游利军, 程秋洋, 康毅力, 田 键, 杨 斌
(西南石油大学油气藏地质及开发工程国家重点实验室,四川成都 610500)
水力压裂形成的裂缝网络使页岩气藏经济开发[1-3]。压裂过程入地液量巨大,但压裂液的返排率普遍低于50%,有的甚至低于10%[4-6]。压裂液滞留储层会产生水相圈闭等一系列储层损害,影响页岩气井压裂改造效果[7-9]。在关井条件下页岩自吸对压裂液滤失和分布有重要作用[10-11]。目前页岩自吸试验主要探究基块和单一裂缝自吸行为,但仍无法合理解释自吸与压裂液滞留、返排率低的关系。考虑压裂液含水率超过90%[12-13],研究页岩裂缝网络水相自吸有助于进一步理解压裂液滤失机制[14-19]。笔者开展页岩基块、单一裂缝、“T”型裂缝网络水相自吸试验,并对比致密砂岩水相自吸,揭示页岩裂缝网络水相自吸与压裂液滤失的联系。
试验页岩岩样取自重庆市彭水县龙马溪组页岩露头,其石英平均含量45.7%,长石平均含量8.7%,黏土矿物平均含量28.5%。黏土矿物以伊利石和伊/蒙间层矿物为主,伊利石平均含量46.0%,伊/蒙间层矿物平均含量42.7%,高岭石平均含量11.3%。页岩岩样物性参数见表1。开展致密砂岩水相自吸对比试验,岩样物性参数见表2。试验流体采用3%KCl溶液。
表1 试验选用页岩岩样的物性参数
表2 试验选用致密砂岩物性参数Table 2 Physical property parameters of tightsandstone sample
自吸试验如图1所示,Ⅰ、Ⅱ表示页岩基块、单一裂缝水相自吸,作为对照组;Ⅲ、Ⅳ为不同组合方式的页岩裂缝-基块水相自吸,模拟页岩气藏水力压裂形成的“T”型裂缝网络[20]。对于试验岩样不同组合定义表述为:裂缝-基块组合代表裂缝岩样在下,基块岩样在上;基块-裂缝组合代表基块岩样在下,裂缝岩样在上。
为增强试验可对比性,在同一岩块上相近位置沿水平页理钻取岩心柱,然后将岩样长度处理约为4 cm;为减小孔渗差异,同一裂缝-基块组合的上、下两块岩样均由同一长岩心柱截取;参照行业标准SY/T53589(2010),烘干过程中每间隔1 h对岩样称重,直至最后两次称重差值小于10 mg,以此判定岩样初始含水饱和度近乎一致。利用自制的岩心造缝机统一沿过轴线截面造缝,保证缝面大小接近,考虑页岩裂缝面黏土矿物粒度小,各人造岩样缝面粗糙度差异近似忽略,利用同一型号的橡胶带固定裂缝岩样,以保证缝宽不变(图2);注入适量3%KCl溶液至自吸试验装置中,保持岩样底端被液体浸没5 mm;观察自吸现象并拍照,利用电子天平(精度为0.1 mg)实时动态对自吸岩样称重。页岩纵向对比试验自吸时间为24 h;页岩与致密砂岩对比分析试验自吸时间为168 h。
图1 页岩裂缝网络水相自吸示意图Fig.1 Schematic of water phase spontaneous imbibition of shale fracture network
图2 页岩和致密砂岩岩样人工造缝缝面Fig.2 Artificial joint surface of shale and tight sandstone samples
图3 页岩自吸量与时间的关系Fig.3 Relationship between shale imbibition amount and time
从页岩单块及组合岩样自吸试验结果(图3)看,在自吸初始3 h内,Ⅰ号基块平均自吸速率0.064 g/h,累积自吸量0.192 g;Ⅱ号裂缝平均自吸速率0.091 g/h,累积自吸量0.273 g。裂缝平均自吸速率相比基块提高42.19%,累积自吸量增加0.081 g。表明裂缝会诱使水相沿裂缝面快速自吸,提高水相侵入深度,增加滤失量[21]。Ⅲ号基块-裂缝组合的平均自吸速率0.049 g/h,累积自吸量1.178 g,而Ⅳ号裂缝-基块组合平均自吸速率0.204 g/h,累积自吸量1.683 g;结合试验现象观察发现,Ⅲ号组合约在13 h时基块-裂缝接触端见水,水润湿接触面积约为1/10,由于下端基块为上端裂缝自吸供液量小,整体自吸速率提高不明显;Ⅳ号组合约在0.7 h时裂缝-基块接触端见水;0.5~0.7 h平均自吸速率0.145 g/h,水迅速润湿整个接触面,0.7~1.2 h平均自吸速率提高到0.172 g/h。对比分析认为,Ⅳ号下端裂缝为水相提供快速自吸通道,使裂缝-基块接触端见水早,促使上端基块自吸相对提前,加速了水相向基块渗吸扩散,也加快了整体自吸进程;在毛管力作用下,水可能进入基块深部。
