钻井液类型对井壁稳定的影响实例与防塌机理分析

针对东海油气田N区块钻井过程中易发生井壁坍塌的问题,通过梳理分析N5区块及周边构造三口探井地质条件、地层特性、测井数据,基于地质力学与岩石力学基本原理计算了井壁坍塌压力;并对使用水基钻井液和油基钻井液的钻井工况进行对比。研究发现在钻井液密度高于坍塌压力的情况下,使用密度相对较低的油基钻井液即能够保持井壁稳定,无阻卡等复杂问题。使用水基钻井液钻井,则部分泥页岩井段井径扩大,起下钻明显阻卡,处理复杂问题耗时较长。分析主要原因,在于油基钻井液能够降低泥页岩水化程度,减缓钻井液向微裂隙中的渗流,抑制微裂隙扩展,提高钻井液对井壁的有效支撑作用。因此,在东海油气田复杂泥页岩地层钻井中,使用油基钻井液能够更好地保持井壁稳定,避免或减少钻井复杂问题。

Cases of the Influence of Drilling Fluid Type on Borehole Stability and Analysis of Anti-collapse Mechanism

Aiming at the problem of wellbore collapse during drilling in Block N of the East China sea, the wellbore collapse pressure is calculated based on the basic principles of geomechanics and rock mechanics by sorting out and analyzing the geological conditions, formation characteristics and logging data of three exploration wells in the Block N5 and its surrounding structures. By comparing the drilling conditions using water-based drilling fluid and oil-based drilling fluid, it is found that when the density of drilling fluid is higher than the collapse pressure, using oil-based drilling fluid with relatively low density can keep the borehole wall stable and avoid complicated problems such as stuck pipe. When water-based drilling fluid is used for drilling, the hole diameter of some shale sections is enlarged, and there is obvious stuck on pipe trip, so it will take a long time to deal with complex problems. The main reason is that the oil-based drilling fluid can reduce the hydration of shale formations, slow down the seepage of drilling fluid into micro-fractures, inhibit the expansion of micro-fractures, and improve the effective support of drilling fluid to the borehole wall. Therefore, in the oil and gas drilling of complex shale formations in the East China sea, the use of oil-based drilling fluid can better maintain wellbore stability and avoid or reduce complex drilling problems.