitude during the hydration process of slickwater is the largest, and the pressure tends to be stable after 3 days, indicating that the expansion of secondary hydration fractures is completed.

[0035] Through comprehensive analysis, the conclusion is: for the artificially fractured core selected in this experiment, the hydration effect of slickwater is the most obvious, it is easy to form hydration fractures, and the optimal hydration time is about 3 days.

[0036] The above embodiments are only used to illustrate the present invention and are not intended to limit the present invention. Those of ordinary skill in the relevant technical field can also make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, all equivalent technical solutions also fall within the scope of the present invention. The scope of patent protection of the present invention shall be defined by the claims.

Claims

1. An on-line nuclear magnetic imaging experimental device for evaluating shale hydration secondary fractures, characterized in that, Including: Core, core holder and on-line nuclear magnetic device; The core is arranged in the core holder, and the core holder is arranged in the on-line nuclear magnetic device; On one side of the core holder, a pressure gauge, a metering pump and a liquid inlet tank are connected in sequence through pipelines, and fracturing fluid is placed in the liquid inlet tank; The other side of the core holder is connected to a waste liquid tank; On another side of the core holder, a hand pump for pressurization is connected.

2. The on-line nuclear magnetic imaging experimental device for evaluating shale hydration secondary fractures according to claim 1, characterized in that, Including: The core holder is made of stainless steel material.

3. The on-line nuclear magnetic imaging experimental device for evaluating shale hydration secondary fractures according to claim 1, characterized in that, Including: A first switch is provided on the pipeline path between the core holder and the waste liquid tank.

4. The on-line nuclear magnetic imaging experimental device for evaluating shale hydration secondary fractures according to claim 1, characterized in that, Including: A second switch is provided in the pipeline path between the pressure gauge and the metering pump.

5. The on-line nuclear magnetic imaging experimental device for evaluating shale hydration secondary fractures according to claim 1, characterized in that, Including: The hand pump is connected to the core holder through an confining pressure pipeline, and a third switch is provided on the confining pressure pipeline.

6. The on-line nuclear magnetic imaging experimental device for evaluating shale hydration secondary fractures according to claim 1, characterized in that, The fracturing fluid is slickwater breaker fluid, guar gum breaker fluid or mixed breaker fluid, and the ratio of slickwater to guar gum in the mixed breaker fluid is 2:8 or 5:

5.

7. An on-line nuclear magnetic imaging experimental method based on the on-line nuclear magnetic imaging experimental device for evaluating shale hydration secondary fractures according to any one of claims 1-6, characterized in that, Including: Step S1, fabricate an artificial fracture core or a core with a central opening; Step S2, clamp the fabricated artificial fracture core or core with a central opening in the core holder, and the core holder is made of stainless steel material; Step S3, place the core holder in the on-line nuclear magnetic device, and connect the liquid tank, hand pump, metering pump, core holder, pressure gauge and switch with pipelines; Step S4, turn on the on-line nuclear magnetic device, conduct nuclear magnetic resonance scanning before hydration, and record the state of the core before hydration; Step S5, inject fracturing fluid into the liquid inlet tank; Step S6, open the third switch, and apply confining pressure to the core holder through the hand pump; Step S7, open the first switch and the second switch, inject fracturing fluid into the core through the metering pump, and when it is observed that fracturing fluid flows out from the liquid outlet pipeline connected to the waste liquid tank, close the first switch and start pressure buildup; Step S8, after observing that the reading of the pressure gauge reaches the target pressure value, first turn off the metering pump, and then turn off the second switch; Step S9, turn on the on-line nuclear magnetic device again, conduct real-time nuclear magnetic monitoring during the hydration process, record the pressure change according to the pressure gauge, and conduct on-line nuclear magnetic resonance and pressure analysis; Step S10, replace the fracturing fluid, and repeat the above steps S4 - S9.

8. The experimental method according to claim 7, characterized in that, It further includes: After the experiment, remove the confining pressure and injection pressure, open the first switch, drain the waste liquid into a waste liquid bucket, remove the core holder after removing the connecting pipelines, and take out the core.

9. The experimental method according to claim 7, characterized in that The fracturing fluid is slickwater breaker fluid, guar gum breaker fluid or mixed breaker fluid, and the ratio of slickwater to guar gum in the mixed breaker fluid is 2:8 or 5:

5.

10. The experimental method according to claim 7, characterized in that In step S9, the on-line nuclear magnetic resonance and pressure analysis method specifically includes: Evaluation of on-line nuclear magnetic imaging results: By comparing the on-line nuclear magnetic signals and signal distribution states after hydration with different liquids, those with obvious signals and uniform distribution represent a large penetration degree of the fracturing fluid and relatively significant hydration effects; One-dimensional nuclear magnetic evaluation: Taking the nuclear magnetic signal relaxation time as the abscissa and the signal intensity as the ordinate, plot the signal curves of the core hydrated with different fracturing fluids. A larger peak area enclosed by the curve indicates obvious hydration effects and is prone to form hydration fractures; Pressure monitoring and evaluation: By comparing and analyzing the pressure changes in the core holder, it is found that the pressure drops significantly during the hydration process, indicating that the hydration effect is obvious and it is easy to form hydration fractures. The pressure becomes stable after decreasing for a period of time, indicating that the propagation of secondary hydration fractures is completed. This time is the optimal hydration time.

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