ABSTRACT:
Closure of hydraulic fractures yields the in-situ stresses, net pressure and fracture geometry. A series of laboratory model experiments using sand samples is conducted with the aid of X-ray CT technique. The observation and analysis of CT images provide the visualized closure characteristics of hydraulic fractures in sand. Furthermore, the image-determined closure pressure is compared with the pressure inferred by the conventional tools of pressure diagnostics.
1 INTRODUCTION
Observation of the closure of hydraulic fractures yields the in-situ stresses, net pressure and fracture width and length. Closure pressure is defined as the fluid pressure at which an existing fracture globally closes. Usually, in practice, the closure pressure is determined by the analysis of the pressure decline curve because direct measurement of fracture width is impossible and the pressure record provides the only way to infer fracture dimensions. Pressure diagnostics is based on the combination of mass balance and linear elastic theory and thus applicable to competent rocks. It however may not provide a reliable reference for soft rock due to the significant plastic deformation as Dam et al. (1998, 2000) indicated that the soft and low permeable rock (soft plaster in his experiments) closed at a pressure lower than the minimum stress in the formation. For a reliable pressure interpretation, therefore, we need to investigate how the closure deviates from the values inferred by the conventional tools of pressure diagnostics. A series of laboratory model experiments has been conducted using sand samples, which are chosen as extreme cases of the soft rocks with highly plastic, permeable properties and without cohesion. In the laboratory, the technique of X-ray CT scanning provides a non-destructive real-time measurement of fracture initiation, propagation and closure during injection experiments. CT images show the visualized fracture geometry and hence provide directly the closure behavior.
