涪陵页岩气水平井工程甜点评价与应用
黄进1, 吴雷泽1, 游园1, 黄晓凯1, 聂彬2, 张辉1    
1. 中国石化江汉油田分公司石油工程技术研究院, 湖北 武汉 430030;
2. 油气钻采工程湖北省重点实验室(长江大学), 湖北 武汉 430100
摘要: 针对涪陵页岩气水平井多段分簇射孔压裂时加砂量符合率低、产气量低的问题,提出了寻找水平段工程甜点、并结合地质甜点进行压裂优化设计的方法。首先利用声波时差和密度测井资料,拟合得到涪陵页岩气藏横波时差计算模型,再结合密度、自然伽马等测井资料,利用地应力剖面计算软件求取涪陵页岩气藏水平井水平段的岩石力学参数,然后通过分析已压裂井段产气剖面测试结果与岩石力学参数的相关性,得到涪陵页岩气藏工程甜点参数为:水平应力差小于8 MPa、脆性指数0.45~0.50。焦页30-1HF井采用了工程甜点与地质甜点相结合的压裂设计方法,其压裂施工压力平稳,总液量和总砂量符合率较好,压后无阻流量88.54×104 m3/d,取得了较好的增产效果。现场试验表明,工程甜点与地质甜点相结合的压裂设计方法,能够提高涪陵页岩气水平井压裂的加砂量符合率和产气量,有助于实现页岩气的高效开发。
关键词: 页岩气    水平井    分段压裂    工程甜点    测井数据    脆性指数    焦页30-1HF井    
The Evaluation and Application of Engineering Sweet Spots in a Horizontal Well in the Fuling Shale Gas Reservoir
HUANG Jin1, WU Leize1, YOU Yuan1, HUANG Xiaokai1, NIE Bin2, ZHANG Hui1    
1. Research Institute of Petroleum Engineering and Technology, Sinopec Jianghan Oilfield Company, Wuhan, Hubei, 430030, China;
2. Hubei Provincial Key Laboratory of Oil and Gas Drilling and Production Engineering(Yangtze University), Wuhan, Hubei, 430100, China
Abstract: Due to the fact that shale gas reservoirs possess heterogeneity, sectional perforation and multi-stage fracturing in horizontal wells in the reservoir may generate very different effects. The engineering sweet spot parameter can affect the crack initiation, extension and reconstruction of hydraulic fractures. Accordingly, the parameter can be used to guide the sectional perforation of shale gas horizontal wells, which can be helpful in improving the reservoir and productivity. With the P-wave transit time and density logging data, the calculation model of S-wave transit time was established for the Fuling shale gas reservoir, and the calculation method of determining the engineering sweet spot was defined for the Fuling shale gas reservoir based on the experimental results of rock mechanics. Combined with the test results for 73 gas production sections in six wells, including Well Jiaoye 7-1HF, the evaluation indicators for the engineering sweet spot in the Fuling shale gas reservoir were summarized, namely, that the horizontal stress difference was less than 8MPa, and the brittleness index ranged from 45% to 50%. Optimum design for fracturing the Well Jiaoye 30-1HF was conducted based on the engineering sweet spot data, and then total liquid volume and total sand content were matched well with stable operation pressure. Post-frac open flow was 88.54×104 m3/d, and better stimulation results were achieved.
Key words: shale gas    horizontal well    section fracturing    engineering sweet spot    log data    brittleness index    Well Jiaoye 30-1HF    

页岩气藏工程甜点是指有利于提高储层改造体积和裂缝复杂程度的岩石力学参数,主要包括水平应力差、脆性指数、最小水平主应力等。国外很多文献都报道过工程甜点参数对水力裂缝扩展和水平井产能的影响[1-2]。文献[3]分析了Barnett页岩气藏一口3级压裂水平井生产测井结果与地应力的关系,发现射孔簇的产量与地应力有直接联系,70%的产气量来自水平段低地应力区的射孔簇,50%的射孔簇不产气,而这些射孔簇全部位于水平段的高地应力区。

