页岩气井压裂液返排对储层裂缝的损害机理
游利军1, 谢本彬1, 杨建2, 康毅力1, 韩慧芬2, 王良2, 杨斌1
1.“油气藏地质及开发工程”国家重点实验室·西南石油大学;
2.中国石油西南油气田公司工程技术研究院

作者简介:游利军,1976年生,教授,博士生导师,博士;主要从事储层保护、非常规油气开发、岩石物理学等方面的科研与教学工作。地址:(610500)四川省成都市新都大道8号。ORCID: 0000-0003-2736-1095。E-mail: youlj0379@126.com

摘要

为了弄清压裂液返排过程中对页岩气储层裂缝的损害机理,选取四川盆地长宁区块下志留统龙马溪组页岩和压裂返排液,利用压裂返排液对造缝岩样开展压裂液返排和气驱压裂液实验,监测压裂液返排流动阶段的岩样液相渗透率、返排液固相粒度分布和浊度变化,对比压裂液气驱前后的气测渗透率,分析压裂返排液对页岩气储层中裂缝的损害机理与损害程度。研究结果表明:①压裂返排液作用后,页岩渗透率损害率介于53.1%~97.6%,返排液固相粒度区间显著缩小,液相滞留所造成的相圈闭损害、固相残渣堵塞、气相携液诱发微粒运移和盐结晶是其主要的损害方式;②气相流阶段,渗透率损害率降至23.1%~80.2%,滞留液相损害有所缓解,但固相残渣堵塞和返排液在裂缝面的盐结晶损害仍然难以避免;③基于页岩气井压裂液返排过程中对裂缝的损害机理,考虑到返排液的处理难度及其对储层裂缝的损害,建议应积极发挥压裂液的造缝能力,优化压裂液性质与用量,尽量做到不返排或少返排压裂液。

关键词: 压裂液返排; 页岩气; 储层损害; 固相滞留; 水相圈闭; 微粒运移; 盐结晶; 四川盆地; 长宁区块; 早志留世
Mechanism of fracture damage induced by fracturing fluid flowback in shale gas reservoirs
You Lijun1, Xie Benbin1, Yang Jian2, Kang Yili1, Han Huifen2, Wang Liang2, Yang Bin1
1. State Key Laboratory of Oil & Gas Reservoir Geology and Exploitation//Southwest Petroleum University, Chengdu, Sichuan 610500, China;
2. Engineering and Technology Research Institute, PetroChina Southwest Oil & Gasfield Company, Chengdu, Sichuan 610017, China
Abstract

In this paper, the Lower Silurian Longmaxi shale samples and the backflow fracking fluid in the Changning Block of the Sichuan Basin were selected to investigate the damage mechanism of retained fracking fluid to fractures in shale gas reservoirs. Thus, experiments were conducted on fracking fluid backflow and gas-driving fracking fluids. The changes of liquid permeability of shale samples, solid particle size distribution and turbidity of the backflow fraking fluid were monitored. The gas permeability before and after fracking fluid gas drive was compared, and the damage degree and mechanism of the backflow fracking fluid to the fractures in shale samples were analyzed. And the following research results were obtained. First, the damage rate of shale permeability after the fracking fluid backflow is between 53.1% and 97.6%, and the range of the solid particle size of the flowback fluid is significantly reduced. The main reservoir damage modes include phase trapping damage caused by liquid phase retention, blockage caused by the solid phase residue, particle migration induced by gas-carrying liquid and salt precipitation. Second, in the stage of gas phase flow, the damage rate of permeability drops to 23.1-80.2%, and the damage caused by liquid phase retention is relieved, but the damage caused by the blockage of solid phase residue and the salt precipitation of flowback on the facture surface is inevitable. Third, based on the damage mechanism of fracking fluid backflow in shale gas wells to fractures, considering the treatment difficulty of the flowback and its damage to reservoir fractures, it is recommended to give a full play to the fracturing capacity of fracking fluid and optimize the properties and dosages of fracking fluid so as to reduce the flowback of fracking fluid as much as possible.

Keyword: Flowback of fracking fluid; Shale gas; Reservoir damage; Solid phase retention; Water phase trapping; Particle migration; Salt precipitation; Sichuan Basin; Changning Block; Early Silurian
0 引言

