Improving Shale Gas Production Using Geomechanics
physico-chemical properties that can provide initial guidance as to which stimulation technique is likely to be appropriate in which shale play, and in which lithological intervals or locations.
A better approach to predict whether stimulation will be effective is to measure the key parameters that control pre- and post-stimulation flow properties. These parameters include the pressure sensitivity of fractures prior to stimulation, the fracture strength, the increase in conductivity as a function of pressure once fractures begin to be stimulated and the decrease in conductivity (hysteresis) when pressure drops back to and below its pre-stimulation value. While the fracture properties that control these parameters can be measured in the laboratory, these properties can also be constrained in situ in a number of ways. One way to characterise fracture properties in situ is by carrying out the appropriate pre-stimulation injection test. Figure 4 shows diagrammatically how injectivity is expected to evolve during such a test. A slow increase with increasing pressure will occur until fractures begin to fail. Above that, pressure injectivity will increase rapidly as increasingly greater numbers of fractures are stimulated. When the pressure drops, injectivity should decrease more slowly, leaving behind a permanent increase.
The pressure sensitivity prior to stimulation controls the slope of the initial line. The ‘knee’ in pressure after which injectivity increases more rapidly with increasing pressure is controlled by fracture strength. This knee will be accompanied by the sudden onset of microseismicity, which could, if detected, provide additional constraints on fracture strength. The slope of the line as the pressure increases above the
knee is controlled by the increase in fracture conductivity that occurs due to stimulation and by the number of additional fractures that are stimulated at each increasing pressure. The slope of the upper red curve is controlled by the pressure sensitivity after stimulation; the difference between pre- and post-stimulation response at original pressure reveals the improvement due to stimulation. For example, it is clear in this case that a significant increase in reservoir performance can be achieved even without creating a hydrofracture. The post- stimulation response can also be extrapolated to pressures below the original reservoir pressure. This makes it possible to predict the reservoir’s response to depletion, which could lead to improved predictions of production decline.
Summary
Although drilling and stimulation techniques that have been developed in the Barnett Shale are often used with considerable success in many of the evolving shale gas plays in North America, modifications have been necessary in nearly all cases to account for differences in their characteristics. By focusing only on the consequences of variations in natural fracture characteristics and the in situ stress state, it is possible not only to understand why these shale gas plays respond differently to stimulation, but also to develop recommendations for operational changes based on that understanding. Once the key parameters controlling stimulation response are quantified, which can be achieved using a fairly simple pre-stimulation test, more quantitative recommendations can be made. This may also make it possible to predict how wells will perform once they are brought into production. n
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