Experimental study of the unblocking of coalbed with single-slit filling by pulse hydraulic fracturing
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摘要: 利用室內自行研制的可調頻脈動水力壓裂系統及預制充填型單縫的煤巖，開展不同頻率條件下水力脈動解堵試驗，研究脈動作用下解堵水壓演化過程和解堵效果。試驗結果表明，水力脈動解堵壓力演化過程可分為三個階段：壓力上升階段、壓力下降階段和壓力波動穩定階段。在疲勞損傷和脈動波的雙重作用下，脈動作用下的煤巖解堵表現出解堵壓力閥值比定常流作用下更低、壓降持續時間更短、壓降幅值更小的特點。定常流作用下煤粉運移集中在壓力下降階段；脈動壓力作用下煤粉在壓力下降階段和壓力波動穩定階段均有運移，且壓力下降時間與運移煤粉總量成正相關關系，但煤粉總的運移量和定常流作用下的相當。脈動作用和定常流作用下徑向滲透主要發生在壓力上升至壓力峰值階段，但脈動流的解堵時間短，則濾液滲透半徑小對儲層的傷害小。綜合考慮解堵壓力、煤粉運移、解堵滲透路徑和解堵滲流深度，特別是解堵壓力和解堵滲流深度作為主要評價因素，在3 Hz條件下解堵效果最好，其具有較低的解堵壓力和最小的解堵滲流深度。Abstract: As an unconventional natural gas resource, coalbed methane has huge reserves in China and has good prospects for exploitation. During the coalbed methane extraction process, a large amount of coal powder is produced. Coal powder accumulates in fissures and blocks them, which is one of the important factors affecting the permeability of coalbed methane. A hydraulic fracturing system with adjustable frequency pulsation and single-slit filling prefabricated samples were developed for testing coalbed fissure unblocking. Experiments were carried out to study the evolution process of the unblocking pressure and the unblocking effect of pulse action under different frequency conditions. Results show that the evolution process of the unblocking pressure can be divided into three stages: pressure rise phase, decline phase, and pressure fluctuation stability phase. Compared to that, under a steady flow, the pressure threshold under the pulse action is lower, pressure drop duration is shorter, and pressure drop amplitude is smaller. The migration of pulverized coal under the action of steady flow is concentrated in the pressure drop stage. Under the action of pulsating pressure, pulverized coal migrates in the pressure drop stage and the pressure fluctuation stable stage. The pressure drop time is positively correlated with the total amount of pulverized coal transported, but the total transport volume of pulverized coal is equivalent to that under the action of steady flow. The fluid infiltration along the radial direction mainly occurs during the pressure rise to the peak pressure. The filtrate penetration radius is smaller due to the shorter unblocking time under the pulsating flow, and the reservoir damage is small. Comprehensively considering the unblocking pressure, coal migration, unblocking seepage path, and unblocking seepage depth, especially the unblocking pressure and unblocking seepage depth, as the main evaluation factors, the best unblocking effect is under the condition of 3 Hz, which has a lower solution.
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圖 2 水力脈動解堵機理示意圖
Figure 2. Schematic diagram of the unblocking mechanism with pulse hydraulic fracturing
圖 3 煤粉充填后的試樣和示意圖（單位：mm）
Figure 3. Sample with coal filling and its schematic diagram (unit: mm)
圖 5 解堵壓力曲線。（a）0 Hz壓力曲線；（b）1 Hz壓力曲線；（c）3 Hz壓力曲線；（d）5 Hz壓力曲線
Figure 5. Unblocking pressure curve: (a) 0 Hz pressure curve; (b) 1 Hz pressure curve; (c) 3 Hz pressure curve; (d) 5 Hz pressure curve
圖 6 解堵過程中煤粉運移情況。（a）定常流壓力下降期；（b）定常流壓力衰減后；（c）脈沖流壓力波動期
Figure 6. Coal powder migration in the unblocking process: (a) period of steady flow pressure drop; (b) after the steady flow pressure decays; (c) pulse flow pressure fluctuation period
圖 8 壓降時間與煤粉運移質量曲線
Figure 8. Curve of the pressure drop time and coal cutting transport quality
圖 9 不同頻率下解堵路徑。（a）0 Hz液流路徑；（b）1 Hz液流路徑；（c）3 Hz液流路徑；（d）5 Hz液流路徑
Figure 9. Unblocking path at different frequencies: (a) 0 Hz liquid flow path; (b) 1 Hz liquid flow path; (c) 3 Hz liquid flow path; (d) 5 Hz liquid flow path
圖 10 不同頻率下滲透狀態。（a）0 Hz滲透狀態；（b）1 Hz滲透狀態；（c）3 Hz滲透狀態；（d）5 Hz滲透狀態
Figure 10. Seepage radius at different frequencies: (a) 0 Hz penetration state; (b) 1 Hz penetration state; (c) 3 Hz penetration state; (d) 5 Hz penetration state
表 1 原巖和相似巖樣物理參數比較
Table 1. Comparison of physical parameters of the original rock and of similar rock samples
Medium Unconfined compressive strength/MPa Tensile strength/MPa Poisson’s ratio Elastic modulus/GPa Friction angle/(°) Cohesion/MPa 12 h expansion rate/% Coal rock 6.41–9.05 0.687–0.716 0.19–0.24 1.37–3.86 18.27–33.8 0.982–1.34 0.35–0.80 Similar sample 7.45 0.58 0.237 3.92 21.55 1.63 0.20 
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表 2 解堵試驗參數設置
Table 2. Unblocking test scheme
Number Frequency/
HzVertical sealing pressure/MPa Horizontal lateral pressure /MPa Test groups 1 0 1.5 0.5 1 2 1 1.5 0.5 2 3 3 1.5 0.5 2 4 5 1.5 0.5 2
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表 3 不同頻率解堵壓力參數
Table 3. Unblocking pressure under different frequencies
Frequency/Hz Steady fluctuation pressure/kPa Critical unblocking pressure/kPa Pressure drop amplitude/kPa Pressure drop duration/s Pressure rise duration/s f·t per cycle 0 146.6 735.6 592.0 9.7 12.52 1 451.9 615.7 163.8 5.6 7.73 7.73 3 237.4 381.0 143.6 2.7 6.32 18.96 5 165.2 291.3 126.1 4.8 9.98 49.90
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