水圧破砕地殻応力計測のためのシャットイン後の横き裂閉口挙動の解析

抄録

Closure process of a transverse crack crossing a wellbore is analyzed to discuss physical meanings of the pressure decay curve after shut-in during hydraulic fracturing tectonic stress measurements, where the crack is assumed to be perpendicular to the wellbore axis and one of the principal axes of the tectonic stress is assumed to be parallel to the wellbore axis. The analysis is based on the linear theory of elasticity and linear fracture mechanics. It is shown that the whole crack closes instantaneously when the downhole water pressure is equal to the compressive tectonic stress acting perpendicularly to the crack plane. The instantaneous crack closure appears as a point of maximum curvature on the pressure decay curve after shut-in. Therefore, the compressive tectonic stress normal to the crack plane is determined as the pressure at the point of maximum curvature. Although the point of maximum curvature appears less clearly on pressure decay curve when the fluid loss coefficient is large, the instantaneous crack closure can always be detected clearly by utilizing the relation of the inverse of the decrease rate of the downhill water pressure Vs the downhill pressure.