Improving Shale Gas Production Using Geomechanics
Figure 3: Effect of Stress State and Fracture Strength on Stimulation Effectiveness
Effect of stress state on simulation pressure
S3
Narrow zone
Sv>SHmax >Shmin
Effect of fracture strength on simulation pressure
S3 Poor efficiency ‘Strong’
Slick-water stimulation is therefore likely to create a broad zone of well-connected fractures in reservoirs that have low, nearly equal horizontal stresses and weak fractures with a wide range of orientations. If fractures are strong, it will be more difficult (require higher pressure) to stimulate these pre-existing fractures than to create hydraulic fractures. If fractures have only a narrow range of orientations, shear stimulation will occur only on an appropriately orientated subset of pre-existing fractures and some benefit will be achieved, but narrower stimulated zones should be expected. Where details of the stress state and fracture distribution have been made public, these tendencies are apparent. In the Barnett, stimulation usually produces a wide zone of seismicity. Where
Broad zone
Sv >SHmax ~Shmin Poor efficiency ‘Weak’
stimulated zones appear to be narrow, SHmax could be slightly higher, as has been proposed elsewhere, or the fracture distribution may change
Critical pressure increases Better efficiency
Stimulation pressure relative to S3, the minimum pressure required to induce hydrofracture growth, in a stress state in which both horizontal stresses are very low (lower left) versus one in which SHmax is significantly larger than Shmin (upper left). On the right is the effect of fracture strength on the number of fractures that could be stimulated before exceeding S3 for the stress state shown in the upper left panel.
Figure 4: Response to Stimulation of this Initially Low-permeability Reservoir
such that only the subset of fractures striking parallel to SHmax are present. A widely studied series of stimulations in the Cotton Valley formation, which has a maximum horizontal stress that is nearly equal to the vertical stress, produced very narrow zones of seismicity trending in the direction in which hydrofracture growth was expected. This is consistent either with a pre-existing fracture set that predominantly
trends in the SHmax direction or with a situation in which the fractures are sufficiently strong and the stress difference is large enough that the pressure in the rock surrounding the fracture was insufficient to cause either failure or opening.
Response to depletion
Shmin Injection pressure
Stimulation of this initially low-permeability reservoir causes a slow injectivity increase until the pressure at which fractures begin to slip, leading to a rapid increase. Injectivity decreases after shear stimulation as the pressure drops to the original reservoir pressure, but more slowly (red curve), leaving behind a permanent injectivity increase.
this mechanism, an appropriate fluid and pumping rate (or pressure) should be used that enables the fluid to easily enter the fractures in their natural state. If fracture stimulation requires pressures above
Shmin, then fractures should be designed to achieve sufficiently high net pressures, although access to the existing fracture network would still require the use of an easily penetrating fluid. This may be the reason for the success of techniques used to increase the width of stimulated zones by stopping primary fracture growth to allow increases in pumping pressure to divert fluid into the surrounding fracture system.
Large numbers of weak fractures can be stimulated (and would have a wide range of orientations enabling formation of a well-connected network) in a low-stress, normal faulting regime such as characterises many parts of the Barnett, as shown on the lower left of Figure 3. On the other hand, if fractures are strong, very few fractures can be stimulated at low pressure, and shear fracturing at modest pressure is likely to be a very inefficient means of enhancing reservoir permeability. If fractures are weak but all three stresses are different, as shown in the upper left of Figure 3, only a subset of fractures will be stimulated at pressures below the pressure required to extend a hydrofracture. These will be similarly orientated, making creation of a wide connected network more difficult.
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In the Marcellus, microseismic activity sometimes extends considerable distances along the expected trend of the primary hydrofracture (this trend in general is sub-parallel to one of the two perpendicular vertical joint sets that are present) before the net pressure is large enough to cause the orthogonal set to shear and open. This behaviour may be characteristic of regions in which the secondary joints are impermeable prior to stimulation. Where these joints have finite conductivity in their natural state, pressure could diffuse through them without inducing shear slip, leading to pockets of microseismic activity at considerable distances away from the stimulated well that have no apparent connection to the main zone.
Simply stimulating a broad well-connected zone that allows access to the gas contained in the extremely low-permeability matrix is not enough to guarantee a productive, long-lived shale gas well. If fractures close up immediately after stimulation or if a small drop in pressure causes a large reduction in conductivity, the reservoir will be poorly stimulated and production will decline rapidly. These are not the only reasons for rapid decline, of course, but they are a virtual guarantee that it will occur. Even if natural fractures are weak, a number of conditions are required in order to achieve sufficient, and sufficiently sustained, stimulation. Rough fractures will open more and will produce more rubble when slip occurs, and thus will be more likely to remain open as the reservoir is depleted. A denser pre-existing fracture network will make access to the matrix more effective. If the shale matrix is more ductile (less brittle), which could be caused by the presence of significant amounts of kerogen, clay or other ductile minerals in the rock frame, self-propping will be ineffective and pressure decline will cause rapid closure. Thus, the appropriate matrix and fracture mechanical properties are necessary to benefit from the presence of pre-existing fractures. These properties can be determined at the well using extensive logging suites including magnetic resonance, geochemical and cross-dipole acoustic logs. Service companies are building libraries of shale play
EXPLORATION & PRODUCTION – VOLUME 9 ISSUE 2
Injectivity
Number of stimulated fractures
Number of stimulated fractures
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