Difficulties and Possible Solutions for Deep Stress Measurements

摘要

The methods of stress determination can be classified mainly into two categories in which (a) thernstress state is measured directly by in-situ borehole tests and (b) it is constrained by stress-relatedrnphenomena observed in boreholes. The in-situ test of hydraulic fracturing (HF) provides the only wayrnto observe stress magnitudes directly, and it is suitable for stress measurements at deep depths.rnAssuming a vertical borehole, the maximum and minimum horizontal stresses, S_(Hmax) and S_(hmin), arerndetermined from the critical borehole pressures of the reopening pressure P_r and the shut-in pressurernP_s observed during the test. P_s is used to determine S_(hmin) and P_r is used to determine S_(Hmax). Pr isrndefined as the borehole pressure at the moment of fracture opening. However, a discrepancyrninevitably occurs between actual and measured values of P_r due to a problem associated with the wayrnof measurement, and sometimes it becomes very significant. For effective measurement of P_r, it isrnnecessary for the fracturing system to have a sufficiently small compliance C (Ito et al., 1999; 2005;rn2006). Deep boreholes generally have a large diameter, and accordingly the test system becomes largernas well. The large size of the test system generally leads to an increase in C. In consideration of theserncircumstances, we are developing a new tool which allows us to do the HF tests in the condition ofrnsufficiently small C regardless of borehole size and test depths. It is assumed here that a drill stringrncontaining a wireline retrievable core barrel assembly has been set in the borehole in advance. Therntool is conveyed on a wireline through the drill string, while the outer barrel remains at the bottom ofrnthe borehole. This way contributes greatly to avoid the risk of trouble occurring in boreholes such asrnthe tool getting stuck. Then the HF test is carried out in a short hole additionally-drilled at the bottomrnof the original borehole, which is referred to the baby hole. The core retrieved from the baby hole canrnbe applied to the Diametrical Core Deformation Analysis, DCDA (Funato and Chen, 2005; Funato etrnal. 2012), and we can have additional information on stress state independently of HF. By combiningrnthose data obtained from HF and DCDA, we can validate the estimated magnitude and azimuth ofrnS_(Hmax).rnThe in-situ testing method is certain way to determine the in-situ stresses. However, it requiresrncomplicated instrumentation and tests at the depth of measurement, and the instrument cannotrnfunction under a high temperature environment such as geothermal fields, since it consists of tworninflatable packer elements that are basically made of rubber. Thus we are proposing a new method tornconstrain the stress state based on the hypothesis that fractures which are critically-stressed forrnfrictional sliding in the current stress field tend to permeable and those which are notrncritically-stressed tend to be impermeable. Fracture depth, dip and strike can be detected fromrnborehole imaging logs such as BHTV/UBI or FMI/FMS. By comparing the depths of fractures andrnthose of thermal anomalies detected on temperature logs, we can identify which of the fractures arernpermeable. Stresses on fracture planes can be estimated from the fracture orientation provided that thernstate of in-situ stress is assumed. Therefore, knowledge of which fracture orientations tend to bernpermeable and hence support critical stress states, and which fracture orientations are not allows us tornmake a grid search over possible states of stress to identify the stress state that best satisfies thernobservations and hypothesis.