图4 页岩自吸量与时间开平方根的关系Fig.4 Relationship between shale imbibition amount and square root of time
页岩和致密砂岩自吸量与时间关系曲线如图5所示。致密砂岩整体自吸效率[23]均高于页岩:致密砂岩Ⅳ号组合自吸量为4.287 g,页岩Ⅳ号组合自吸量为1.454 g,前者自吸量是后者的3倍;致密砂岩Ⅲ号组合自吸量为5.691 g,页岩Ⅲ号组合自吸量为1.825 g,两者自吸量之比也接近3。因致密砂岩岩样孔隙度平均为15.42%,页岩岩样平均孔隙度为5.75%,前者与后者数值比为2.7。分析认为,致密砂岩更好的孔渗条件不仅提供水更大的赋存空间也促进水快速扩散分布,有效降低了自吸前缘含水饱和度,为自吸提供了动力,表现出更高的自吸效率。
图5 页岩和致密砂岩自吸量与时间的关系Fig.5 Relationship between imbibition amount and time of shale and sandstone
自吸进行到第10 min时,页岩Ⅳ号与致密砂岩Ⅲ号组合自吸量曲线出现交点,累积自吸量为0.160 g。相交前,页岩Ⅳ号组合自吸量一直高于Ⅲ号致密砂岩组合,证实裂缝增加了页岩有效渗透率,提高了自吸速率;相交后,同时间节点Ⅲ号致密砂岩组合自吸效率均高于Ⅳ号页岩组合。分析认为,页岩孔隙结构复杂[24]、非均质性强、润湿不均匀[9,25],阻碍了裂缝-基块间水相扩散渗流,削弱了裂缝水相自吸产生的积极效应,降低了整体自吸效率;而致密砂岩非均质性相对较弱,润湿比较均匀,水相自吸液面高度呈现活塞式推进,良好的孔渗条件有利于水相扩散渗流。此外,页岩孔隙中吸附气含量高[26],水相逆流自吸进入孔隙置换吸附气发生置换,气体排出过程可能也会阻碍水相自吸。
裂缝与基块两种不同尺度下的自吸存在相互促进机制:裂缝增大水相与基块接触面,为水相进入基块提供快速供液通道;自吸降低裂缝内含水饱和度,反馈促进裂缝持续快速的自吸。如图6所示,页岩Ⅳ号组合下端裂缝岩样自吸量始终高于Ⅱ号裂缝岩样,同时页岩Ⅲ号组合下端基块自吸量也一直高于Ⅰ号基块岩样;另一方面,Ⅳ号组合上端基块最终自吸量0.656 g,Ⅲ号组合上端裂缝仅0.397 g,前者高于后者0.259 g(图7)。水相未穿透下端岩样时会优先沿着页岩页理自吸[26],整体自吸高度呈现不均匀推进;水相自吸穿透下端岩样后,上端岩样为水相的进一步自吸提供了动力和通道,同时强化下端岩样最终的水相自吸深度。
图6 各组合下端岩样和单岩样自吸量与时间的关系Fig.6 Relationship between imbibition amount and time of bottom of combination and single samples
图7 各组合上端岩样自吸量与时间的关系Fig.7 Relationship between imbibition amount and time of top sample from different combination
在原地条件下,水相自吸存在两个动力:饱和度差异影响的自吸毛管力和孔渗差异引起的自吸液重新分布扩散动力。水力压裂形成的页岩裂缝网络为压裂液滞留提供了赋存空间,且裂缝网络越发育,由于次级裂缝与初级压裂缝含水饱和度的差异,自吸势提高越明显,最终吸入量越大[27-28];同时缝网面积越大,压裂液通过裂缝向基块渗流扩散范围越广,裂缝-基块间跨尺度的导流能力差异和页岩储层超低含水饱和度特征[29]有利于压裂液重新分布以降低自吸前缘含水饱和度,提升自吸动力。页岩储层裂缝-基块跨尺度水相自吸行为,彼此自吸相互促进,提高页岩整体水相自吸量。
页岩储层页理结构发育,以伊利石和伊/蒙间层为主的黏土矿物平行页理沉积分布[30],页理胶结程度低、渗透性好,属于高渗透带;同时垂直页理方向分布亲油有机质会抑制水相渗吸[31],故水优先沿平行页理自吸。水迅速润湿页理面后黏土矿物表面易发生水化作用,水分子渗透进入伊/蒙间层晶层,引起晶层间距显著膨胀扩大,压缩孔隙体积,但因页岩自身孔隙度低,当压缩孔隙无法完全消耗水化膨胀[32]的能量时,能量过剩将引发岩石局部爆裂,产生新裂缝。
