Small volume hydraulic fracture tests, herein defined as microfracs, were conducted in order to determine the variation of minimum principal stress with depth at various locations in and adjacent to oil sands formations in Alberta, Canada. Although hydraulic fracturing has become widely accepted as an in situ stress measurement method, no consensus or standard exists for the interpretation of hydraulic fracture pressure data. Consequently, this paper presents a microfrac in situ stress determination method developed in response to the major geomechanical and stratigraphic characteristics of oil sands formations. This method is an adapt ion of well test analysis methods used by petroleum reservoir engineers. Three microfrac case histories were examined revealing that the graphic procedures used in this method give consistent minimum principal stress determinations. The results from this paper should be applicable to most unconsolidated granular formations.
Hydraulic fracturing has been applied in the design of enhanced thermal bitumen recovery schemes for deep (>150 m) oil sands formations in Alberta, Canada. Hydraulic fracture simulators used to investigate the effect of large injection treatments require a reliable estimate of the in situ stresses at these depths. As with other deep hydrocarbon deposits, the hydraulic fracture stress measurement test is currently the only method for determining in situ stress at great depth. Unfortunately the analysis of in situ stress based on hydraulic fracture data is Often complicated by many extraneous factors 1,2,3 which require more involved interpretation methods. This problem is compounded in oil sands tests because of the unique geomechanical and stratigraphic properties of oil sands formations 4,5. Often only the minimum principal stress can be reliably derived from hydraulic fracture data. The objective of this paper is the analysis of three sets of small volume (<3m3) hydraulic fracture stress measurement tests (defined here as microfracs) performed in and adjacent to oil sands formations in Alberta. A secondary objective is the development of an hydraulic fracture test interpretation method based on well test analysis in order to account for the unique behaviour of oil sands. The objectives were achieved by reviewing previous studies on oil sands hydraulic fracturing, by proposing a method for analyzing hydraulic fracture pressure-time data and by analyzing microfrac data for both oil sands formations and adjacent strata.
The oil sands deposits of Alberta exhibit unique geomechanical properties which vary from the classic isotropic, homogeneous linear elastic materials assumed in most hydraulic fracture theories. In general, oil sands consists of a dense, interpenetrative fabric of quartz grains which are water-wet and pore space occupied by viscous bitumen (3.0xl06 mPa. a), water and dissol ved gas. No cementation exists and thus oil sands are cohesionless. The bitumen is immobile under in situ conditions (T=10–15 C) but its viscosity is highly temperature dependant. Formation stratigraphy varies widely and ranges from thick sequences of rich oil sands to interbedded oil sands and shale. Isolated gas zones, thin water lenses and cross bedding have also been observed.