二氧化氯作为煤储层压裂液破胶剂的可行性实验研究

郭红玉,夏大平,苏现波,马俊强,张双斌,王 振

(1.河南省瓦斯地质与瓦斯治理重点实验室——省部共建国家重点实验室培育基地,河南焦作 454000;2.河南理工大学能源科学与工程学院,河南焦作 454000)

二氧化氯作为煤储层压裂液破胶剂的可行性实验研究

郭红玉1,2,夏大平2,苏现波1,2,马俊强2,张双斌2,王 振2

(1.河南省瓦斯地质与瓦斯治理重点实验室——省部共建国家重点实验室培育基地,河南焦作 454000;2.河南理工大学能源科学与工程学院,河南焦作 454000)

针对煤层温度低、高黏压裂液无法及时破胶返排的难题,探讨二氧化氯作为压裂液破胶剂的可行性及其对煤层气开采的影响。实验选择清洁压裂液、交联冻胶压裂液和二氧化氯溶液对不同煤阶煤样进行处理,对处理后煤样再分别进行光学显微镜、等温吸附和压汞等测试。结果表明:残留的压裂液会对煤层造成污染伤害,二氧化氯不但具有良好的低温破胶作用,同时还可以提高大孔和中孔的孔容比,使得孔隙连通性得到改善,进而提高储层渗透率;经二氧化氯处理后煤样的朗格缪尔体积显著降低,表明了煤的亲甲烷能力显著降低,从而使得煤层气临界解吸压力、含气饱和度与采收率均得到不同程度的提高。研究表明二氧化氯可以作为煤储层压裂液的破胶剂。

煤层气;二氧化氯;压裂液;破胶剂;亲甲烷能力;采收率;含气饱和度;临界解吸压力

郭红玉,夏大平,苏现波,等.二氧化氯作为煤储层压裂液破胶剂的可行性实验研究[J].煤炭学报,2014,39(5):908-912.doi:10.13225/j.cnki.jccs.2013.0638

Guo Hongyu,Xia Daping,Su Xianbo,et al.Experimental study on the feasibility of chlorine dioxide as fracturing fluid gel-breaker for coal reservoirs[J].Journal of China Coal Society,2014,39(5):908-912.doi:10.13225/j.cnki.jccs.2013.0638

我国煤储层渗透率普遍较低,煤层气的开采面临诸多不利条件[1-2]。水力压裂是煤储层增透的主要措施,压裂液性能是影响压裂成败的关键因素之一[3-6]。目前,我国煤层气井压裂主要采用活性水,因其黏度接近清水,无法携带更多支撑剂进入地层裂缝[7-9]。若添加增稠剂提高黏度,又面临煤层低温条件下破胶困难的问题,一旦压裂液破胶不彻底而滞留地层裂缝,储层污染严重,甚至抵消压裂改造的效果[10-13]。为解决这一矛盾,亟需选择一种低温快速破胶剂,这对高黏压裂液的推广应用尤为重要。二氧化氯主要在油气田上用以解堵,即利用二氧化氯强氧化性和杀菌效率高的特点,解除压裂液、钻井液滤液中有机高分子堵塞、溶除铁垢、杀死细菌,达到解堵的目的,已在国内外得到成功实践,并取得较好的经济效益[14-17]。依据强氧化性的特性,选择二氧化氯作为煤层气井压裂液的破胶剂,并通过压裂液污染、破胶、等温吸附和压汞等实验手段,探讨二氧化氯作为煤层压裂液破胶剂的可行性对煤层气开采的影响,为高黏压裂液的破胶剂选择提供实验支撑。

1 样品与实验方法

1.1 样品制备

(1)煤样。分别在焦作古汉山煤矿、阳泉寺家庄煤矿、柳林沙曲煤矿、义马千秋煤矿和大同泉岭煤矿井下采集新鲜煤样,进行工业分析与反射率测试(表1)。

(2)压裂液。清洁压裂液配方:表面活性剂(0.8%VES)+胶束促进剂(0.2%SYN)+防膨剂(盐) (1.0%KCl)。交联冻胶压裂液配方:0.3%羟丙基瓜胶(HPG)+0.01%润湿剂+2.0%KCl+0.02%硼砂。

