Development of experimental apparatus for real-time observation of hydraulic fracture in unconsolidated sands by x-ray CT method

In order to clarify the characteristics of hydraulic fracture in unconsolidated sands, we have been developing laboratory experimental apparatus to observe the hydraulic fracture behavior by the X-ray CT method. The apparatus should be designed satisfying conditions such as (i) specimens have a size to be sufficiently large for the fracture growth, i.e. more than few hundreds mm, (ii) the inside of such a large specimen is visualized within a time interval to be sufficiently short compared with a time period of few hundreds seconds required for fracture initiation and growth, and (iii) the specimen is subjected to tri-axial compression for simulating subsurface conditions. Considering those requirements, we developed several types of experimental apparatus, and finally succeeded to develop an apparatus which allows us to obtain within an interval of few seconds a CT image of a longitudinal slice of the fracture along the growing direction. The results clearly show characteristic behaviors of hydraulic fracture. For example, the fracturing fluid reached the fracture tip and invaded farther into the rock matrix; therefore, a non-wetted zone did not develop at the fracture tip, in contrast to that observed in hard rocks.