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首页> 外文期刊>International Journal of Rock Mechanics and Mining Sciences >Drilling-induced tensile wall-fractures: implications for determination of in-situ stress orientation and magnitude
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Drilling-induced tensile wall-fractures: implications for determination of in-situ stress orientation and magnitude

机译：钻井引起的拉伸墙裂缝：对确定原地应力方向和大小的启示

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Detailed investigation of failure of the borehole wall in two scientific drilling projects, the German KTB (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland) and the European geothermal research project at Soultz-sous-Forets, France, has lead to new insight in the phenomena of tensile fractures induced in the wellbore wall during drilling. Comparison of the orientation of the fractures with the orientation of the horizontal principal stress known from breakout and hydraulic fracturing analysis demonstrates that these fractures are reliable indicators of the orientation of the maximum horizontal principal stress S_H. A model for the initiation of the fractures is presented which points out the important influences of (a) the tectonic stress state, (b) increased mud pressures during drilling operation and (c) thermal stresses induced by circulation of relatively cold drilling mud. Analysis of drilling-induced fractures in the GPK1 borehole at Soultz-sous-Forets (where the magnitude of S_H is known from hydraulic fracturing experiments) demonstrates the validity of this model for the initiation of the fractures. Further, a new method is proposed to estimate the magnitude of S_H from the occurrence of drilling-induced fractures and knowledge of thermally induced stress and pumping pressure during drilling. The method is successfully applied to both KTB boreholes. An independent method to estimate the magnitude of S_H based on the analytical calculation of the stress intensity factor for drilling-induced fractures taking into consideration both, increased mud pressure and thermal stress, is also presented. Application of this method confirms the results derived with the analysis described above. Additionally, the evaluation of the orientation of the fractures with respect to the wellbore axis indicates that over major depth sections of the investigated wells the vertical stress is a principal stress.
机译：在两个科学钻探项目中，德国KTB（德国的Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland）和法国Soultz-sous-Forets的欧洲地热研究项目，对井壁的破坏进行了详细的研究，从而得出了关于拉力断裂现象的新见解。在钻井过程中在井眼壁中引起的。从裂缝和水力压裂分析获知的裂缝方向与水平主应力方向的比较表明，这些裂缝是最大水平主应力S_H方向的可靠指标。提出了裂缝萌生模型，指出了以下重要影响：（a）构造应力状态;（b）钻井作业过程中泥浆压力增加;（c）相对冷的钻探泥浆循环引起的热应力。对Soultz-sous-Forets的GPK1井眼中的钻井诱发的裂缝进行分析（通过水力压裂实验可知S_H的大小）证明了该模型对于引发裂缝的有效性。此外，提出了一种新的方法来估计S_H的大小，该值由钻井引起的裂缝的发生以及钻井过程中的热致应力和泵送压力的知识来确定。该方法已成功应用于两个KTB钻孔。还提出了一种独立的方法来估算S_H的大小，该方法是基于对钻井引起的裂缝的应力强度因子的解析计算，同时考虑了泥浆压力和热应力的增加。该方法的应用证实了通过上述分析得出的结果。另外，相对于井眼轴线的裂缝取向的评估表明，在所研究井的主要深度部分上，垂直应力是主要应力。

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