页岩气水平井固井质量对套管损坏的影响

Influence of cementing quality on casing failures in horizontal shale gas wells

四川盆地西南部长宁—威远国家级页岩气示范区采用水平井方式开发页岩气藏,部分井由于套管损坏导致压裂过程中不能顺利下入桥塞、连续油管不能顺利钻磨桥塞等问题,甚至部分井段被迫放弃压裂作业,直接影响了气井产量的提高。为此,以WH1井为例,在分析该井压裂过程中套管损坏问题的基础上,认识到固井质量差是该井套管损坏的主要原因。然后,应用ABAQUS有限元软件,结合现场实际参数建立模型,对水泥环窜槽缺失、套管偏心和井径变化等3种固井质量差的形式,进行了数值计算,得出了固井质量对套管应力的影响规律:①水泥环缺失时会在套管内壁上产生较严重的应力集中,且套管应力随缺失角度的增加而增加;②套管偏心和水泥环缺失同时作用会极大增加套管应力,甚至达到套管屈服值;③井径变化对套管应力影响较小。该研究成果可对今后页岩气开发提供有益的指导作用。

In the Changning-Weiyuan national shale gas demonstration area, southwestern Sichuan Basin, due to casing failures in some wells, some problems occur, like the bridge plug neither being smoothly put into the hole nor being drilled through during the fracturing operation. And in some serious cases, fracturing operations have to be abandoned in some well sections. All these will sig- nificantly affect the gas well production. A thorough analysis of the casing failures in Well WH1 shows that the poor cementing quali- ty is the main factor. By use of the finite element software--ABAQUS, and in combination with the actual site model parameters, a model was built and numerical calculation was performed on the three forms of poor cementing quality, namely cement sheath fail- ure, casing eccentricity, and borehole diameter variation. The influence rule of the above three forms on casing stress is concluded here., in the fracturing process, the cement sheath failure will result in undesirable stress concentration on the inner wall of the cas- ings and the more serious the cement sheath failure is, the more stress it will cause; the concurrence of cement sheath failure and casing eccentricity will greatly increase casing stress, even reaching the casing yield value; borehole diameter variation has little effect on casing stress. These research results provide beneficial guidance for shale gas development in the future.