其次,试验过程发现,自吸萌生了宏观新裂缝,白色可溶盐晶体沿新裂缝析出;致密砂岩虽未出现裂缝,但因孔渗相对较好,白色可溶盐晶体从岩样孔隙中析出(图8)。试验采用的3%KCl溶液常用于常规储层岩样自吸评价试验,但页岩自身可溶盐含量高,自吸流体与岩石不配伍导致水-岩作用明显。在孔隙度约为10%的岩石中渗透水化力高达30 MPa,且水化应力有随孔隙度降低而增加的趋势[33],鉴于页岩低孔低渗特性,且有机质孔隙度更低,常规手段难以精确测量。水相自吸进入页岩基块甚至有机质孔隙,可溶盐溶解后构成矿化度差异,产生较高渗透水化力,一旦作用在页理弱结构面和裂缝尖端,势必促使微米级裂缝扩展导致宏观页理缝出现。
考虑页岩工程地质的特殊性,宏观尺度上定义水-岩作用促使页理缝萌生;而在微米尺度上应客观定义为微米缝的扩展延伸。其作用机制为:水进入黏土矿物晶层,水-岩作用导致黏土微结构破坏和颗粒间黏结力减小,即岩石胶结强度降低。宏观上表现为岩石内聚力和内摩擦角降低,导致岩石强度或岩石I型断裂韧性下降,阻止裂纹失稳扩展的能力被削弱[34]。当水化作用产生水化应力、毛细管力、孔隙压力共同作用微米缝尖端时,引起裂纹尖端处应力集中,使应力强度因子增加;当应力强度因子大于断裂韧性时裂纹扩展或延伸。多条微裂纹汇合贯通后形成宏观裂纹,宏观裂纹进一步发展形成裂缝[35]。分析认为水相侵入触发水-岩作用促进了宏观页理缝的萌生。
图8 页岩与致密砂岩自吸过程实物图Fig.8 Physical map of spontaneous imbibition process of shale and tight sandstone
水力压裂形成了由初级压裂缝、天然裂缝和水-岩作用产生的次级衍生缝组成的复杂裂缝网络[36]。压后焖井期间,滑溜水和支撑剂填充初级裂缝,仅有滑溜水渗吸进入次级裂缝。高返排压力下,导流能力较强的初级裂缝中的压裂液返排率高;次级裂缝因自身导流能力差,受到的返排压力又比较低,返排困难导致大量压裂液滞留。目前对压裂液大量滞留诱发的储层损害和产能削弱问题,矿场主要采取经验焖井和快速返排措施以降低损害程度,但若焖井时间设计不合理会削弱压裂液与页岩作用对储层改造的积极效应。
基于页岩水相自吸试验结果分析认为,页岩裂缝网络水相自吸扩大了页岩储层中压裂液的分布范围,拓宽了压裂液与页岩接触面积,有利于增强水-岩作用萌生新裂缝(图8),进一步优化改造储层。在地层条件下,水-岩作用协同时间效应导致页岩胶结强度降低,裂缝尖端流体压力与地应力和岩石强度间的平衡被打破,焖井所维持的流体超压条件还能促进裂缝扩展延伸和微裂缝萌生,使简单的裂缝网络(图9(a))演变成次级裂缝发育的复杂裂缝网络(图9(b))。在毛管力起主导作用下,次级裂缝中的水通过逆流自吸进入基块,促进气体从基块中解吸-扩散-渗流进入裂缝,有助于提高页岩储层早期产气量[37]。与之相对的,压裂液返排率越高,压裂液滞留量越少,水-岩作用对裂缝网络的积极改造程度就越低,且水相逆流自吸对基块内气体解吸-扩散-渗流进入裂缝的作用也越弱,一定程度上削弱了焖井对早期产气量的积极作用。
页岩气井水力压裂过程的入地液量巨大,压后返排率低,大量压裂液滞留储层。压后焖井的出发点就是在降低压裂液返排量和污水处理成本的同时,利用水-岩作用对储层的积极改造的优势,通过优选入井压裂液,将可能产生储层损害的滞留压裂液转化为进一步改造储层的动力[5,38]:一方面,凭借水-岩作用消耗部分滞留压裂液,缓解水相圈闭、结垢堵塞等损害;另一方面,利用水-岩作用产生的微裂缝提高裂缝网络密度,改善气体渗流通道,同时促进水相逆流自吸使基块内气体的解吸-扩散-渗流进入裂缝,提高气体产量,达到变害为利的目的。
图9 页岩储层裂缝网络示意图Fig.9 Schematic of simple and complex fracture network of shale reservoir
(1)页岩裂缝网络快速自吸为基块自吸充足供液,基块自流扩散为裂缝进一步自吸提供动力,裂缝与基块自吸相互促进利于压裂液重新分布,增加了压裂液滤失量。
(2)页岩气藏天然裂缝和页理发育,水相易沿页理自吸诱发页理缝,提高了缝网密度,改善了渗吸路径,扩大了水-岩作用范围。
(3)优选压裂液配方,利用水-岩作用消耗储层滞留压裂液,变害为利降低压裂液滞留对储层的损害;发挥水-岩反应对储层裂缝网络的积极改造作用,改善气体渗流通道,强化页岩气解吸-扩散-渗流过程,提高产气量。
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