长北气田CB21-2井长水平段煤层防塌钻井液技术实践与认识

CB21-2井是一口壳牌中国勘探与生产有限公司长北气田二期开发井，地处工区边缘，储层地质复杂，特别是下二叠统山西组煤层易碎、易塌，水平井段更为突出。为此，基于长北气田和苏里格气田普遍应用的低伤害钻井液体系，重点考虑泥岩、碳质泥岩、煤层交替出现的水平井段安全钻井的需要，研制了适合该区块的有机盐加重钻井液体系（0.2%～0.3%提切剂+0.1%～0.2%提黏剂+0.3%～0.5%降失水剂+0.1%～0.2%抗盐降失水剂+0.5%～1.0%G301-SJS+18%～20%有机盐+4%～5%无机盐+高效润滑剂+防腐剂+除氧剂+缓蚀剂+石灰石），CB21-2井第2条分支钻穿多套煤层，没再出现卡钻等井下复杂情况，平均井径扩大率只有1.0%。由此总结出该区山西组长水平段煤层防塌技术对策：①钻井液密度提高至1.22 g/cm³，平衡地层坍塌压力；②提高钻井液抑制性，防止地层水化膨胀；③强化钻井液体系封堵能力，封堵煤层裂缝，降低钻井液滤失量；④控制钻井液流变性，保持井眼清洁和润滑性。

Application of anti sloughing drilling fluid technology to the long horizontal coalbed interval of Well CB21 2, Changbei Gas field

Well CB21 2 is a development well drilled during the second stage of the development of the Changbei gas field operated by the Shell China Exploration and Production Company Limited. This well is located at the edge of this gas field with complicated geological properties, especially in Shanxi whose coal beds are fragile and easy to collapse, particularly at its horizontal intervals. Therefore on the basis of the low damage drilling fluid systems commonlyused at the Changbei and Sulige gas fields, a weighted organic brine drilling fluid system is developed for this focusing on the requirements of safe horizontal drilling through the interbedded siltstone, carbargilite and coal zones. After this system was used to drill the second lateral of CB21-2 (Leg 2) through multiple coal beds, no sticking ever occurred, the average wellbore enlargement ratio was only 1%. In summary, the anti sloughing measures for the directional drilling with long horizontal intervals through Shanxi coal beds are as follows: to increase the mud weight up to 1.22 g/cm³ to balance the well sloughing pressure, to improve the drilling fluid inhibition to prevent the formation from hydration swelling, to reinforce the plugging performance of the drilling fluid system to seal the coalbed fissures and reduce the drilling fluid loss, and to regulate the drilling fluid rheology to keep the wellbore clean and maintain proper lubricity. And it is also realized that the drilling fluid performance should be optimized further, especially the mud weight can be increased to meet the engineering needs.