压力衰竭地层地应力变化及影响因素

压力衰竭地层易发生井漏、井塌等井下复杂，其主要原因是油气压力下降后岩石特性参数以及地应力发生改变。如果沿用压力未衰竭时的安全钻井液密度窗口将会导致钻井液密度选用不合理，增大井下复杂情况的发生几率。为解决上述问题，考虑岩石特性参数变化对地应力的影响，假设地层岩石为横观各向同性弹性材料，建立了新的地应力计算模型，以DF气田的DF111调整井为例，分析地应力影响因素。结果表明:岩石弹性模量增加会导致水平地应力差增大，且其对最小水平主地应力的影响随井深增加而变大；泊松比的变化不会改变水平地应力差；DF111调整井储层压力由1.03 g/cm3衰竭到0.46 g/cm3，最小水平主地应力下降20.5%，最大水平主地应力下降16.1%，地层发生井漏的主要原因是地应力降低导致岩石有效剪应力增大。

Study on the variation of in-situ stress and influencing factors in pressure depleted formations

It is easy to occur the complex situation in the formation of pressure depletion, such as the leakage of the well, the collapse of the well, etc. The main reason is that the stress and rock characteristic parameter will both be changed. So if the initial drilling safety mud density window is used, the choice of the density of the mud will be not reasonable, the probability of borehole problems will increase. In order to solve the above problems, on the basis of considering the influence of the change of rock characteristic parameters on the ground stress, a new calculation model of ground stress is established by assuming the formation rock is the transversally isotropic elastic material. Taking the DF111 adjusting well of DF gas field as an example, the influencing factors of ground stress are analyzed.The results show that the increase of the elastic modulus of rock will lead to the increase of the horizontal ground stress difference, and its influence on the minimum horizontal main ground stress will increase with the increase of the well depth. The change of Poisson’s ratio will not result in the change of horizontal stress difference. The reservoir pressure of DF111 adjusted well collapsed from 1.03 g/cm3 to 0.46 g/cm3, and the minimum horizontal main stress decreased by 20.5% and the maximum horizontal main stress decreased by 16.1%. The main reason for formation leakage is that the reduction of ground stress leads to the increase of effective shear stress of rock. It provides theoretical guidance for drilling in pressure-depleted formations.