Abstract
It has been almost 70 years since the first hydraulic fracturing job was carried out. Since then, hydraulic fracturing has made it possible to produce oil and clean burning natural gas from shale where conventional technologies are ineffective. However, in fracking the shale, the mechanism of increased gas production after the prolonged well shut-in has yet to be understood. The objective of this paper is to address this issue.
To shed light on this issue we have studied and experimented with Pierre shale in detail. In our experimental work we suspended approximately 15 grams of shale cubic samples in deionized water. We have measured the changes in pH, Eh, (Redox potential), and Temperature and simultaneously have recorded the process of gas bubble flow under microscope, using a video system. We plotted our 1024 data points taken every three seconds. We used the Fourier Transform of the data to construct the Power Spectrum for extracting the hidden information in the data in relation to the release of the first bubble of gas from the shale mass.
The results of time series analysis in frequency domain reveals the following information: (1) depending on the type of shale, it takes a certain amount of time for water molecules to saturate/activate the shale capillaries. This is analogous to the prolonged post frac well shut-in before flow-back commences. From there on the process follows the Fick's law of diffusive transport and (2) the pH, Temperature, and Eh show the release of the first gas bubble to be the result of diffusion of water in shale capillaries where it takes a certain amount of driving energy by water molecules to displace the sorbed gas from the shale macro, meso, and micro capillaries. Interestingly, the gas production mechanism follows the predictable Activation Energy accurately as described by Arrhenius Activation Energy equation followed by rigorous application of Diffusion equation. Therefore, from both the results of theoretical and experimental work we may conclude that (a) the diffusion time, diffusion constant, and the flux rate of water molecules into shale capillaries can be predicted for determining the optimal well shut-in time in post-frac period and monitoring the advance of displacement front of the advancing frac fluid into the walls of the hydraulic fracture toward achieving higher gas production; we call this process Shale Pickling, or Capillary Saturation/Activation and (b) the desorbed gas production is a cyclic flow.
The practical application of our paper is (1) we propose a simple and cost effective methodology for determining the post frac optimal shut-in time and (2) once this optimal shut-in time is implemented the industry may benefit from realizing higher gas production from their prospects.
