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截至2019年7月底，涪陵页岩气田已投产水平井417口，其中45口气井水平段长度超过2 000 m。受地层起伏影响，部分气井水平段呈下倾型，其中B、A靶点垂深差大于100 m的气井共有116口，垂深差最大达到745 m，水平段倾角主要为5~15°，占比83%。生产过程中因压裂液返出，井筒中处于气、液两相流流动状态，随地层能量降低，气井携液能力变差，液体滑脱至井底不断积聚，最终造成气井水淹停产。对于生产后期的下倾型水平井来说，液面往往在油管鞋以下位置，而气举、泡排、柱塞气举等常规排水采气工艺均要求油管鞋以上具有一定液面，因此该方法无法对水平段积液发挥作用。目前现场多采用间歇生产和放喷带出积液，造成气井生产时率低和资源的浪费。因此，有必要研究解决下倾型页岩气井水平段积液的排水采气问题，实现页岩气井低压阶段连续稳定生产。
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为解决下倾型页岩气井水平段携液困难的问题，考虑在原有生产管柱内，优选尺寸更小的连续油管下至水平段，一方面提高水平段携液能力，另一方面可通过连续油管开展泡排、气举工艺，排出水平段积液，形成由小直径内管和原完井管柱构成的具有实现多种生产模式的排水采气工艺管柱[4-8]。
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涪陵页岩气田油管结构主要有两种：(1)导锥+筛管+定压接头+陶瓷堵塞器+N80倒角油管+XN型工作筒+N80倒角油管+X型工作筒+N80倒角油管+双公变扣+油管挂；(2)喇叭口+筛管+破裂盘+倒角油管+破裂盘+XN型工作筒+N80倒角油管+X型工作筒+N80倒角油管+变扣短节+油管挂。考虑到要将连续油管下至水平段，原油管结构优选第2种结构。
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为使连续油管在原油管中安全、顺利下入，要求连续油管与外管之间至少留有4 mm的安全间隙，且由于原油管中安装有X、XN型工作筒，因此要保证下入连续油管能够顺利通过工作筒，则Ø73.0 mm油管(内径62.0 mm)中可下入Ø50.8 mm和Ø38.1 mm连续油管，Ø60.3 mm(内径50.66 mm)油管中仅可下入Ø38.1 mm连续油管，如表1所示。
表 1 不同规格外管中可下入的连续油管尺寸
Table 1. Sizes of coiled tubing that can run in the outer pipes of different dimensions
外管 内管(连续油管) 油管内径/
mmX工作筒
内径/mmXN工作筒
内径/mm最大外径/
mm内径/
mm壁厚/
mm62.0 58 56 38.1 30.74 3.68 50.8 42.88 3.96 50.66 47 45 38.1 30.74 3.68 -
一般认为油管尺寸越小，气井携液能力越强，但井筒摩阻越高，制约气井高产。统计涪陵页岩气田Ø50.8 mm和Ø38.1 mm连续油管生产时每增加1×104 m3气井筒压力损耗增加值如图1所示。可以看出，管柱内径越小，井筒压力损失越大。同时，水气比对井筒压力损失的影响较大，采用Ø50.8 mm连续油管生产时，当水气比大于1.5 m3/104 m3时，每万方气井筒压力损耗增幅显著增加；采用Ø38.1 mm连续油管生产时，当水气比大于1 m3/104 m3时，每万方气井筒压力损耗明显增大。
图 1 Ø50.8 mm与Ø38.1 mm连续油管生产压力损耗对比
Figure 1. Comparison of production pressure loss between Ø50.8 mm and Ø38.1 mm coiled tubing
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为使连续油管能够有效排除水平段积液，对于水平段下倾幅度较大的气井，一方面考虑将连续油管尽可能下至水平段深处，另一方面要计算连续油管的抗拉强度，达到抗拉安全要求，通常要求安全系数大于1.5(安全系数=抗拉屈服强度/井口处油管拉力)。如图2，在满足安全系数大于1.5的条件下，不考虑井斜时，Ø38.1 mm连续油管下深应不超过3 300 m，Ø50.8 mm连续油管下深应不超过4 700 m。对于水平井而言，垂直方向受力为重力的分量，减小了井口处油管拉力，连续油管可适当下入更深位置。对于钻塞返排率低、出砂严重的气井，连续油管不宜下深过大，防止油管砂埋。
图 2 连续油管下入深度与抗拉安全系数的关系(不考虑井斜)
Figure 2. Relationship between the setting depth of coiled tubing and the tension safety coefficient (without considering well deviation)
连续油管的下深同时要考虑井筒压力损耗，统计涪陵页岩气田不同水气比区间内Ø50.8 mm油管生产井每万方气的井筒压力损耗，如图3所示。可以看出，井筒压力损耗与连续油管下入深度呈正相关，水气比大于1.5 m3/104 m3后，连续油管下入深度对井筒压力损失的影响增加，水气比越高，井筒压力损耗随连续油管下入深度的增加幅度越大。
图 3 不同水气比条件下井筒压力损耗与连续油管下入深度的关系
Figure 3. Relationship between the well pressure loss and the setting depth of coiled tubing at different water/gas ratios
