节理煤层井壁稳定性的评价模型

Evaluation model of wellbore stability in coal seam

对节理煤层的井壁失稳机理分析表明,近井壁的裂纹受到地应力和裂纹应力的双重影响,比煤岩块体更容易遭到破坏。用应力强度因子描述了裂纹的受力集中程度,利用强度准则构建了裂纹扩展破坏的判据,据此提出了节理煤层井壁稳定评价模型。该模型可以定量判断节理煤层直井或水平井井壁是否稳定,对其他节理地层的井壁失稳评价也具有借鉴意义。应用该模型并结合实例评价了煤储层水平井的井壁稳定,分析了裂纹倾角、井底压力、裂纹长度和井眼半径对应力强度因子的影响。对于节理煤层水平井,随着井底压力的增加,应力强度因子KⅠ和KⅡ呈线性减小,井壁稳定性变好;随着裂纹长度的增加,应力强度因子KⅠ和KⅡ也显著增加,井壁稳定性变差;裂纹倾角增大,应力强度因子KⅠ减小,而KⅡ增加,裂纹的抗张能力增强,抗剪能力降低,井壁的整体稳定性变差。当井径增加时,KⅡ增加,抗剪能力降低,井壁的整体稳定性变差。节理煤层中井眼存在尺寸效应,煤岩中存在裂纹是其重要原因。裂纹与井径的比值越大,尺寸效应越明显。

The analysis on wellbore instability mechanism shows that both the in-situ stress and the crack stress control the cracks near wellbore in coal seam, and the cracks get failure more easily than coal rock. The evaluation criterions for crack propagation and failure were presented according to the Drucker-Prage criterion and stress intensity factor used to describe the stress concentration of cracks. An evaluation model for the wellbore instability was developed on the above methods and criterion. This model can be used to assess the wellbore stability of coal seam in vertical or horizontal wells quantatively and may be helpful in other rocks. The effects of crack obliquity, bottom pressure, crack length and borehole radius on the stress intensity factor were analyzed. With the increase of bottom pressure, the stress intensity factors KⅠ and KⅡ decreased linearly, the wellhore stability was improved. With the increase of crack length, the factors KⅠ and KⅡ increased remarkably, and the wellbore stability got worse. As the crack obliquity increased, KⅠ decreased and KⅡ increased, the anti-tension capacity was improved while the anti-shear capacity was reduced and the stability of overall borehole got worse. With the increase of borehole diameter, KⅡ increased and anti-shear capacity was reduced, the wellbore stability got worse. The size effect happened in the borehole of coal seam and was resulted from the cracks existing in the coal bed. When the ratio of crack length to borehole diameter is greater, the size effect will be more obvious.