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作者
赵欣 孙昊 邱正松 黄维安 徐加放 钟汉毅
关键词
油气井工程;油田开发;复合盐层;井壁失稳机理;多元协同;稳定井壁;水活度窗口;钻井液
摘要
The complex features of the compound-salt formation result in wellbore instability during drilling operations. The mechanism of wellbore instability in compound-salt formation is studied by the analysis of mineral compositions, microstructures, physical-chemical properties, and interface mechanical properties of compound salts from different areas. The results show that the existing micro-scale pores and fractures, creep and dissolution of salt, hydration of gypsum-mudstones are the key causes of wellbore instability in compound-salt formation. In order to stabilize the wellbore in compound-salt, we propose the drilling strategies based on the synergism of multiple methods of strengthening the wellbore by the sealing and consolidating effect of drilling fluid, inhibiting the dissolution of salt and the hydration of gypsum-mudstones, balancing the water activity between rocks and drilling fluid, providing effective stress to support the wellbore. Based on the effect of water activity on the shale hydration swelling and dispersion properties, the design basis of the safe window of water activity for drilling fluids is determined. According to the wellbore instability problems when drilling in compound-salt formations in M Oil Field in the Middle East, H-S Oil Field of Uzbekistan, and Xinjiang Uygur Autonomous Region of China, the drilling fluids are designed respectively based on the proposed wellbore stabilizing technology. The field applications show that the drilling fluid technology greatly improves the wellbore stability in compound salt formations and reduces downhole problems, thereby improving drilling efficiency.
正文
表1 不同地区盐膏层的矿物组成Table 1 Mineral composition of salt formations in different areas
1)括号中数值为矿物的平均质量分数.
部分层状盐岩中,存在石膏和泥岩夹层,盐岩和膏、泥岩界面清晰,称为突变界面[12].界面处盐岩和泥岩相互嵌入,结构紧密,理论上具有较高的力学强度.而无突变界面的盐、膏、泥混层更为常见.根据中东地区M油田和AZ油田、中国塔里木油田、吐哈油田以及胜利油田的膏泥岩、含泥盐岩以及盐膏泥混层岩样扫描电镜照片(图1)可以看出,不同地区复合盐层微观构造各不相同,但存在一个共同点,即混层中发育有微孔隙或裂缝,孔缝尺寸大多介于2~80 μm,如中东M油田的膏盐岩(图1(a))及塔里木油田含泥盐岩图片(图1(b)).孔隙和裂缝的存在导致钻井液滤液极易进入地层内部,引起盐膏泥岩溶解及水化等,进而影响周围井壁岩石的力学状态.因此,复合盐层中微孔隙和裂缝发育是造成井壁失稳的重要原因之一.
图1 复合盐层扫描电镜图 (a)中东地区M油田膏盐岩(1300倍);(b)塔里木油田含泥盐岩(3000倍)
1.3.1 溶解和分散特性
复合盐层的理化性质比较复杂,可能发生石盐溶解、石膏溶解、膏泥岩水化膨胀及分散等.本研究利用页岩滚动分散实验方法,分析了77 ℃下中东地区AZ油田深度为2 229 m的石膏岩(1#)和2772 m的盐岩(2#),M油田深度为2571 m的盐膏泥混层(3#)和2723 m的盐膏岩(4#),吐哈油田深度为1210 m的盐膏泥混层(5#),塔里木油田深度为3990 m的膏泥岩(6#),塔河油田含泥盐岩露头岩样(7#)以及胜利油田深度为2979 m盐膏泥混层(8#),共8块盐膏层岩样的溶解与分散特性.图2复合盐层岩样滚动回收率结果表明,岩样在清水中滚动16 h后回收率均较低(0%~15.1%),表明盐岩地层岩石遇水极易溶解或水化分散,在钻井液设计中需重点考虑钻井液抑制盐膏溶解及膏泥岩水化分散能力.
图2 复合盐层岩样的滚动回收率
无机/有机盐是调节钻井液水活度的主要处理剂.利用LabSwift-aw水活度测定仪,测试了不同质量分数的氯化钠(NaCl)、氯化钾(KCl)、甲酸钠(HCOONa)和甲酸钾(HCOOK)盐水的水活度.分别采用水化膨胀实验和岩屑滚动回收率实验,测试了不同水活度溶液中泥页岩沿岩心轴线方向的膨胀率(图3)和滚动回收率(图4).选用岩样为胜利油田泥页岩夹层,主要矿物组成及质量分数分别为:黏土矿物(42%)、石英(36%)、斜长石(10%)、方解石(8%),其中,黏土矿物主要为伊/蒙间层(70%)和伊利石(25%).图3结果表明,泥页岩水化膨胀率随着水活度降低而显著降低.由于K+的晶格固定作用,相同水活度下KCl和HCOOK溶液中岩样膨胀率比NaCl和HCOONa更低,水活度为0.94时即可将膨胀率从11.5%降低至4.0%以下,而NaCl和HCOONa水活度需要降低至约0.80才能将岩样的膨胀率降低至4.0%以下.图4结果表明,随着溶液水活度降低,泥页岩滚动回收率显著增加.KCl和HCOOK溶液水活度为0.95时即可将回收率从42.5%提高至90.0%以上,可有效抑制泥页岩水化分散;NaCl和HCOONa溶液水活度约为0.86时泥页岩回收率可提高至90%以上.因此,降低钻井液水活度可有效抑制泥页岩水化膨胀、分散,提高井壁稳定性.但是,钻井液的水活度并不是越低越好,过低的水活度会导致岩石发生去水化,强度降低.理论上,可将岩石膨胀率为0对应的水活度设为维持井壁稳定的钻井液水活度值安全窗口下限aw1,当水活度低于aw1时泥页岩会发生去水化;可将地层岩石中水的活度值作为钻井液水活度窗口上限,此时钻井液与岩石中的水活度达到平衡,不存在化学渗透压驱动力.在实际作业中,可结合钻井设计及实际施工可接受的地层岩石的最高膨胀率Smax和最低滚动回收率Rmin指标要求,确定对应的钻井液水活度值或为水活度安全窗口上限,如图5,aw1—即为水活度安全窗口.
图3 泥页岩膨胀率随溶液水活度的变化
图4 泥页岩滚动回收率随溶液水活度的变化
图5 钻井液水活度安全窗口设计示意图
参考文献