When a horizontal well is drilling through the No.15 coal seam in the Qinshui Basin, coal seam collapse and instability and reservoir contamination tend to happen. To deal with these problems, degradable thickening agent and low-temperature oxidizing breaker were developed. Then, a set of sold-free drilling fluid system which can guarantee both borehole stability and reservoir protection was prepared. And finally, this system was evaluated by performing experiments on its rheological property, filtration property, inhibition property, reservoir protection and gel breaking capacity. It is shown that this system is characterized by stable property, strong inhibition and good compatibility with the formation fluid. The recovery ratio of coal rock is higher than 96％ and its relative expansion ratio is lower than 0.3％. The gel breaking ratio is 95％ at 30 ℃, and the recovery ratio of coal rock permeability after gel breaking is over 96％. This drilling fluid system was applied on site in 4 U-shape horizontal wells in Qiyuan Coal Mine. The drilling operation was smooth and no accident happened. The skin factor of well testing after drilling was the lowest (-2.17). And thus, the double targets of borehole stability and reservoir protection are realized.%针对沁水盆地15#煤层水平井钻井过程中煤层坍塌失稳和储层污染的难题,通过研发可降解的稠化剂和低温氧化破胶剂,开发出一套既能稳定井壁又能保护储层的无固相保护煤层钻井液体系.通过钻井液流变性、滤失性、抑制性、储层保护性能和破胶性能等实验对该体系进行了评价.实验结果表明,该体系性能稳定,抑制性强,煤岩回收率大于96％,煤岩相对膨胀率低于0.3％,与地层流体配伍性良好,30℃下破胶率达到93％ 以上,破胶后煤岩渗透率恢复值达到96％ 以上.该钻井液在七元煤矿4口U型水平井现场应用,现场施工顺利,未发生任何事故复杂,完钻后试井表皮因数最低-2.17,达到了稳定井壁和保护储层的双重目的.
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