冀中坳陷潜山内幕油气藏钻井井壁稳定分析

冀中坳陷潜山内幕油气藏钻井过程常面临恶性井漏、井塌和卡钻等复杂情况。通过对华北油田冀中坳陷20余口井的地质工程等资料进行分析,总结了潜山内幕油气藏影响井壁稳定因素:下第三系地层稳定性差(硬脆性泥页岩易垮塌,微米级微裂缝极为发育,岩性复杂多变和地层存在多套压力系统),潜山内幕碳酸盐岩储层孔洞裂缝极为发育,断层和潜山风化壳形成破碎带以及深层井底高温影响。针对井壁稳定影响因素提出相应技术措施:增强钻井液对下第三系硬脆性泥页岩的抑制和封堵性,其中对微裂缝的封堵是关键,异常压力和复杂岩性段要合理设计井身结构,优化钻具组合;碳酸盐地层重点是强化裂缝封堵,且密度是防漏控制因素,与地层压力近平衡的低密度水包油乳化钻井液防漏效果好;利用软化点适当及粒度分布合适的可变形封堵材料对破碎地层形成快速封堵并适当提高钻井液密度有利于井壁稳定;对井底高温情况要选用合适的抗高温钻井液体系,关键要维护好高温高压条件下钻井液的性能,提高其抗温能力,维持高温高压下胶体稳定,稳定流变性能和滤失量。

Countermeasures for Wellbore Stability in Drilling Procedures of Inner Buried-hill Reservoir in Jizhong Depression

Terrible leakage, wellbore collapse and sticking were encountered repeatedly during drilling procedures of inner buried-hill reservoir in Jizhong Depression. Through the analysis of geological and drilling engineering data taken from more than twenty wells in Jizhong Depression, Huabei Oilfield, some factors which affect wellbore stability were summarized, i.e., the poor stability of Lower Tertiary stratums(hard brittle shale easily collapse, micron-sized microfracture is highly developed, lithology is complex and multiple pressure systems exist), fractures and solution pores can be seen everywhere in carbonate reservoir of inner buried hill, fracture zones as the result of weathering and denudation of buried hill surface has been drilled, besides, the effect of high temperatures should be given enough attention . Some countermeasures are raised for the mentioned influence factors: strengthening the inhibition and sealing capability of mud when drilling hard brittle shale, and plugging the microfractures is of great importance. Stratums with abnormal pressure and complex lithology require an optimized design of well casing program and downhole BHA. Sealing is important for carbonate formations, in addition, mud density is one of the governing factors which may led to leakage and near-balanced drilling with oil-in-water drilling fluid is effective in preventing mud lost. Deformable materials, which can used to seal fractures and pores quickly, with appropriate softening point and particle size distribution are conducive to wellbore stability when combined with increasing the density of drilling fluid. Selecting appropriate drilling fluid and maintaining its fine properties under high temperature high pressure condition, the ability of resisting high temperature should be improved to maintain colloidal stability and stabilize rheological property as well as filtration of drilling fluid.