Economical development of the vast oil sands reserves of Alberta is contingent upon the ability of operators to design efficient hydraulic fracturing strategies. The prediction of fracture growth in unconsolidated reservoirs however, is difficult as conventional mathematical models fail to incorporate the important behavioural characteristics of uncemented clastic sediments. In order to better understand the hydraulic fracturing phenomenon in uncemented sands, a large scale laboratory simulation has been carried out The experiment demonstrates the potential application of large scale laboratory tests to examine the physical mechanism of hydraulic fracturing. Unlike field tests which are subject to several unknown factors, laboratory data may be used directly to enhance the mathematical models for fracture analysis.
The bitumen contained within the oil sands reservoirs of Alberta is highly viscous and will not flow at formation temperatures. Bitumen production from deeply buried oil sands formations requires mobilizing the bitumen by in situ stimulation techniques. The most common methods include cyclic steam stimulation, steam drive and in situ combustion. For these procedures to be effective, however, it is necessary to overcome the low intrinsic permeability of the formationby hydraulically fracturing.
Unlike consolidated reservoir rocks, the individual sand strains in the oil sands matrix are not constrained by cementation. As injection fluid is forced rapidly into the oil sands matrix, individual sand grains slide relative to one other to accommodate the induced mechanical and/or thermal strain. Structural changes to the oil sands matrix are generally highly non-linear and inelastic. Volumetric changes occur and significantly alter the in situ stress field in the fracture zone. The initiation, propagation and orientation of fractures in unconsolidated sediments such as oil sands is therefore complex, and a better understanding of the inter-related physical process is desireable for the future exploitation of oil sands.
One method to examine the physical processes in more detail is through laboratory testing, and there are test data reported in the literature (e.g. Raisbeck & Currie, 1981). A limitation of experimental work carried out in the laboratory to date is that the specimen size has been small, in the order of a few centimeters. Thus fracture initiation can be examined, but fracture propagation and fluid leakoff are not modelled because of the small specimen size. In addition, the proximity of the sample boundary to the injection point may influence the initiation of the fracture. This paper describes a laboratory fracture test on d sufficiently large specimen that the limitations notes above are avoided. The fracture fluid was injected into the centre of a cylindrical sample. 1.4 m in diameter by 1 m high.
The purpose of the fracture test described in this paper was firstly to demonstrate the feasibility of carrying out tests on this scale. It is noted that the ultimate objective is to examine the details of the physical processes that occur during hydraulic fracturing of dense uncemented sands. This objective should be distinguished clearly from carrying out a small scale simulation of in situ fractures in natural geologic materials.
