Managed Pressure Well Cementing Techniques for Wells with Narrow Safe Drilling Windows in Sichuan Basin
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摘要: 四川盆地地质构造复杂，以川西地区为例，井深7000 m以上，安全密度窗口仅0.05~0.08 g/cm3，固井漏失风险高，通常被迫反挤水泥浆补救，固井质量段长合格率仅39.6%。基于此，开展控压固井工艺研究，以川西地区为例，分析了井筒工作液密度、钻井液流变性、顶替排量、环空控压值对固井防漏和顶替效率的影响。研究表明，控压固井前钻井液等井筒工作液密度下调范围宜在0.05~0.08 g/cm3；钻井液动切力宜低于6 Pa；固井顶替排量应不低于22 L/s，即环空返速为0.9m/s，同时顶替后期应根据薄弱层位压力当量密度，采取变排量顶替技术；采用控压下套管工艺和分段憋压候凝技术解决常规下套管工艺和候凝工艺的不足。控压固井技术在四川盆地窄密度窗口超深井应用26井次，创造了多项应用指标记录，最大井深7793 m，最小密度窗口0.05 g/cm3，一次上返率为100%，固井合格率为100%，复杂易漏失井固井质量段长优质率由21.45%提高到44.58%，较好地解决了固井漏失低返问题。Abstract: The Sichuan Basin is characterized by complex geological structures. Take the Chuanxi area (West Sichuan) as an example, wells drilled in this area have depth of at least 7000 m, and the safe drilling windows are only 0.05-0.08 g/cm3. Losses of cementing slurry have been frequently encountered, reverse squeezing of cement slurry has to be performed for the well cementing to be certified. Percent qualified cementing job was only 39.6% (calculated by cemented length). Laboratory study has been performed on managed pressure well cementing technology to improve the quality of the well cementing job. Effects of the wellbore fluid density, drilling fluid rheology, displacement flow rate and the magnitude of managed annular pressures on the losses of cement slurry and displacement efficiency were analyzed. It was found that prior to managed pressure well cementing job, the density of the wellbore fluid should be decreased by 0.05-0.08 g/cm3, the yield point of the drilling fluid should be less than 6 Pa, the displacement rate should be at least 22 L/s, that is, the annular velocity should be 0.9 m/L or higher. Also, the displacement rate at the last stage of displacement should be varying based on the equivalent density of the weak zones along the wellbore. To overcome the shortage of conventional casing running operation and WOC techniques, managed pressure casing running technique and staged pressurizing/cementing techniques were used in the field operations. Managed pressure well cementing technology has been used 26 times on wells with narrow safe drilling windows in the Sichuan Basin, and many operation records were set; the deepest well cemented with this technology was 7793 m in depth, the narrowest safe drilling window was 0.05 g/cm3, rate of one-time return of cement slurry was 100%, rate of certified well cementing job was 100%, percent length of excellent well cementing job quality was increased from 21.45% to 44.58%. The managed pressure well cementing technology helped resolve the problems of cement slurry lossed and low return rate encountered in well cementing operations in the past.
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