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Characterisation of Stress and Strength Dependent Fracture Flow Properties in Carbonate Reservoirs

Daniel Moos¹, Colleen Barton¹, and Thomas Finkbeiner²
¹Baker Hughes Inc., Palo Alto, CA, USA
²Baker Hughes, Inc., Dubai, UAE

The vast majority of carbonate reservoirs are naturally fractured at various intensities and scales. These fractures are usually the main pathways for fluid flow of both hydrocarbons and water, and thus their properties control to a large degree both the production profile from producing wells and the well injectivity and sweep efficiency during secondary recovery. Because most fractures are stress-sensitive, their hydraulic conductivities will change with changes in bottom-hole-flowing and reservoir pressures causing variations in production profiles between wells. More specifically, fractures can hydraulically (partially) open or close due to a decrease (caused by injection) or increase (caused by production) in the effective stress. Because flow properties are a function of effective fracture aperture it is possible to predict reservoir behavior using the relationship between the mechanical behavior of natural fractures (in response to in situ stress and pore pressure changes) and their hydraulic properties. One obvious effect is by mode I or tensile opening of the natural fractures perpendicular to their plane which enhances their ability to transmit fluids. Another effect is that of mechanical shear failure whereby slip (i.e., shear failure) along the rough surface of an otherwise nearly closed fracture causes self-propping by natural asperities due to damage and the miss-match of the offset surfaces providing conduits for fluid flow (see the discussion of “critically-stressed fractures” in Barton et al. 1995). In carbonate reservoirs, where fracture opening can be enhanced by dissolution, it may also be that initially open fractures which fail in shear will tend to close, causing a decrease in their conductivity.

The ability of natural fractures to become critically-stressed and to remain hydraulically open due to shear slip is controlled by their intrinsic strength. The stronger the natural fracture, the more difficult it is to slip. Hence, strong natural fractures may remain hydraulically closed (or open where dissolution has created a connected pathway for fluid flow along the fracture) even for high ratios of shear to normal stress. Furthermore, fractures which are initially strong also tend to be initially stiffer than those that are initially weak, and stiffness can increase (by self-propping of initially nearly closed, weak fractures) or decrease (by collapse of initially strong, open fractures) after slip has occurred. Thus fracture stiffness and strength, both prior to and following shear slip, must be determined in order to model production or injection. Since carbonate reservoirs often contain multiple sets of natural fractures, and these may have different origins and thus different orientations and properties, the mechanical behavior and characteristics of each fracture set must be considered separately in order to predict the overall flow characteristics of the reservoir over time (i.e., with changing pressure). For example, in one reservoir critically-stressed fractures may dominate flow behavior while in another, pre-existing, high-aperture and stiff joint sets predominate. The latter case is more likely to result in apparent stress insensitive behavior. The fracture orientation with respect to the present day stress field, the pore pressure, and the mechanical fracture properties together determine which processes are potentially active on an individual fracture plane or set.

 

AAPG Search and Discovery Article #120034©2012 AAPG Hedberg Conference Fundamental Controls on Flow in Carbonates, Saint-Cyr Sur Mer, Provence, France, July 8-13, 2012