高压膏盐层定向井钻井关键技术

Jeribe—Kirkuk储层为伊拉克哈法亚油田的主力储层之一,上覆厚约500 m、地层压力系数为2.25的Lower Fars膏盐层作为其盖层。因该高压膏盐层的存在,导致第一口Jeribe—Kirkuk定向井多次卡钻2次侧钻,第一口水平井提前下套管完井,Lower Fars高压膏盐层钻井面临极大挑战。为此,通过分析Lower Fars膏盐层的力学特性、孔隙压力、地应力大小以及岩石矿物特征,建立Lower Fars层不同岩性地层井壁稳定和井眼变形规律的数学模型和判定规则,模拟钻井过程中的井眼变形规律,有针对性地研制适用于该膏盐层定向钻井的高密度饱和盐水钻井液体系、优化定向井井眼轨迹与施工方案。研究结果表明:(1)Lower Fars膏盐层最易失稳地层为泥岩层,卡钻的主因是泥岩水化后流变性增大,导致井眼缩径,并拖拽相邻硬石膏和盐岩坍塌,加剧卡钻的风险导致卡钻;(2)Lower Fars层井壁变形失稳随井斜角和井眼钻开时间的增大而增大,随着井眼钻开时间的延长,井壁失稳的风险急剧增加;(3)筛选了适合高密度饱和盐水的聚胺抑制剂BZ-HIB及聚合物稀释剂JNJ,优化钻井液配方,可有效提高钻井液在膏盐层钻进过程中的抑制性及流变性;(4)Jeribe—Kirkuk定向井造斜点由Lower Fars层提升至Upper Fars层,Upper Fars井段造斜、Lower Fars井段稳斜,降低Lower Fars段的井斜角和定向段长度,从而降低卡钻风险。截至目前,22口井的现场应用效果表明,该区?311.2 mm井眼事故复杂得到了有效控制,其中,2016年平均钻井周期为35.8 d,同比缩短52.8%,机械钻速从平均3.3 m/h提高至7.33 m/h,提高122%,技术优化取得了明显的效果。

Key technologies for directional well drilling in high-pressure anhydrite salt layers

The Jeribe-Kirkuk reservoir is one of the major payzones in the Halfaya Oil Field, Iraq, and its overlying 500 m-thick Lower Fars anhydrite salt layer with a formation pressure coefficient of 2.25 acts as the cap-rock sealing. That’s just why the first Jeribe-Kirkuk directional well experienced multiple drill pipe stickings and two side trackings, and the first horizontal well had to be completed with casing ahead of the schedule. Obviously, the drilling in the Lower Fars high-pressure anhydrite salt layer is faced with severe challenges. In this paper, the Lower Fars anhydrite salt layer was analyzed from the aspects of mechanical property, pore pressure, ground stress and rock mineral characteristics. Then, a mathematical model and criterion rules were established for borehole stability and wellbore deformation in the Lower Fars formations with different lithologies and the wellbore deformation was simulated in the process of well drilling. Finally, the high-density saturated salt water drilling fluid was specially developed for directional drilling in anhydrite salt layers, and the well trajectory and drilling program of directional wells were optimized. The following results were obtained. First, the most unstable zone in the Lower Fars anhydrite salt layer is composed of mudstones. As to the main reason for drill pipe sticking, the increase of rheological property after mudstone hydration leads to tight holes, so the neighboring anhydrites and salt rocks are pulled to collapse, which ultimately results in drill pipe sticking. Second, borehole deformation and instability in the Lower Fars layer intensify with the increase of hole deviation angle and wellbore opening time, and the risk of borehole instability increases sharply with the increase of wellbore opening time. Third, polyamine inhibitor BZ-HIB and polymer thinner JNJ are selected to optimize the formula of the high-density saturated salt drilling fluid, thus making its inhibition and rheology during the drilling in anhydrite salt layer improved effectively. Fourth, the kick off point of the Jeribe-Kirkuk directional well is moved upward to Upper Fars from Lower Fars. Specifically, the buildup section is in Upper Fars, the angle holding section is in Lower Fars, and the hole deviation angle and directional section length in Lower Fars are decreased, so the risk of drill pipe sticking is reduced. So far, it has been practically applied to 22 wells, showing that the drilling complexities in Ø311.2 mm hole have been effectively controlled. In 2016, the average drilling time was 35.8 days, 52.8% shorter than that in 2015, and the average ROP was increased by 122% from 3.3 m/h to 7.33 m/h. In conclusion, technological optimization has achieved good application results.