目前,涪陵页岩气水平井在进行分段压裂设计时,主要考虑了地质甜点参数,如总有机碳含量(TOC)、渗透率、岩性和密度等,对工程甜点的认识不深入,导致部分井段加砂符合率低、同一层段内加砂量差异较大[4-8]。统计涪陵页岩气田已压裂井的资料发现,压裂后产气量低的井总加砂量符合率较低(平均为65.3%),储层的压裂改造效果未达到预期要求。研究与现场实践表明,工程甜点参数、井筒起裂点对页岩气水平井压裂裂缝的延伸扩展起着关键作用[7],在进行页岩气井压裂设计时,必须要考虑储层的工程甜点参数。

1 工程甜点参数研究

页岩气藏的工程甜点参数,如水平应力差、脆性指数(利用杨氏模量、泊松比计算[9]),其实质就是岩石力学参数。目前,求取岩石力学参数的方法主要有2种[10-12]:一种是岩心试验测定法,另一种是利用常规测井及偶极声波测井、成像测井等资料进行计算。岩心试验测定法数据准确,但取心困难、成本高。测井资料求取法具有连续性强、地质信息丰富、数据准确等优点。因此,目前普遍利用测井资料求取工程甜点参数。

声波时差反映地层的抗压、抗剪等特性,是求取杨氏模量和泊松比等岩石力学参数的基础数据。求取岩石力学参数时,应用的是横波时差,而在常规测井中大多只测量纵波时差。为此,笔者选取进行了横波时差测井的重点探井的测井资料,通过回归密度纵波时差比与横波时差的关系,求取未进行横波时差测井的开发井的横波时差测井曲线。

焦页1井密度纵波时差比与横波时差的关系曲线如图 1所示。对图 1中的曲线进行拟合,得到密度纵波时差比与横波时差的关系式为:

其中
式中:Δts为横波时差,μs/m;ρ为岩石密度,g/cm3;Δtp为纵波时差,μs/m。

图1 焦页1井密度纵波时差比与横波时差的关系曲线 Fig.1 Relationships between density/P-wave transit time and S-wave transit time in Well Jiaoye 1

将焦页11-4HF井和焦页41-5HF井的纵波时差和密度测井数据代入式(1)计算横波时差,并将计算出的横波时差与实测横波时差进行对比,结果见图 2图 3。由图 2图 3可知,利用式(1)计算的横波时差准确度较高,与实测横波时差相比,相对误差在5%以内。

图2 焦页11-4HF井横波时差计算值与实测值对比 Fig.2 Calculated and measured S-wave transit time for Well Jiaoye 11-4HF

图3 焦页41-5HF井横波时差计算值与实测值对比 Fig.3 Calculated and measured S-wave transit time for Well Jiaoye 41-5HF

利用式(1),根据密度和纵波时差测井数据求取横波时差数据后,再结合密度、自然伽马等测井数据,利用地应力剖面计算软件求取涪陵页岩水平井的岩石力学参数。图 4为利用地应力剖面计算软件求取的焦页12-1HF井水平段岩石力学参数。由图 4可知,该井计算出的最小水平主应力为49~51 MPa,与岩石力学试验测得的最小水平主应力(48~52 MPa)吻合较好。由于图 4中的杨氏模量和泊松比是动态的,而岩石力学试验测得的杨氏模量和泊松比是静态的,因此,通过相关转换得到焦页12-1HF井的静态杨氏模量为32.0~35.2 GPa、静态泊松比为0.20~0.22,与通过岩石力学试验测得的杨氏模量(32.0~34.0 GPa)和泊松比(0.21~0.23)相吻合。由此可知,根据式(1)的计算结果求取的岩石力学参数与岩石力学试验测得岩石力学参数的相对误差小于5%,计算结果准确可靠。

