维持井壁稳定的充气钻井液密度确定方法研究

随着我国西部和海洋深层天然气勘探开发不断加快、深入，钻井不断遇到高温、高压、气侵环境，受气体侵入的井筒钻井液其密度随温度和压力的变化而变化，这导致常规井壁稳定研究确定的当量静态钻井液密度不能有效地阻止井下井壁坍塌、缩径引起的复杂情况。国内外高温高压条件下钻井液密度计算模型存在着明显的问题：①没有考虑气体在环空中的影响，此时环空中是气液两相流体的流动，不能用单相液体的情况来对待；②井筒温度用地温梯度来代替不合理。为此，在确定有气侵、压耗和温度影响的有效安全钻井液密度时，分析了气液两相钻井流体受井筒压力、温度、气侵量与钻井液密度的相互影响关系，结合地层参数、钻井水力参数和钻柱结构，通过对温度场与压力场的耦合求解，获取了有效安全钻井液密度的下限和上限，计算结果在实际钻井中得到了较为成功地应用。

A METHOD OF DETERMINING AERATED DRILLING FLUID DENSITY FOR WELLBORE STABILITY

With the fast and in depth development of natural gas in West China and deep oceans, cases of high temperature, high pressure and gas invasion are met frequently in drilling. The density of gas invaded drilling fluid changes with temperature and pressure, which leads to complex downhole problems such as wellbore collapse and shrinkage, if conventional static ECD calculation methods are used. The traditional models for determining mud density of HTHP wells are not perfect in two aspects: ① the effects of gas in the annulus is not considered, and single phase hypothesis should be replaced by gas fluid two phases; ② geothermal gradient is used to represent wellbore temperature. In this paper, for determining the safe effective mud density influenced by gas invasion, pressure loss and temperature, the relationships between mud density and wellbore pressure, temperature, gas invasion volume are analyzed. By taking into consideration the formation parameters, drilling hydraulic parameters and drilling string configuration, with temperature field and pressure field coupled, the both limits of the safe effective mud density are obtained. The study results are successfully applied in the field drilling.