A precise estimation of the in-situ stress tensor is important for any underground excavation. It is particularly crucial in petroleum engineering when reservoir condition of pressure and temperature are usually high. Leak-off tests (LOT) are regularly performed in new wells where the formation characteristics have not been established yet. A number of technical communications have stated that leak-off test can be used for measuring the minimum in-situ stress magnitude, which is not completely correct due to the operational procedure itself and the inaccurate guidelines adopted for interpreting leak-off tests. The authors first performed critical reviews of the leak-off test and the extended leak-off test, and then studied the three-dimensional stress distribution around the bottom of the borehole. Instead of creating a vertical fracture which can be expected from the solution of an infinite borehole, leak-off test procedures may initiate a horizontal fracture along the bottom of the wellbore before the fracture turns in the direction according to the orientation of maximum principal stress prevailing in the reservoir.
A precise estimation of the in-situ stress tensor is important for any underground excavation. It is particularly crucial in petroleum engineering when reservoir conditions of pressure and temperature are usually high. Various types of tests for in-situ stress measurement in downhole formation can be performed such as strain-relief, overcoring, and hydraulic fracturing. To measure rock stress distribution at depths of several thousands of meters from a surface access, ISRM has suggested hydraulic fracturing (HF) and/or hydraulic testing of pre-existing fractures (HTPF) as methods for rock stress estimation [1]. Hydraulic fracturing is a technique first developed by the petroleum industry in order to stimulate production from reservoirs. Further analysis of its mechanics adopted it for use as a technique of stress determination. Hydraulic fracturing is now widely used for in situ stress determination. However, special test equipment and personnel qualification are essential for its success.
Leak-off tests (LOT) are regularly performed in new wells where the formation characteristics have not been established yet [2]. The principal purposes of such leakoff tests are to determine the strength or fracture pressure of the open formation, usually conducted immediately after drilling below a new casing shoe. A number of technical communications have stated that leak-off tests can be used for measuring the minimum in-situ stress magnitude due to their similarity to small volume hydraulic fracturing method, which is not completely correct due to the operational procedure itself and the inaccurate guidelines adopted for interpreting leak-off tests [3, 4]. Extended leak-off tests (ELOT/XLOT) are improved LOTs by repeating the pressurization cycles as in hydraulic fracturing methodology to overcome the tensile strength and stress concentration around the borehole. However, XLOTs still maintain the same test configurations as LOTs and three-dimensional stress distributions around the bottom of the borehole are not considered.
This paper will initially review LOT and XLOT for stress estimation, and compare them to hydraulic fracturing.
