An analytical method based on the finite element variation principle and fracture mechanics is proposed to assess the in-situ fracture of oil sands formations. The concept on which the method is based involves the optimization of: (1) the wellbore geometry with "fracture starters", and (2) the stress fields ahead of the crack fronts produced by the optimized fracture driving forces originating from the fracture starters. The primary function of the fracture starters is to control the direction of the initial crack propagation, while the driving forces will create desirable stress fields ahead of the propagating crack, and thus control the subsequent propagation paths.
While there is no explicit fracture criteria available for the in-situ oil sands at the present time, two models for determining oil sands fracture are proposed. Numerical results from a case study will provide engineers with a comprehensive description of the fracture pattern in the oil sands formation with given geometries of the wellbore and the fracture starter, as well as the loading history. It will be demonstrated that extensive horizontal fracture is possible with particular combinations of initial geometry and loading conditions.
The enormous amounts of deep oil sands reserves make a suitable in-situ recovery technique extremely attractive. There are however, two major problems associated with this method [1,2]: These are: (1) the lack of communication between wells, due to low effective permeability of the deposit; (2) the high viscosity of the in-place bitumen.
The methods which are currently used to overcome the first problem include [3 – 5]:
inducing hydraulic fracture by hot steam or cold gelled water;
using natural communication channels, e.g., a high permeability water zone;
using existing solid permeability, if it exists.
In some areas where a zone having a high water saturation underlies the oil sands formation, it is possible to heat the oil zone by heat conduction from the steam flooded water zone without fracturing the formation. This approach, however, requires a thin water zone underlying a relatively thick oil zone. Such a condition exists only in a few areas among the vast oil sands deposits in Canada.
For the majority of Canadian oil sands in-situ projects which have been carried out or have been planned for, a type of hydraulic fracture process has been included as a necessary first step for inter-well communication which facilitates, mainly through heat conduction, a more efficient mobilization and displacement of the oil. However, in most cases it induces a limited fracture system through which fluids could flow. Consequently, in some cases, sweep efficiency tended to be very low [2,6]. Many in-situ processes such as fire flooding, in-situ combustion, in-situ hydrogenation, and the use of an aqueous emulsifying fluid have been considered as potential recovery tools of viscous fluids. However, such processes necessitate a prior extensive fracture of the sands in order to provide the necessary distribution of the active agent, a condition which is not provided by the classical hydrofracture processes.