SPE Members

Abstract

An experimental study has been carried out to investigate the influence of the cementing chronology. That is to say, cementing of a perforated casing in a stress-free borehole or a loaded borehole resulted in a difference in near wellbore fracture geometry.

Starter fractures, propagating into multiple fractures, were observed when the casing was set and cemented in a stress-free borehole. Cementing the casing in a stressed borehole resulted in a continuous fracture along the wellbore, which sometimes ignored the perforations. Therefore, hydraulic fracturing experiments with the aim to represent field situations should be carried out when the casing is cemented in a stressed borehole.

Introduction

In physical model tests of hydraulic fracture initiation at cased and perforated boreholes the stress situation will play an important role. For the sake of experimental play an important role. For the sake of experimental convenience model tests are often done while the casing is already cemented in the wellbore before loading of the model block. However, cementing in field situations is done in a stressed borehole. In a model test the stress distribution present under field conditions cannot develop because the borehole is unable to converge.

The geometry of the fracture near the wellbore is of the utmost importance for communication of the wellbore with the far field fracture. Several previous workers have performed laboratory experiments to investigate the fracture geometry near the wellbore. They observed that fractures induced in deviated wellbores initiated in a plane determined by the orientation of the wellbore. plane determined by the orientation of the wellbore. Further away from the wellbore the fracture reorientates to the plane perpendicular to the least principal stress; the preferred fracture plane. Similar experiments showed preferred fracture plane. Similar experiments showed the presence of starter fractures at each perforation, resulting in a discontinuous fracture. In all these experiments the casing was set and cemented under stress-free conditions. Experiments with a casing cemented in a stressed borehole and perforations shot afterwards showed primary fractures which initiated from the wellbore by ignoring the perforations. This observation was made for perforations which lie more than 10deg. out of the preferred fracture plane. The presence of starter fractures was explained by Yew et presence of starter fractures was explained by Yew et al. by using elastic stress analysis, which ignored the casing and the perforations.

Although the circumstances during the experiments described by Hallam et al. (1990) and Behrmann et al.(1991) were different, some of the differences in their results showed that cementing chronology is of influence on the fracture geometry. We performed several hydraulic fracturing experiments in which only the cementing chronology was changed. The model blocks were loaded in a true tri-axial machine.

P. 259

This content is only available via PDF.
You can access this article if you purchase or spend a download.