Borehole Stability in Drilling the Paleogene System and Inner Buried Hill in Huabei Oilfeld
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摘要: 华北油田古近系地层以砂泥岩为主体,间或有玄武岩、煤层等,存在不同压力系统,复杂情况以垮塌、漏失、遇阻、划眼为主,潜山带灰岩地层裂缝发育,卡钻和井漏事故多发,钻井复杂事故占全井的73.62%。从矿物组分分析、岩石力学特征、地应力测试、钻井液浸泡的影响入手,探索出古近系地层井壁失稳机理。古近系地层黏土矿物含量高,岩石水化膨胀严重,地层岩石黏聚力和内摩擦角变化幅度大(6~25 MPa、26°~45°),长时间浸泡后易形成缝网,当钻井液液柱压力高于坍塌压力达到某种程度时,裂隙宽度呈几何倍数增加,导致井壁掉块;奥陶系和蓟县系灰岩地层地应力差相对较大,岩石微裂缝发育,高地应力作用下易产生微裂缝,且多沿弱面破坏,而引起坍塌和漏失。为解决以上问题,在KCl-聚磺钻井液中引入了聚胺抑制剂和纳米防塌封堵剂BZ-PNP,提高抑制性和封堵能力,并增大润湿角,降低岩石亲水能力。该技术在阳探1、文安101x、安探1x等深井古近系地层进行了应用,取得了井壁稳定、钻井复杂事故为零的效果,其中阳探1井顺利钻穿邻井垮塌严重的大段泥页岩地层,平均井径扩大率1.8%,最大井径扩大率14.82%;安探1x风险探井钻井液密度最大为1.50 g/cm3,低于邻井的1.69 g/cm3。得出,在华北古近系地层使用密度过高的钻井液钻井,会增大微裂隙开启程度,并增加地层的吸水量,建议在井控安全前提下以高过坍塌压力当量密度15%为宜。Abstract: The lithology of the Paleogene system in the Huabei oilfeld is mainly sandstone and mudstone, intercalated occasionally with basalt and coal bed. Different formation pressure systems have been found in this oilfeld. Time required for coping with troubles during drilling accounted for 73.62% of the total time required for coping with troubles encountered in the whole process of drilling operation. The destabilization mechanism of the micro-fractured Paleogene system has been studied from mineral analysis, rock mechanics, formation stress measuring to mechanism of borehole wall destabilization. Large variations of cohesion (6-25 MPa) and angle of internal friction (26°-45°) of formation rocks and high formation stress cause micro fractures along the weak planes in rocks, resulting in borehole collapse and lost circulation. Abundant clays cause the formation to hydrate and swell, and after long time of soaking in water, network of fractures result. When pressure of mud column is greater than the collapse pressure of formation, the widths of the fractures increase exponentially, resulting in borehole wall sloughing. To stabilize borehole wall, Polyetheramine and a nano plugging agent were introduced into KCl drilling fluid. Polyetheramine inhibit osmotic hydration of clay through ether bonds and hydrogen bonds, and the unique action of amino group. This technology has been used in several wells such as Yangtan-1, Wen'an-101x and Antan-1x which penetrated the Paleogene system, no downhole troubles have occurred. The well Yangtan-1 successfully penetrated the long section shale formation which experienced severe borehole wall collapse in adjacent wells. The average hole enlargement of the well Yangtan-1 was only 1.8%, and the maximum hole enlargement, 14.82%. The well Antan-1x had maximum mud density of 1.50 g/cm3, 0.19 g/cm3 less than the mud density used in adjacent wells. Furthermore, excessive mud density is disadvantageous to borehole wall stabilization. It is suggested that the mud density should only be 15% higher than the equivalent density of collapse pressure, provided that there is no well control risk.
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