泸州地区深层页岩气水平井套变成因机理探讨

Discussion on formation mechanism of casing deformation in horizontal wells in deep shale gas of Luzhou area

四川盆地南部泸州地区深层页岩气储量丰富,是继长宁、威远等中浅层页岩气之后重要的天然气资源接替领域。但在泸州地区深层页岩气开发过程中水平井套管变形(以下简称套变)频发,对水平井压裂施工产生明显影响,制约了该区深层页岩气的规模建产。业内普遍认为套管变形是地层中的断裂发生滑移并对水平井产生剪切和挤压作用引起的,但基于该理论的套变模型与诸多实测数据不相吻合。为了进一步明确套管变形机理,提高套变的防控效果,以泸203井区18口套管变形水平井为对象,基于多臂井径成像测井,逐个对比套变形态、套变层位以及平面断裂发育情况,建立了断裂滑移和层理面滑移2种地质模型,并分析了2种地质模型下的不同套管变形特征。研究结果表明:(1)断裂滑移诱发的套变具有以下规律,即平面上套变仅出现在水平井与断裂带的交会处;套管的变形长度理论上应在10 m以内;套管的变形程度应呈近似对称关系,中部变形强,向两侧逐渐减弱。(2)该区多数套变不在预测的断裂带内,且部分套变段变形长度与形态不符合断裂滑移的特征。(3)层理面滑移可诱发多个相似的套变,在该机制下套管变形长度等于井筒与滑移面相交的距离,且层理面滑移的垂向分布与地层的页理发育程度一致。(4)层理面滑移与断裂滑移具有相似的力学机理,通过三维莫尔圆可以分析钻井和压裂过程中诱发断裂/层理面滑移的关键参数变化。结论认为,除断裂滑移外,层理面滑移是诱发页岩气水平井套管变形的另一重要机理,该研究成果可为深层页岩气规模效益开发的套变防控提供理论指导。

The Luzhou area in the southern Sichuan Basin is rich in deep shale gas reserves,which makes it an important replacement field of natural gas resources after the middle and shallow shale gas in Changning and Weiyuan.During the development of the northern deep shale gas zone of Luzhou area,however,casing deformation happens frequently in horizontal wells,which has a significant impact on the fracturing construction of horizontal wells and restricts the efficiency of largescale productivity construction of deep shale gas in this area.Casing deformation is generally believed to be caused by the slip of faults in the strata and its shear and compression effects on horizontal wells,but the casing deformation model based on this theory seldom matches the measurement data.In order to further clarify the mechanism of casing deformation and improve the casing deformation preventing and controlling effect,this paper mainly analyzes 18 horizontal wells with casing deformation in the Lu 203 well block.Based on multi-arm caliper logging,they are compared one by one from the aspects of casing deformation morphology,casing deformation horizon and plane fault development.Finally,two geological models of fault slip and bedding plane slip are established,and different characteristics of casing deformation in these two geological models are analyzed.And the following research results are obtained.First,the casing deformation induced by fault slip presents the following regularities.The casing deformation on the plane only occurs at the intersection of a horizontal well and a fault zone.The theoretical length of casing deformation should be within 10 m.The degree of casing deformation should be approximately symmetrical,with strong deformation in the middle and gradually weakening towards both sides.Second,most of the casing deformations in this area are beyond the predicted fault zones,and the deformation lengths and morphologies of some casing deformation segments do not conform to the characteristics of fault slip.Third,the bedding plane slip can induce multiple similar casing deformations.Under this mechanism,the length of casing deformation is equal to the distance between the wellbore and the slip plane,and the vertical distribution of bedding plane slip is consistent with the degree of bedding development.Fourth,bedding plane slip and fault slip have similar mechanical mechanisms,and the key parameter changes that induce fault/bedding plane slip during drilling and fracturing processes can be analyzed through three-dimensional Mohr circles.In conclusion,besides fault slip,bedding plane slip is another important mechanism that induces casing deformation in shale gas horizontal wells.The research results can provide theoretical guidance for the prevention and control of casing deformation during the large-scale benefit development of deep shale gas reservoirs.