ABSTRACT:

Observations of the tip of hydraulic fractures in sand have indicated sometimes a blunt tip. That would have important consequences for modeling fracture propagation. CT imaging shows that the shape of the fracture tip in soft, permeable samples depends on the leak-off of the fluid. Using cross-liked gel as injection fluid, the fracture tip closes smoothly. Within the resolution of the CT-image (of 0.5 mm) we found no evidence of blunting of the fracture tip. For particle-laden fluids with high leak-off we indeed found a blunt tip due to dehydration of the slurry and plugging of the tip by particles.

1. INTRODUCTION

Observations of the tip of hydraulic fractures in sand have indicated sometimes a blunt tip [1]. That would have important consequences for modeling fracture propagation [2, 3, 4, 5]. In competent rock, the stress near the fracture tip is governed by the process zone over which the tensile bearing capacity of the rock decreases from the tensile strength to zero. That implies that the fracture closes smoothly. However, the failure mechanism in unconsolidated rock may be different because shear failure might be dominant and the fracture tip may be blunt. The shape of the fracture tip is coupled to the fluid transport in the fracture, which depends strongly on the width profile. According to lubrication theory, for laminar flow inside a hydraulic fracture, the pressure drop along the fracture length is inversely proportional to the cube of the width. Hence, the change of fracture pressure in the vicinity of the fracture tip depends on how the width converges to the tip point. If there is a smooth transition for a narrow tip, the pressure varies smoothly as well, although the pressure drop from the wellbore to the tip will occur mostly near the tip. The leak-off of cross-linked gel is always beyond the fracture tip in the case of fracturing sand, a process which is observed during the injection tests. This then results in a smooth change of effective stress along the fracture length and also a smooth change beyond the tip. If the geometry at the tip is blunt, i.e., if the width changes sharply over a very short distance, there is a large effective stress concentration. For the numerical model it will be important to ensure that the geometry of the tip is correctly described. We show that the shape of the fracture tip in soft, permeable samples depends on the leak-off of the fluid and the particle concentration in the fluid. Unconsolidated sand is chosen as an extreme case of soft rocks, and the technique of X-ray CT scanning is aiding to provide the real-time measurement of the fracture geometry during the experiments.

2. CT SCANNING OF FRACTURING EXPERIMENTS

The pressure cell is made of high strength aluminum to hold a confining pressure of 20 MPa, and has appropriate dimensions to fit the CT scanning machine. The cell, which was obtained from Schlumberger Cambridge Research, is shown in Figure 1 [6].

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