页岩储层基块致密, 通常需要利用水力压裂才能实现页岩气藏的经济开发[1, 2, 3]。气井压裂后经过几天到数周时间焖井开井返排[4], 一些研究成果表明, 在返排初期由于储层压力降低导致裂缝闭合, 使有效裂缝体积损失大约30%[5], 且压力降低导致支撑剂嵌入裂缝壁面或被压碎随压裂液一起返排到地面[6, 7], 造成气井出砂[4]。矿场数据显示压裂液返排液普遍呈现出三高特征[8, 9], 即高化学需氧量、高总溶解性固体和高总悬浮物含量, 由于返排液仅占入井液的10%~50%[10, 11], 大量压裂液滞留储层对气藏造成严重损害, 如裂缝壁面附近含水饱和度升高, 造成水相圈闭损害使基质气体难以解吸[12, 13, 14]; 返排液中高浓度的二价离子(Ca2+、Mg2+、Sr2+、Ba2+等)会在储层中生成碳酸盐和硫酸盐沉淀, 从而使储层结垢[15, 16, 17, 18], 堵塞储层孔隙和裂缝空间, 增加气体传质阻力[17, 19, 20]; 压裂液破胶后大量残渣会和其他固相残渣“ 抱团” 堵塞裂缝, 造成渗透率损失[21]。由此可见, 尽管水力压裂创造了复杂的裂缝网络, 但只有一小部分裂缝有效[5], 压裂后各类损害相互叠加严重制约了页岩气藏的高效开发[22, 23, 24]

现有的室内实验主要通过评价页岩裂缝导流能力、基块损害程度、压裂液残渣含量、助排性能(考虑表面张力、吸附性等因素的影响)、压裂液在地层条件下的乳化和破胶性能、黏土稳定性(考虑速敏、水敏、盐敏、碱敏等敏感因素的影响)来研究压裂液对页岩气储层的损害[25, 26, 27], 但压裂液与含可溶盐的页岩作用后压裂液返排过程中对裂缝的损害没有被重视, 缺乏相关研究。

为此, 选取四川盆地长宁区块下志留统龙马溪组井下页岩, 利用压裂液返排液对造缝岩样开展压裂液返排和气驱压裂液实验, 通过监测驱替过程中压裂液浊度和粒度分布变化以及裂缝渗透率, 分析返排液对裂缝岩样的损害程度和损害机理。

1 实验样品与方法
1.1 实验样品

1.1.1 压裂液返排液

选用长宁区块某页岩气井不同返排时间(20 d和35 d)收集的压裂液返排液作为实验流体。压裂液返排液呈偏黄色的悬浊液体, 其组成复杂, 悬浮物多, 摇匀后其浊度介于55~260 NTU。除了各类可溶盐离子外, 胶体沉淀、聚合物残渣、破碎支撑剂、页岩裂缝面破碎所产生的有机质粉末等也是返排液的常见成分[28, 29]

1.1.2 实验岩样

实验岩样与所取压裂液返排液属同一口井。X射线衍射分析结果表明岩样矿物组分以石英、方解石和黏土矿物为主。石英平均含量为51.7%, 方解石平均含量为12.7%, 黏土矿物平均含量为27%。黏土矿物主要有伊利石、高岭石、绿泥石、伊/蒙间层矿物, 它们的相对含量分别为16.0%、21.7%、52.2%和10.1%。有机碳含量介于1%~5%, 镜质组反射率约为1.6%, 属成熟— 过成熟阶段。选取矿物组成和物性相近的岩样, 平行层理方向钻取6块长度约30 mm、直径约25 mm的岩心柱塞, 人工造缝后, 测定其基础参数。

1.2 实验方法

为了模拟储层条件下压裂液返排对页岩气藏裂缝的损害情况, 利用在矿场取得的压裂液返排液, 对人造裂缝岩样开展压裂液返排和气驱压裂液实验。实验装置如图1所示, 具体步骤如下:①将烘干后的岩心放入岩心夹持器, 施加围压3 MPa, 老化处理24 h, 以消除应力敏感, 待围压稳定后利用高纯氮气正向驱替测量岩心气测渗透率; ②将摇匀的压裂液返排液倒入中间容器中, 利用氮气向中间容器施压, 反向驱替压裂液返排液, 模拟压裂液返排过程, 驱替时间设置为24 h, 驱替压力设置与岩心气测渗透率实验相同, 实验驱替过程中同时监测岩心渗透率变化以及出口端液相浊度; ③利用高纯氮气正向驱替页岩岩心, 开展自然返排实验, 模拟气井生产返排过程, 返排时间设置为24 h; ④返排实验后将岩心取出, 60 ℃烘箱中烘干后再次采用氮气正向驱替测量岩心烘干后的气测渗透率, 实验方法同步骤①。

图1 压裂液返排和气驱压裂液实验装置示意图

利用式(1)[30]计算返排液驱替后和岩样烘干后的渗透率损害率。

$a=1-\frac{K_{d}}{K_{i}}$ (1)