表1 煤样基础数据Table 1 Basic data of coal samples

(3)二氧化氯溶液。按照比例称取A试剂和B试剂,加入小口锥形瓶中,加入蒸馏水充分搅拌至无颗粒或块状物;按比例加入C添加剂,充分搅拌溶液,直至溶液呈浅黄色,即二氧化氯溶液,并对其密封保存,本实验采用的二氧化氯的浓度为4 000×10-6。

1.2 实验方法

(1)光学显微镜实验。大同泉岭煤矿和柳林沙曲煤矿煤样抛光处理后放入交联冻胶压裂液中浸泡6 h,在光学显微镜下观察浸泡前后的表面特征(图1)。

(2)压汞实验。选择千秋煤矿煤样,放入浓度为4 000×10-6的二氧化氯溶液中浸泡72 h,取出自然风干后采用Auto poreⅣ9505全自动压汞仪对煤的孔隙结构进行测试,为避免水分对测试结果的影响,在实验前首先对煤样进行干燥处理(图2)。

(3)等温吸附实验。每个煤样粉碎至60～80目,平均分为3份,每份质量不小于150 g,其中一份保持原样、一份被清洁压裂液浸泡48 h、另一份放入清洁压裂液和浓度为4 000×10-6的ClO2混合溶液浸泡48 h,分别烘干和称重,然后在25.6℃下在IS-300型等温吸附仪测试,实验严格执行《煤的高压等温吸附试验方法》(GB/T 19560—2008)标准。等温吸附实验结果见表2,等温吸附曲线拟合如图3所示。

图1 冻胶浸泡前后煤表面污染对比Fig.1 Surface pollution comparison of coal samples before and after gel immersion

图2 煤样进-退汞曲线Fig.2 Mercury injection and ejection curves of coal samples

表2 等温吸附实验结果Table 2 Results of isothermal adsorption

图3 不同方式处理煤样的等温吸附曲线Fig.3 Isothermal adsorption curves of coal samples with different actions

2 实验结果分析

2.1 二氧化氯破胶效果及其作用机理

煤层温度一般仅20℃左右,远远低于油气储层,油气田储层改造常用的高黏压裂液的破胶剂主要是过硫酸铵、双氧水和过硫酸钾等,但其普遍存在温度低于40℃释放游离氧速度减慢的问题,难以达到低温煤层压裂液彻底破胶的目的。从图1可知,煤经过冻胶压裂液浸泡后,表面有许多附着物,表明若压裂液不能及时破胶返排,滞留在煤储层中,就会堵塞空隙和裂隙等煤层气运移产出的通道,造成储层伤害,严重影响压裂增透的效果。二氧化氯依靠强氧化性能使大分子有机物断链降解为有小分子机物或无机物,可以在低温条件下对高黏压裂液进行快速破胶。

2.2 二氧化氯对煤样孔隙结构的影响

由图3可知在相同压力下,处理煤样的进汞量增大,大孔与中孔的孔容比增加(表3)。原煤样进-退曲线存在明显的拐点,为“细颈瓶”孔;处理后煤样拐点消失,为开放型孔。说明二氧化氯有助于提高煤层的孔隙连通性,这对改善煤层渗透率有重要意义。

表3 千秋煤矿煤样孔隙结构分析结果Table 3 Pore structure analysis results of coal samples in Qianqiu Coal Mine

2.3 二氧化氯对煤样吸附性的影响

从图3可看出,经清洁压裂液处理后,煤的吸附能力也相应降低,这与文献[18]得到的结论是一致的。但二氧化氯作为破胶剂加入到清洁压裂液,煤样经过该破胶液浸泡处理,对甲烷的吸附能力将进一步降低,其中义马千秋煤矿和焦作古汉山煤矿的煤样降低幅度更为明显。该实验不但证实了清洁压裂液对煤吸附能力影响的事实,更说明二氧化氯作为清洁压裂液的破胶剂将进一步降低煤的亲甲烷能力。二氧化氯作为压裂液破胶剂降低了煤的亲甲烷能力,在煤层含气量相同的条件下,吸附能力降低意味着煤层含气饱和度与临界解吸压力两项参数均有提高,缩短了排采阶段见气时间,且含气饱和度和临界解吸压力增加对提高煤层气井的单井产量也有重要作用。

