A Method for Accurate Calculation of Pore Pressure in Fractured Formations of Shale Gas Reservoirs
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摘要:
在钻遇页岩气藏裂缝区地层时，正钻井的地层压力与原始地层压力有较大差异，压裂后地层压力比压裂前地层压力有显著升高，利用传统孔隙压力预测方法求取的页岩气藏裂缝区地层孔隙压力不够准确。针对该问题，分析了不同工况下页岩气藏裂缝区地层孔隙压力动态变化的原因以及页岩储层的压裂增压机理，并提出了一种钻遇天然裂缝区及压裂作业区时求取动态变化的页岩气储层孔隙压力的方法。应用实例表明，该方法原理简单、机理明确，与现场测试结果吻合程度高，为一种准确求取页岩气藏裂缝区动态变化孔隙压力的有效方法，具有较高的实用价值。
Abstract:When drilling through the fractured formations of a shale gas reservoir, the formation pressure will exhibits a significant difference from normal pressure as well as after fracturing. It is hard to obtain accurate formation pore pressure in fractured formations of shale gas reservoirs by using traditional pore pressure prediction methods. To tackle this problem, we analyze the causes of dynamic changes in formation pressure in fractured formations under different operating conditions. We also analyze the large increase of formation pressure after shale gas reservoirs stimulation. Based on those analyses, we propose a method of calculating pore pressure change in both naturally fractured and stimulated shale gas reservoirs. Applying the method shows it has asimple principle and clear mechanism, and gives results which highly agree with those from field tests. As an effective method for accurately calculating dynamic pore pressure in fractured formations of shale gas reservoirs, this method offers high practical value.
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Keywords:
- shale gas reservoir /
- natural fracture /
- artificial fracture /
- pore pressure
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图 2 同平台钻进地层压裂波及区与未波及区渗透率对比
Figure 2. Comparison on the permeability of fractured area and unswept area reached from the same platform
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图 3 不同工况下页岩裂缝区的地层孔隙压力
Figure 3. Formation pressure in shale fractured area under different operation conditions
表 1 压裂增压计算结果
Table 1 Calculation of formation pressure from stimulation
Vdc/
（104m3）Va/
（104m3）Vgs/
（104m3）Vgf/
（104m3）Vfl/
（104m3）孔隙压力当量
密度/（kg·L–1）压裂前 压裂后 5 250 80% Vdc 20 748.0 51.03 3 1.37 1.52 70% Vdc 18 154.5 44.65 1.54 60% Vdc 15 561.0 38.28 1.57 
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表 2 反演所输入参数及模型计算结果
Table 2 Parameter inputs for inversion and calculation results of model
输入参数 计算结果 井深/m 声波时差/（μs·m–1） 密度/（kg·L–1） 天然地层渗透率/mD 泊松比 天然裂缝孔隙压力当量密度/
（kg·L–1）压裂后孔隙压力当量密度/
（kg·L–1）3 648 217.37 2.62 0.14 0.21 1.25 1.58 3 649 216.89 2.58 0.12 0.21 1.22 1.54 3 650 216.35 2.61 0.09 0.21 1.26 1.56 3 651 215.77 2.63 0.08 0.21 1.28 1.58 3 652 215.06 2.59 0.10 0.20 1.22 1.54 3 653 214.00 2.60 0.11 0.20 1.23 1.54 3 654 212.85 2.61 0.10 0.20 1.24 1.55 3 655 212.31 2.60 0.09 0.20 1.23 1.54 注：取测试气层中段的测井数据用于反演，地层基质渗透率K=0.08 mD，压裂后等效渗透率Kc=0.4 mD，压裂前测试孔隙压力当量密度为1.28 kg/L，压裂后测试孔隙压力当量密度为1.54 kg/L。
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