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在原气井井口装置中原1#阀与原4#平板阀之间加装一个油管头四通，油管头两翼均有两个平板闸阀，用于控制速度管柱与原油管间小环空生产，内置连续油管悬挂器，用以悬挂连续油管。由于下入连续油管后原1#阀无法关闭，需要更换采气树时存在井控风险，因此在新增油管头与原4#平板阀之间新增一个平板闸阀，发挥原1#主阀的井控作用。改造后的井口流程能实现连续油管、连续油管与原油管间小环空、油套大环空3种管位的生产、放喷和加药功能，如图4所示。
图 4 连续油管排水采气管柱结构与井口流程
Figure 4. String structure and wellhead process of coiled tubing based drainage gas recovery
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涪陵页岩气田JY-6HF井A靶点测深3 880 m(垂深3 135 m)，B靶点测深5 093 m(垂深3 483 m),水平段呈下倾型，A、B靶点垂深差达348 m。该井于2016年5月16日投产，2016年7月31日带压下入Ø73.0 mm普通油管，下入深度3 738.75 m，油管为“喇叭口+筛管+双工作筒”结构。随着气井产能降低，受水平段积液影响，自2017年2月起，气井生产表现为开井套压快速下降，产气量与压力变化极不稳定，期间开展放喷、气举、增压开采等措施效果不佳，如图5所示。
图 5 JY-6HF井生产曲线
Figure 5. Production curve of Well JY-6HF
2019年7月29日在原Ø73 mm普通油管中下入Ø38.1 mm连续油管至井深4 300 m，并实施气举和多次放喷，共排出液体40 m³，平均生产套压5 MPa↗11 MPa，油套压变化稳定，气井得以复产。该井自实施连续油管排水采气工艺后，目前已稳定生产294 d，累计增气503×104 m3，能够维持低气量下气井稳定生产。
通过PIPSIM软件进行节点分析，JY-6HF井在当前地层静压20 MPa、井口外输压力4.5 MPa的条件下，采用原Ø73.0 mm油管协调产量为2.2×104 m3/d，无法达到Ø73.0 mm临界携液气量2.7×104 m3/d。下入Ø38.1 mm连续油管至井深4 300 m后，气井协调产量为1.9×104 m3/d，能够达到Ø38.1 mm临界携液气量1.1×104 m3/d，满足携液要求，如图6所示。
图 6 措施前后气井节点分析对比
Figure 6. Node analysis of gas well before and after the measure application
从井筒内持液率分布来看(图7)，当采用原Ø73.0 mm油管生产时，在井深2 303 m(井斜40°)处发生持液率突变，即出现液体滑脱，而采用Ø38.1 mm连续油管生产时，井筒内持液率分布较为均匀，说明此时气井携液能力较好。
图 7 措施前后井筒持液率分布
Figure 7. Liquid holdup distribution in the well before and after the measure application
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(1)连续油管排采工艺能够满足页岩气井低液量后期排水采气要求，有效解决下倾型页岩气水平井水平段积液问题。
(2)连续油管尺寸的选择应综合考虑管柱的可下入性、携液能力与井筒压力损耗，Ø73.0 mm油管中推荐下入Ø50.8 mm连续油管，优选水气比小于1.5 m3/104 m3气井；Ø60.3 mm油管中仅可下入Ø38.1 mm连续油管，优选水气比小于1 m3/104m3气井。
(3)连续油管的下深需要综合考虑井筒摩阻损失、抗拉强度和井下复杂情况，不宜一味追求深度。
Coiled tubing of drainage gas recovery technology used in shale-gas downdip horizontal wells
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摘要: 页岩气井水平段采用Ø139.7 mm套管完井，受地层构造影响，部分气井B、A靶点垂深差大，呈现下倾型特征，水平段携液能力差，随地层能量衰竭，积液易堆积在油管鞋以下水平段，造成气井水淹，采用气举、柱塞、泡排等工艺难以复产。在原有生产管柱内，优选更小尺寸的连续油管下至水平段，增大气体流速，提高气井携液能力，同时可实现小直径管+气举+泡排复合排水采气，排出水平段积液。研究表明，Ø50.8 mm连续油管适用于水气比小于 1.5 m3/104 m3气井，Ø38.1 mm连续油管适用于水气比小于1 m3/104 m3的气井。现场应用表明，下倾型水平段积液气井下入连续油管至水平段中部后，油套压变化稳定，气井连续携液气量降低，井筒内气液分布均匀，滑脱损失降低。连续油管排水采气工艺能够有效解决下倾型页岩气水平段积液问题，实现页岩气井低产阶段连续稳定生产。Abstract: The horizontal section of shale gas well is completed using Ø139.7 mm casing, and due to stratigraphical and structural conditions, target A and B in some wells are more different in vertical depth, which leads to the emergence of downdip characteristics. The liquid carrying capacity in the horizontal section is poor, and with the depletion of formation energy, the liquid loading tends