Numerical simulation of gas production for multilayer drainage coalbed methane vertical wells in southern Qinshui Basin
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摘要: 随着煤层气勘探开发的深入,多煤层合层排采受到广泛关注。合层排采管控工艺是确保煤层气合采井高产稳产的关键,而多煤层组合条件下复杂的地质条件增加了合层排采管控的难度。数值模拟技术是研究煤层气井合层排采管控工艺的有效手段,科学、可靠的模拟结果可为合采井排采管控提供依据。考虑温度效应、煤基质收缩效应、有效应力作用对煤层流体运移规律以及渗透率等煤层物性参数的影响,建立煤层气直井合层排采生产动态过程多物理场耦合数学模型,并进行有限元法的多物理场耦合求解。通过对沁水盆地南部郑庄区块煤层气合采井组的模拟,探讨不同排采速率下煤层气直井合层排采产气效果及渗透率等煤层物性参数动态演化特征,提出煤层气直井合层排采工程建议。模拟结果显示,郑庄区块3号、15号煤层整体含气量较高,煤层气合采井组具有较大增产潜力,提高排采速率对提高煤层气采收率的效果不显著;排采过程中,煤基质收缩效应对渗透率的影响强于有效应力作用,是提高煤层气井排采速率的保障,在确保排采速率不超过煤层渗流能力上限的基础上,适当提高排采速率可实现煤层气井增产。基于模拟结果,建议排采速率的调整以控制动液面或液柱压力为主;以3号、15号煤层气合采井增产为目标,产水阶段和憋压阶段,郑庄区块煤层气直井合层排采速率以液柱压力降幅0.12~0.20 MPa/d或动液面降幅12~20 m/d为宜,既可实现煤层气增产,又可避免储层伤害。Abstract: With the development of CBM (coalbed methane) exploitation, multilayer drainage of the CBM well has received wide attention. The production control technology is the key to ensure high and stable gas production of CBM well multilayer drainage. However, the complex geological conditions of multiple coal seams increase the difficulty of production control of multilayer drainage. Numerical simulation technology is an effective method to study the production control technology of CBM well multilayer drainage. The scientific and reliable simulation results can provide a basis for production control. In this study, considering the influence of temperature effect, shrinkage effect of coal matrix, and effective stress on the fluid migration law in the coal seam, permeability and other coal seam physical parameters, the multi-physical field coupling mathematical model for the dynamic process of CBM vertical well multilayer drainage was established. The coupled solution of the multiple physical fields was obtained by using the finite element method. Then, the gas production effect of CBM vertical well multilayer drainage at different drainage rates and the dynamic evolution characteristics of permeability and other coal seam physical parameters were discussed by simulating a multilayer drainage CBM well group in Zhengzhuang block, Qinshui Basin, and corresponding engineering proposals were also put forward. The simulation results show that the gas contents of No.3 and No.15 coal seams in Zhengzhuang block are relatively high, and the CBM well group has a great potential to increase production, causing an insignificant effect of increasing the drainage rate on improving CBM recovery. In the production process, the effect of coal matrix shrinkage on permeability is stronger than that of effective stress, which is the guarantee to improve the drainage rate of CBM wells. On this basis, an appropriate increase in the drainage rate can increase the production of CBM wells when the drainage rate does not exceed the upper limit of the seepage capacity of the coal seam. Based on the simulation results, it is suggested to adjust the drainage rate mainly by controlling the working fluid level or fluid column pressure. To increase gas production of multilayer drainage CBM wells in No.3 and No.15 coal seams, the drainage rate of vertical well multilayer drainage in Zhengzhuang block is decreased by 0.12− 0.20 MPa/d in the fluid column pressure or by 12− 20 m/d in the working fluid level in the water production stage and pressure holding stage, which can not only increase production but also avoid reservoir damage.
