温度对气层气体钻井井壁稳定的影响

气体钻井过程中气体经过钻头喷嘴后产生焦耳-汤姆逊冷却效应,导致井底温度远小于地层温度。当气层被钻开后井壁围岩温度、孔隙压力和应力分布发生改变,容易引起井下复杂事故。为此,分析了井底低温对致密砂岩气藏气体钻井井壁稳定的影响。研究表明,井眼钻开后井底低温产生拉热应力,使近井壁有效应力减小,有利于防止岩石剪切失稳,但增加了岩石产生拉伸破坏的可能。气层气体产出使近井壁地层孔隙度和渗透率减小,不考虑井底低温时减小程度偏大。气体钻井近井壁可能存在塑性区,低温使塑性区向地层延伸更容易导致井壁失稳。塑性区的形成不仅取决于水平地应力,而且与垂向地应力有关。通常致密砂岩气藏埋藏较深,垂向应力影响大,其弹塑性分析需考虑三维主应力的影响。

Influence of Temperature on Wellbore Stability during Gas Drilling through Gas Formations

Joule-Thomson cooling effect in gas drilling causes a significant temperature drop at bit. Large changes in pore pressure, temperature, and the stress state when drilling through high pressure gas reservoirs normally lead to downhole problems. The influence of low temperature on wellbore stability during gas drilling was investigated. The results reveal that low bottom hole temperature tends to decrease the effective radial and tangential stresses suggesting enhanced wellbore stability with respect to shear failure, whereas low temperature of gas also increases the potential for tensile failure. Porosity and permeability drops of near wellbore are captured as a result of high pressure gas outburst from formations, and those drops are more pronounced without consideration of low bottom hole temperature. Gas formations may undergo plastic deformation during gas drilling, and the plastic zone expands further into the formation due to low temperature further aggravating wellbore stability. The formation of plastic zone depends not only on the horizontal stress but on vertical stress. Generally due to deep burial depth of tight sandstone gas reservoirs, elastoplastic analysis demands consideration of full three principal in-situ stresses.