钻井液液压作用下裂缝性定容封闭体地层压力的变化规律

Changing laws of formation pressure of constant-volume fractured enclosed reservoirs under the hydraulic pressure of drilling fluid

裂缝性地层在钻进过程中经常出现漏喷同存的复杂情况,目前对其机理只能采用经验描述,缺乏基础理论研究和模型描述。为此,以伊朗雅达油气田大量现场实钻录井和测试资料为例,采用基于真实裂缝的液—液置换可视化实验装置对液—液定容置换行为进行了模拟实验和CFD单裂缝定容仿真验证,分析了钻井液密度、井口回压、裂缝宽度等参数对地层压力的影响规律。研究结果表明:(1)置换现象随钻井液密度的增大而变得更加明显且地层压力迅速下降,随着置换时间的推移,地层压力基本稳定;(2)地层压力随着回压升高而逐渐增大;(3)当高密度钻井液抵达缝板后,地层压力逐渐增加并达到最大值,随着置换的发生,地层压力重新建立起新的平衡;(4)裂缝越宽地层压力变化越大,置换形态越不易受裂缝面形态的影响,置换推进速度越大。结论认为,对于定容性油气藏地层压力与所用钻井液密度和回压呈正相关,现场宜采用封堵和井口控压结合的方式进行钻进,采用泄压法实施压井作业。

The complex situation of leakage and blowout coexistence often occur during the drilling of fractured strata. At present, their mechanisms are described only based on experience without basic theoretical studies or model descriptions. In this paper, a great number of actual drilling, logging and testing data of the Yada Oil Field in Iran were taken as an example to carry out simulation experiments and CFD single-fracture constant volume simulation verification on the liquid-liquid constant volume displacement behavior by means of the visual liquid-liquid displacement experiment device based on the real fractures. Then, the influential laws of drilling fluid density, wellhead back pressure and crack width on formation pressure were analyzed. And the following research results were obtained. First, with the increase of drilling fluid density, the displacement phenomenon gets more and more obvious and the formation pressure drops rapidly. And as the displacement goes, the formation pressure is basically constant. Second, the formation pressure increases gradually with the increase of back pressure. Third, the formation pressure rises gradually to the maximum value when high-density drilling fluid reaches the crack plate. And as the displacement goes, the formation pressure reaches a new balance. Fourth, the wider the crack is, the greater the formation pressure changes, the less affected the displacement form is by the morphology of crack surface and the faster the displacement advancing is. It is concluded that the formation pressure of constant-volume oil/gas reservoirs is positively related to the drilling fluid density and back pressure. Therefore, it is recommended to combine plugging with wellhead pressure control in drilling and to apply the pressure relief method to well killing.