Fracture Mechanics Interpretation of Leak-Off Tests

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.

Review of the Leak-off Tests with a Focus on Automation and Digitalization

“…Furthermore, the role of drilling mud type in LOT should not be neglected because different mud types have different effects on the stable fracture growth and fracture tip. In case of using Water Based Mud (WBM), the fracture tip can be isolated effectively by buildup of external filter cakes, while by using the Oil Based Mud (OBM) or Synthetic Based Mud (SBM), very low filtration loss will be caused due to the wettability contrast between the rock and the mud [10].…”

“…Estimating reservoir parameters and reservoir characterization are vital in all stages of reservoir evaluation, from exploration to exploitation, which is done by different methods and tools and for other purposes [1][2][3][4][5][6][7][8][9]. One of the most common jobs done during drilling operations for decades is the Leak-off Test (LOT) [10][11][12]. Different types of downhole formation tests could be conducted to characterize the newly drilled wellbores, especially the geomechanical properties and the state of in-situ stresses.…”

“…LOTs are widely used to interpret the minimum stress magnitude, stress determination [15][16][17][18], and in the geomechanical studies [19,20]. Aadnoy et al [10] developed one model for interpreting LOT, including evaluating after borehole fracturing. They concluded that Leak-off Pressure (LOP) correctly displayed the in-situ stress state and the tensile rock strength.…”