原生裂隙水压致裂原地应力测量的理论与实践新进展

A new theory and application progress of the modified hydraulic test on pre-existing fracture to determine in-situ stresses

HTPF法测试所需的数目众多的原生裂隙面大大制约了这种方法的推广应用。针对这一问题,通过为原生裂隙面上广泛存在的剪应力建立力学方程,利用最小二乘法和计算机程序试错搜索原生裂隙面摩擦系数的办法反演原地应力张量。理论上基于每个原生裂隙面的水压致裂测试结果可以建立两个力学方程,那么只需要3条原生裂隙就可以求解原地应力张量,但为了保证计算机程序反演收敛,至少需要5条原生裂隙,这一方法被定名为M-HTPF法。将这种方法在山东某科研钻孔的原地应力测量作业中进行了应用,通过利用5条原生裂隙面上的水压致裂测试得到的关闭压力和方位角数据,反演得到原地应力张量:σ_1=8.85 MPa,方位角为N58.12°W∠14.18°;σ_2=6.61 MPa,方位角为N26.2°E∠-21.54°;σ_3=5.01 MPa,方位角为N62.86°E∠63.86°。通过与同一钻孔内的经典水压致裂法的测量结果对比可知,两种方法得到的最小主应力和中间主应力非常接近,最大主应力则相差较大;两种方法获得的最大、最小主应力方位角基本一致。该方法为单孔三维水压致裂原地应力测量提供了新的思路和途径。

Since many pre-existing fractures（i.e. more than 15） are required by the hydraulic testing of pre-existing fractures（HTPF） method, the application of HTPF method to various stress measurement conditions is restricted. To overcome the shortcomings, a mechanical equation is developed to describe the shearing stresses intrinsically occurring on geological discontinuity planes. The least square method and the trial searching algorithm code are used to calculate the frictional coefficient of pre-existing fractures and further to calculate in-situ stress tensors by the inversion technique. Theoretically, the hydraulic test on each pre-existing fracture is utilized to establish two mechanical equations, and three tests on pre-existing fractures is used to determine the in-situ stress tensor. Practically, to guarantee the convergence of the inversion code, at least five pre-existing fractures are required. Thus, the developed method is called the modified-HTPF method, which is further applied to measure in-situ stress in Weifang area, Shandong Province. During the process of stress measurement, the complete in-situ stress tensor is determined by the shut-in pressures from the hydraulic fracturing test on pre-existing fractures and the azimuth and dip angle data from Televiewer logging of five geological fractures. The in-situ stress tensors are characterized as σ1 = 8.85 MPa, N58.12°W∠14.18°; σ2= 6.61 MPa, N26.2°E∠ - 21.54°; and σ3= 5.01 MPa, N62.86°E∠63.86°. Compared with the data by the classic hydraulic fracturing（HF） method, the medium and minimum principal stresses determined by these two methods are similar, but there exists a large difference between the maximum principal stresses. The orientations of the maximum and minimum principal stresses determined by the modified-HTPF method are in good agreement with the classic HF method. The modified-HTPF method offers a new access to determine a complete stress tensor using a single borehole.