Abstract
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.
1. INTRODUCTION
In recent petroleum engineering, hydraulic fracturing techniques are applied not only to brittle formations but also to unconsolidated one for enhancement of production. The unconsolidated sands have properties of rather higher permeability more than 10 mD and fairly weaker bond between particles compared with hard rocks. For those reasons, fracturing behavior in unconsolidated sands is not suitably represented by existing models for brittle, linear-elastic rocks. Thus we have been trying to clarify the fundamental mechanism of hydraulic fracturing in unconsolidated formations by laboratory experiments [1-4] and numerical simulations based on the Discrete Element Method [5, 6]. As one of those studies, we have been developing laboratory experimental apparatus to observe the hydraulic fracture behavior by the X-ray CT method. In this study, big improvements have been made recently, and it allows to have CT images of a longitudinal slice of hydraulically induced fracture along the growing direction within an interval of few seconds. This paper will summarize new findings obtained by using the apparatus.