图4 焦页12-1HF井水平段岩石力学参数计算结果 Fig.4 Rock mechanics parameters calculated for the horizontal section in Well Jiaoye 12-1HF

对焦页7-1HF井、焦页12-1HF井、焦页6-2HF井、焦页8-2HF井、焦页4HF井和焦页10-4井等6口井压裂后73段的产气剖面测试结果与岩石力学参数的相关性进行分析,发现产气剖面测试结果与水平应力差、脆性指数的相关性较好(见图 5图 6)。从图 5可以看出:随着水平应力差增大,单段产气量逐渐降低;水平应力差低于8.0 MPa时,单段产气量不小于6 000 m3的概率较大。从图 6可以看出:脆性指数在0.45~0.50时,单段产气量不小于6 000 m3的段数较多;脆性指数大于0.50时,单段产气量小于6 000 m3的段数反而增多。综合以上分析结果,得到涪陵页岩气藏的工程甜点参数:水平应力差小于8.0 MPa,脆性指数为0.45~0.50。

图5 产气量与水平应力差的关系 Fig.5 Relationship between gas production and the horizontal stress difference

图6 产气量与脆性指数的关系 Fig.6 Relationship between gas production and the brittleness index
2 现场试验

利用焦页30-1HF井的测井资料计算其水平段的岩石力学参数,并根据涪陵页岩气藏的工程甜点参数评价指标,找到焦页30-1HF井的工程甜点区域(见图 7)。依据预测的工程甜点区,在分段时,尽量将工程甜点井段整合在一起,使工程甜点井段得到充分压裂改造;射孔位置选择在最小水平主应力低值区,并且考虑同一段内的应力平衡,射孔簇之间的应力差应尽可能小;在优化压裂规模时,将大规模压裂段设计在工程甜点区。该井的压裂设计结果如图 8所示。由图 8可知,该井8段大规模压裂段中有6段设计在工程甜点区(红色竖线、蓝色竖线分别表示大、小规模压裂段,大规模压裂段设计液量1 900 m3、加砂量70 m3;小规模压裂段设计液量1 700 m3、加砂量60 m3)。

图7 焦页30-1HF井预测工程甜点区 Fig.7 Forecasted engineering sweet spot area in Well Jiaoye 30-1HF

图8 焦页30-1HF井压裂设计结果 Fig.8 Schematic diagram of the fracturing scale designed for Well Jiaoye 30-1HF

焦页30-1HF井整个压裂过程中压力平稳(52~75 MPa),压裂液用量34 938.74 m3,符合率较好(99.1%);总加砂量1 099.90 m3,符合率较高(86.3%),平均单段加砂量61.10 m3,高于已压裂试气井平均单段加砂量54.70 m3。该井压裂后采用12.0 mm油嘴测试,平均无阻流量88.54×104 m3/d,取得了较好的增产效果。

3 结 论

1) 由页岩气井压裂段产气量与岩石力学参数的相关性分析可知,涪陵页岩气藏工程甜点参数为水平应力差小于8 MPa、脆性指数0.45~0.50。

2) 现场试验表明,通过评价得到页岩气井的工程甜点,并与地质甜点相结合,对压裂设计进行优化,可以提高页岩气井的压裂效果。

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文章信息

黄进, 吴雷泽, 游园, 黄晓凯, 聂彬, 张辉
HUANG Jin, WU Leize, YOU Yuan, HUANG Xiaokai, NIE Bin, ZHANG Hui
涪陵页岩气水平井工程甜点评价与应用
The Evaluation and Application of Engineering Sweet Spots in a Horizontal Well in the Fuling Shale Gas Reservoir
石油钻探技术, 2016, 44(3): 16-20
Petroleum Drilling Techniques, 2016, 44(3): 16-20.
http://dx.doi.org/10.11911/syztjs.201603003

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收稿日期: 2015-12-08
改回日期: 2016-03-02

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