to accumulate in the horizontal section below the tubing shoe. As a result, gas wells are watered out and their production can be hardly restored by using gas lift, plunger and foam drainage technologies. To solve this problem, this paper selected a smaller coiled tubing and ran it into the horizontal section through the original production string to increase gas flowing velocity and improve the liquid carrying capacity in the well while applying the composite drainage gas recovery of small diameter tubing & gas lift & foam drainage to drain the liquid loading out of the horizontal section. It is indicated that Ø50.8 mm coiled tubing is suitable for the gas wells with water/gas ratio less than 1.5 m3/104 m3 and Ø38.1 mm coiled tubing is suitable for the gas wells with water/gas ratio less than 1 m3/104 m3. Field application shows that after the coiled tubing is ran into the middle part of horizontal section in the downdip gas well with liquid loading in the horizontal section, the change of tubing and casing pressure is stable, its gas rate for continuous liquid carrying is decreased, gas and liquid is distributed uniformly in the well, and slippage loss is reduced. In conclusion, the coiled tubing based drainage gas recovery technology can effectively solve the problem of liquid loading in the downdip horizontal section of shale gas well and ensure the continuous stable production of shale gas wells in the low-yield stage.
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图 1 Ø50.8 mm与Ø38.1 mm连续油管生产压力损耗对比
Figure 1. Comparison of production pressure loss between Ø50.8 mm and Ø38.1 mm coiled tubing
图 2 连续油管下入深度与抗拉安全系数的关系(不考虑井斜)
Figure 2. Relationship between the setting depth of coiled tubing and the tension safety coefficient (without considering well deviation)
图 3 不同水气比条件下井筒压力损耗与连续油管下入深度的关系
Figure 3. Relationship between the well pressure loss and the setting depth of coiled tubing at different water/gas ratios
图 4 连续油管排水采气管柱结构与井口流程
Figure 4. String structure and wellhead process of coiled tubing based drainage gas recovery
图 6 措施前后气井节点分析对比
Figure 6. Node analysis of gas well before and after the measure application
图 7 措施前后井筒持液率分布
Figure 7. Liquid holdup distribution in the well before and after the measure application
表 1 不同规格外管中可下入的连续油管尺寸
Table 1. Sizes of coiled tubing that can run in the outer pipes of different dimensions
外管 内管(连续油管) 油管内径/
mmX工作筒
内径/mmXN工作筒
内径/mm最大外径/
mm内径/
mm壁厚/
mm62.0 58 56 38.1 30.74 3.68 50.8 42.88 3.96 50.66 47 45 38.1 30.74 3.68 
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