Fracture Mechanics Interpretation of Leak-Off Tests

Abstract

Abstract This paper presents a model for leak-off interpretation that includes evaluation after the borehole is fractured. The Kirsch equation is valid up to the conventional leak-off point, but beyond leak-off the hole is fractured, and an elastoplastic bridge model applies to ultimate failure. Also presented is an evaluation of in-situ stress interpretations from leak-off tests. Findings of the new model are that the leak-off point using Kirsch is correctly defined by the in-situ stress state and the rock tensile strength. The model also explains the circulation loss problem with oil based drilling fluids where the wellbore strength reduces to the minimum horizontal stress level. Introduction The models presented in this paper applies to a typical drilling operation, using particle laden drilling muds with filtrate control, performing leak-off tests with a large annular volume and relative small fractures opening during the tests. The model is not applicable for massive hydraulic fracturing with a clean penetrating fluid, as performed during well stimulation operations. This is an important definition as many publications either confuse or use a model for penetrating clean fluid instead of a non-penetrating model for drilling mud. Performing numerous fracturing experiments on hollow concrete cores Aadnoy and Belayneh(2004) found that for most drilling fluids the Kirsch equation underpredicted the ultimate fracture pressure significantly. Figure 1 shows three fracture tests. Mud B and C are common commercial waterbased drilling fluids. For these the ultimate fracture pressure is 2–3 times higher than predicted by the Kirsch equation. Mud A is a special mud with less particles but a particular chemistry. It is in accordance with Kirsch.