2.4 二氧化氯对煤层气开采的影响

采收率、临界解吸压力和含气饱和度是影响煤层气开采的3个主要参数。依据表2中的等温吸附实验结果、煤层含气量、储层压力和废弃压力等对上述3个参数分别进行计算(表4),其中废弃压力根据美国目前的经验,取0.70 MPa[19]。

表4 二氧化氯对煤层气开采的影响Table 4 Impact of chlorine dioxide on CBM development

由表4可看出,清洁压裂液对煤层气开采3个参数有不同程度的增加;经清洁压裂液和二氧化氯的破胶液处理煤样的3个参数在清洁压裂液影响的基础上进一步提高,说明二氧化氯能有效降低煤的亲甲烷能力,促进煤层气解吸脱附,这有利于煤层气开采。

3 结 论

(1)二氧化氯具有良好的低温破胶作用,同时还可以提高煤体大孔和中孔的孔容比例,在采用高黏压裂液进行水力压裂后期注入适量的二氧化氯溶液,不但可以使高黏压裂液迅速破胶返排,减少储层污染,同时还能在水力压裂形成裂缝增透的基础上进一步提高煤基质渗透性,有利于煤层气的运移产出。

(2)等温吸附实验表明二氧化氯可以有效降低煤吸附甲烷的能力,这有助于提高煤储层的临界解吸压力与含气饱和度,缩短排采初期的见气时间。二氧化氯对煤层气的吸附/解吸性能的改变,促进煤层气脱附,有利于煤层气开采。因此,二氧化氯具有低温破胶、提高含气饱和度、临界解吸压力和采收率等多重效果,可以作为压裂液的破胶剂。

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Experimental study on the feasibility of chlorine dioxide as fracturing fluid gel-breaker for coal reservoirs

GUO Hong-yu1,2,XIA Da-ping2,SU Xian-bo1,2,MA Jun-qiang2,ZHANG Shuang-bin2,WANG Zhen2

(1.State Key Laboratory Cultivation Base for Gas Geology and Gas Control(Henan Polytechnic University),Jiaozuo 454000,China;2.School of Energy Science and Engineering,Henan Polytechnic University,Jiaozuo 454000,China)

Based on the difficulty that fracturing fluid with high viscosity is unable to break and flow back timely in the low-temperature coal seams,the feasibility of chlorine dioxide(ClO2)as gel-breaker of fracturing fluid and its impact on coalbed methane(CBM)development were studied.Different rank coal samples were treated with clean fracturing fluid,cross-linked gel fracturing fluid and ClO2solution,and were tested by optical microscope,isothermal adsorption and mercury porosimetric method,etc.The experiment results show:First,residual fracturing fluid will damage the coal seam.ClO2not only has a good effect on gel breaking at low temperature,but also it can increase the pore volumes ratio of macropores and mesopores for coal samples treated by ClO2.It improves the pore connectivity and further enhances the permeability of coal reservoirs.Second,the Langmuir volume of coal samples soaked by ClO2decreases significantly,indicating the coals’affinity for methane has fallen.As a result,the critical desorption pressure,gas saturation and the recovery factor of CBM will increase to some degrees.The study show that ClO2can be used as gel-breaker of fracturing fluid for coal reservoir.

coal bed methane;chlorine dioxide;fracturing fluid;gel-breaker;affinity for methane;recovery factor;gas saturation;the critical desorption pressure

P618.11

A

0253-9993(2014)05-0908-05

2013-05-16 责任编辑:韩晋平

国家自然科学基金资助项目(41002047);山西省煤层气联合研究基金资助项目(2013012004);河南省教育厅科学技术研究重点项目(12A440005)

郭红玉(1978—),男,河南遂平人,副教授,博士。Tel:0391-3987981,E-mail:ghy1026@126.com。通讯作者:苏现波(1963—),男,河南孟津人,教授,博士生导师。Tel:0391-3987981,E-mail:1054608403@qq.com