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参数 数值 3号煤层 15号煤层 煤层平均厚度/m 6.00 5.00 煤层初始压力/MPa 6.00 7.00 煤层初始温度/K 300 303 基质孔隙率/% 4.00 4.50 裂隙孔隙率/% 1.00 1.50 裂隙渗透率/10−3 μm2 0.514 0.754 煤的弹性模量/GPa 0.713 1.414 煤泊松比 0.240 0.250 基质弹性模量/GPa 8.47 10.32 裂缝刚度/(GPa·m−1) 2.80 2.85 基质热膨胀系数/K−1 2.4×10−5 2.4×10−5 Langmuir压力pL/MPa 2.07 2.07 Langmuir体积VL/(m3·kg−1) 0.025 6 0.025 6 CH4温度系数/K−1 0.021 0.025 CH4压力系数/MPa−1 0.071 0.075 克林肯伯格因子/MPa 0.76 0.74 CH4吸附热/(kJ·mol−1) 33.4 33.4 水相温度系数/[kg·(m3·K)−1] 0.022 8 0.022 8 表 2 数值模拟方案与初始条件、边界条件
Table 2 Numerical simulation cases, initial conditions, and boundary conditions
步骤 模拟内容 时间/d 初始条件 边界条件 第1阶段 历史拟合 实际井底流压降幅 220 煤层初始压力 实际煤层气井井底流压;
其他边界均为恒压边界第2阶段 不同排采制度下煤层
气井生产效果模拟实际动液面降幅,
2、5、7倍实
际动液面降幅1 800 煤层初始压力为排采
220 d后的煤层压力实际动液面降幅为“基准”、
“基准”降幅的2、5、7倍;
其他边界均为恒压边界表 3 排采各阶段动液面降幅模拟参数
Table 3 Simulative hydraulic pressure drop in different production stages
模拟方案 基准 动液面降幅 排水阶段 动液面降幅倍数 1 2 5 7 液柱压力降幅/(MPa·d−1) 0.04 0.08 0.20 0.28 动液面降幅/(m·d−1) 4.08 8.16 20.41 28.56 结束时液柱压力/MPa pads pads pads pads 憋压阶段 持续时间/d 60 60 60 60 液柱压力降幅/(MPa·d−1) 0.04 0.08 0.20 0.28 动液面降幅/(m·d−1) 4.08 8.16 20.41 28.56 结束时液柱压力/MPa 0.20 0.20 0.20 0.20 套压降幅/(MPa·d−1) 0.04 0.04 0.04 0.04 结束时套压/MPa 2.40 2.40 2.40 2.40 产气阶段 持续时间/d 236 236 236 236 结束时液柱压力/MPa 0.01 0.01 0.01 0.01 套压降幅/(MPa·d−1) 0.01 0.01 0.01 0.01 结束时套压/MPa 0.04 0.04 0.04 0.04 结束时液柱压力/MPa 0.01 0.01 0.01 0.01 套压/MPa 0.04 0.04 0.04 0.04 注:产气阶段包含控压产气阶段、稳产阶段和产气量衰减阶段;pads为临界解吸压力。 表 4 模拟井组日产气量平均历史拟合误差
Table 4 Average history fitting error statistics of daily gas production of simulation well group
井号 No.1 No.2 No.3 No.4 No.5 No.6 误差/% 0.66 1.59 1.88 3.82 3.07 1.72 井号 No.7 No.8 No.9 No.10 No.11 No.12 误差/% 12.43 1.60 1.31 1.01 4.07 2.23 表 5 模拟井组预测累计产气量增幅
Table 5 Prediction of cumulative gas production increase of simulation well group
井号 累计产气量增幅/% 2倍动液面降幅 5倍动液面降幅 7倍动液面降幅 3+15号煤层 3号煤层 15号煤层 3+15号煤层 3号煤层 15号煤层 3+15号煤层 3号煤层 15号煤层 No.1 12.09 13.06 10.80 32.22 32.08 32.41 52.51 53.74 50.88 No.2 9.04 10.06 7.68 21.52 21.38 21.70 33.43 34.61 31.86 No.3 12.42 13.29 11.24 33.88 33.76 34.05 55.65 56.76 54.15 No.4 8.93 9.91 7.62 21.36 21.23 21.53 33.18 34.31 31.67 No.5 6.31 7.30 5.02 13.41 13.27 13.58 20.20 21.23 18.84 No.6 23.97 24.92 22.67 62.11 61.95 62.33 62.63 63.76 61.09 No.7 7.13 8.05 5.83 15.83 15.70 16.00 24.10 25.10 22.71 No.8 6.08 7.05 4.82 12.81 12.69 12.97 19.24 20.26 17.93 No.9 8.80 9.75 7.50 21.01 20.87 21.19 32.62 33.70 31.15 No.10 13.27 14.23 11.96 36.50 36.37 36.67 60.70 61.97 58.97 No.11 7.63 8.50 6.42 17.67 17.57 17.81 27.04 27.98 25.72 No.12 1.21 1.96 0.13 0.17 0.06 0.33 0.54 1.21 -0.43 -
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