式中Ki表示岩心初始气测渗透率, mD; Kd表示返排液驱替后和岩样烘干后的渗透率, mD。

2 实验结果

如图2所示, 岩样初始气测渗透率和烘干后气测渗透率均受到气体滑脱效应的影响[31, 32, 33], 随着岩样入口端压力增加(出口端接大气), 气体滑脱效应减弱, 气测渗透率逐渐降低。利用式(1)评价返排液驱替作用前后的渗透率损害率, 如表1所示, 返排液作用之后, 岩样渗透率明显降低, 即使经过24 h的气驱返排, 岩样渗透率损害率仍然较高(53.1%~97.6%), 表明仍有返排液和固体残渣等堵塞了裂缝渗流通道。再将返排液作用后的岩样放入60 ℃烘箱中烘干, 测试其气测渗透率, 渗透率有所恢复, 渗透率损害率下降(23.1%~80.2%), 说明在解除液相滞留所造成的相圈闭损害后, 固相残渣堵塞造成的页岩渗透率损失仍较为严重, 且难以恢复。

图2 返排液作用前后岩样气测渗透率变化曲线图

表1 返排液驱替损害评价实验结果对比表
3 讨论

页岩微裂缝发育, 压裂后大量压裂液滞留在裂缝中难以返排, 严重制约了气井产能[10, 11]。分析认为压裂液破胶后残渣的固相堵塞、液相滞留造成的相圈闭损害、气相携液诱发微粒运移和气井生产过程中可溶盐在裂缝面结晶析出是页岩气藏压后主要的损害方式。

3.1 固相堵塞

页岩储层通过水平井和体积压裂技术形成了复杂的裂缝网络, 且大多为微米级裂缝, 与返排液固相粒度处于同一量级(表1), 潜在固相损害严重。通过监测压裂液在裂缝中返排流动过程岩样液测渗透率和出口端液相浊度显示(图3), 返排液驱替实验开始后, 岩样液测渗透率快速降低, 但岩样出口端监测的浊度却有升有降。分析认为渗透率较高的岩样(yx-17、yx-18)裂缝缝宽较大, 裂缝不能有效捕获颗粒残渣并形成封堵带, 随着中间容器中固相沉积, 监测的液相浊度逐渐升高; 而渗透率较低, 缝宽较小的岩样(yx-19、yx-20、yx-21及yx-22)在驱替实验开始后, 裂缝能快速捕获流体中的部分残渣颗粒, 导致岩样出口端所监测到的液相浊度比较平稳。

图3 实验岩样返排液驱替过程液测渗透率和出口端液相浊度变化曲线图

对比岩样yx-22驱替前后返排液的粒度分布曲线(图4), 可以看出, 驱替后返排液固相粒径d(10)、d(50)、d(90)全部减小, 且d(90)降低幅度最大。驱替前90%的固相粒径小于21.87 μ m, 10%粒径分布在21.87~100 μ m之间; 驱替后90%的固相粒径小于6.73 μ m, 10%粒径分布在6.73~20 μ m之间, 粒径分布区间显著缩小, 表明压裂液驱替过程有大量的固相残渣滞留在了裂缝中。根据裂缝内“ 0.8倍架桥理论” [34], 当返排液固相粒度为裂缝水力学缝宽的0.8倍时, 返排液中固相会在裂缝中架桥, 形成桥堵, 小粒径固相随之充填入大粒径固相架桥形成的孔隙, 逐渐形成堵塞带。实验所选取的压裂液返排液部分固相粒径与页岩裂缝宽度处于同一量级, 固相滞留严重, 致使岩样烘干后气测渗透率损害率仍然高达23.1%~80.2%, 驱替后的岩心照片也进一步证实了上述分析(图5)。

图4 yx-20岩样驱替作用前后压裂液返排液固相颗粒粒度分布曲线图
注:图中d(10)、d(50)、d(90)分别表示颗粒粒度累积分布曲线上累积体积百分数为10%、50%、90%所对应的颗粒直径

图5 实验岩样压裂液驱替作用后岩样裂缝面图像

3.2 水相圈闭与微粒运移

页岩纳米级孔隙发育, 黏土矿物含量高且存在超低含水饱和度现象[35], 低矿化度压裂液侵入会使裂缝壁面附近含水饱和度升高造成水相圈闭损害, 同时水— 岩作用还会引发页岩黏土矿物水化膨胀诱发微粒运移[12, 13, 14, 36, 37]。从气驱压裂液返排实验结果(图6)可知, 部分岩样经过24 h气驱返排后渗透率有较显著的恢复, 渗透率恢复率分别为30.7%、10.3%、42.1%、46%、14.2%、0.2%, 说明通过自然返排能够有效缓解水相圈闭损害。在返排后期yx-20、yx-21、yx-22等岩样渗透率出现快速降低的趋势, 岩样入口端压力传感器监测的压力显示, 在返排后期, 压力传感器监测的岩样入口端压力呈波浪式跳动。分析认为这是由于在返排后期氮气的汽化携液作用已经带走了裂缝面大部分滞留的水相, 滞留在裂缝中的返排液固相黏结力降低, 在高返排压差条件下, 气流冲刷裂缝面和裂缝中滞留固相, 打破了裂缝面微粒和滞留固相的受力平衡, 使裂缝面吸附的返排液固相和页岩颗粒脱落并堆积在裂缝狭窄部位, 改变微粒的“ 架桥” 和沉积方式, 堵塞渗流通道, 使岩样渗透率快速降低[36]。这也提示管理者在制订页岩气井返排制度时, 考虑最优化解除水相圈闭损害的同时还要防止过大的生产压差诱发储层微粒运移。

