基于多孔介质热弹性理论的井壁诱导缝成因

Genesis of induced fractures on borehole walls based on the thermo–poroelasticity theory

常规纯弹性理论通常把钻井过程中直井井壁诱导缝的形成归结为钻井液密度过高或者水平主应力差值过大,然而该理论却并不能完全合理地解释深井井壁诱导缝的成因。为揭示井壁诱导拉伸缝形成机理,考虑钻井液循环期间低温钻井液与高温井壁围岩的热交换,基于多孔介质热弹性力学理论,建立了适于深部地层的井壁稳定分析模型,研究了流固热耦合作用下的井周周向有效应力、孔隙压力与温度分布计算模型。计算结果表明:(1)在最大水平地应力方位,井眼钻开初期多孔弹性作用居于主导,孔隙压力在近井壁地带出现谷值,对诱导缝的形成起到了抑制作用;(2)然而随着钻井液与地层热交换的进行,钻井液的冷却作用愈加重要,井周周向有效应力由挤压状态逐渐转变为拉伸状态,从而导致近井壁地带诱导缝的形成。结论认为,为避免诱导缝的形成,在工程上一方面需加强钻井液的封堵性,控制液柱压力向井周的扩散;另一方面也可以通过调整优化钻井液密度与流变性能,控制井底钻井液当量密度(ECD)。

According to the conventional pure elasticity theory, induced fractures on the wall of a vertical well in the process of drilling are generated due to the fact that the density of drilling fluid is too high or the horizontal principle stress difference is too large. However,this theory cannot provide a reasonable explanation for the induced fractures on the wall of a deep well. For revealing the formation mechanisms of induced tension fractures on borehole walls, the heat exchange between low-temperature drilling fluid and high-temperature rocks around the borehole wall in the process of drilling fluid circulation was analyzed. Then, a wellbore stability analysis model suitable for deep layers was built according to the thermo–poroelasticity theory. Finally, a calculation model for circumferential effective stress, pore pressure and temperature distribution under the effect of fluid–solid thermal coupling was analyzed. It is shown that at the beginning of drilling, poroelasticity is dominant in the direction of the maximum horizontal principle stress and valley pore pressure appears near the borehole wall, inhibiting the formation of induced fractures. Besides, along with the heat exchange between drilling fluid and strata, however, the cooling effect of drilling fluid becomes dominant and the circumferential effective stress transforms into a tensile state from a compression state, leading to the formation of induced fractures near the borehole wall. It is concluded that two measures can be taken to avoid the formation of induced fractures from the aspect of engineering. On the one hand, it is necessary to strengthen the plugging capacity of drilling fluid to prevent fluid column pressure from propagating around the borehole. On the other hand, it is necessary to control the equivalent circulating density （ECD） of drilling fluids at the bottom hole by modifying and optimizing its density and rheological property.