Abstract
Shale gas has become an increasingly significant source of natural gas in the United States. It is puzzling to many that most of the shale gas wells decline very rapidly. Our study shows that in hydraulic fracturing of shale-gas wells operators often use "slick water". The "slick-water", as carrier fluid, comprises plain water and some combination of polymer gel, gel breakers, cross-linkers, and surfactant or surfactant/water alone. However, in lessons learned from the oil field personnel and our own years of research and field practice, we see that first Polymers in general upon hydration retain a water layer for a long time, second Shale absorbs or allows this water layer in the shale body under suction (negative) pressure, then, it converts the suction pressure to swelling pressure reaches which closes the micro fractures, and third, to complicate the matters, some surfactants simply disperse the shale-clays. The net effect of these mechanisms is rapid production decline.
One of the most important mechanisms is water absorption. Shale absorbs water due to the capillary effects of micro fractures and the shale-clay potential. However, there is no model to show the relationship among capillary effect, the shale-clay type potential, the morphology of the shale aggregate, and finally the production decline. We propose a model which connects the above mentioned four elements together and shows the way to the root cause of rapid production decline.
Based on the results we conclude: first the proposed model shows a good correlation between the above four elements in terms of shale fractal dimension and the shale hydration index, all used for calculating the excess hydration stress, and second the excess hydration stress accelerates micro fracture closure, hence the rapid gas production decline.
The benefits of our work follows: one is the combined HHI and the shale fractal dimension can be used to optimize the shale hydraulic fracture conductivity toward long term gas production and the other is the correlation offered here shows it to be a better method for measuring the shale-water activity by using the shale characteristics, namely, the shale water content, the Hydration Index, and the shale fractal dimensions.