图6 实验岩样渗透率随返排时间变化曲线图

3.3 盐结晶损害

页岩气藏在成藏过程中通过压实排水、生烃消耗和汽化携液作用消耗了储层大量的原始地层水, 使可溶盐滞留在了页岩孔隙或天然裂缝中[35]。气藏通过体积压裂后压裂液与储层的相互作用会溶解天然裂缝或孔隙中充填的可溶盐, 或与页岩储层高矿化度盐水混合使压裂后返排液矿化度随返排时间快速上升[38, 39] 。如加拿大Horn River盆地页岩气井返排液矿化度在40 000~70 000 mg/L之间, 而美国Marcellus页岩气井开井返排90 d后Cl浓度就高达170 000 mg/L。如表2所示, 四川盆地龙马溪组某两口页岩气井开井返排仅数小时返排液矿化度就超过10 000 mg/L, 且矿化度随返排时间的增加有逐渐递增的趋势。滞留储层的高矿化度压裂液中的水相会在气藏降压开采过程以气态形式存在于烃类气体中蒸发[40, 41]。蒸发作用促进了液相中可溶盐析出, 盐结晶充填页岩裂缝空间, 缩小有效裂缝体积(图5), 造成岩样渗透率降低[20], 且初始渗透率越低, 损害程度越严重[42]。页岩气井压裂液配方和返排制度优化中, 除了考虑固相残渣堵塞、微粒运移和滞留液相的相圈闭损害外, 还应考虑高矿化度返排液的潜在盐结晶损害。

表2 四川盆地龙马溪组某两口页岩气井返排液离子组成和矿化度数据表
4 页岩气井压裂液返排损害阶段分析

页岩气井压裂液返排过程各种损害因素是相互叠加的, 但返排各阶段储层损害的主控因素各不相同(图7)。开井初期压裂液返排阶段(液相流阶段)主要是返排液中悬浮固相对气藏微裂缝的堵塞作用和水相圈闭损害; 随着水相的排出, 页岩储层压力降低, 页岩基块孔隙壁上的吸附气开始解吸, 扩散至页岩微裂缝中, 裂缝内含气饱和度升高, 气相渗透率持续增加, 水相渗透率不断下降, 气藏出现气、水两相流[43, 44]; 气相携液作用使裂缝水相圈闭损害得到有效缓解, 但滞留微裂缝中固相之间的黏结力却显著降低, 储层中开始发生微粒运移; 在返排后期, 随着气井产气, 滞留压裂液中的水相被逐渐蒸发, 蒸发作用促进了液相中的可溶盐析出, 结晶盐则会在裂缝壁面析出造成盐结晶损害。

图7 页岩气藏压裂液返排各阶段损害示意图

页岩气井压裂液返排过程中各种损害因素是难以避免的, 且除水相圈闭损害外大多是不可逆的。现有研究表明页岩与水相流体作用后将发生明显的裂缝起裂扩展现象并对储层进行持续的微改造[45, 46], 这也成为矿场普遍采用压后焖井的理论依据。考虑到返排液的处理难度[28, 29, 47, 48]和返排过程对裂缝损害的负面影响, 应积极发挥滞留压裂液的造缝能力[49, 50], 优化压裂液体系与用量, 调控压裂液渗吸能力, 实现压裂液不返排或少返排。

5 结论

1)压裂液返排液中固相残渣易滞留在页岩微裂缝中, 堵塞气体渗流通道, 降低气相渗透率。

2)压裂液滞留在储层中会对页岩气藏造成相圈闭损害, 通过自然返排裂缝渗透率可以部分恢复, 但在大压差与压裂液长期浸泡下裂缝表面微粒会发生脱落运移, 堵塞裂缝。

3)页岩气井压裂液返排后期, 压裂液溶解的可溶盐会发生盐析, 盐结晶填充页岩裂缝空间, 影响裂缝渗透率。

4)优化压裂液体系与用量, 积极发挥压裂液的造缝能力, 调控压裂液渗吸能力, 不返排或少返排压裂液, 不仅避免或弱化压裂液返排对裂缝导流能力的损害, 而且可以解决返排液处理问题和减少对环境产生的负面影响。

The authors have declared that no competing interests exist.

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