Abstract

The first hydraulic fracturing was implemented in conventional reservoirs. However, this technology was not applied to Unconventional resources such as Shale reservoirs until recently. As a result of its application, along with the advances in design of the environmentally acceptable fracturing fluid, we are witnessing a remarkable increase in production of oil and gas in US. In this work, we show our experimental and theoretical work related to heating the frac fluid by a few degrees for increasing gas production even more.

To address the issue of the increased gas production from shale through shortening the post-frac shut-in time, we have studied and experimented with Pierre shale in detail. In our experimental work we suspended few grams of the shale samples in heated, deionized water and measured the changes in pH, Eh, (Redox potential), and Temperature. Using a video recording system, we simultaneously recorded the process of gas bubble flow under the microscope. We plotted our 210 data points. We used the Fourier Transform of the raw data to construct a Power Spectrum which we use to extract the hidden information from frequency domain.

The results are: 1. it takes a short characteristic time for the heated water molecules to saturate and activate the shale capillaries. This part of the plot of Eh Vs Time shows that at the beginning of the experiment the capillary saturation and activation is controlled by hydraulic potential. The dominating frequency associated with this stage is the mean frequency of the whole power spectrum. At the end of the early process, a Diffusive transport of the cold or heated water into the shale mass begins. This section follows the Fick’s second law of diffusion. Diffusion process; using heated water instead of cold water, is much faster than that of cold water, and 2. The Eh show the release of the first gas bubble to be the result of hydraulic potential spearheading the rest of the process. After completion of the first stage of the process, the diffusion of the cold or heated water in the shale capillaries begins.This is where the addition of a small amount of heat energy to the frac water displaces the sorbed gas from the shale macro, meso, and micro pores. Interestingly, the gas production mechanism follows the predictable Activation Energy accurately, as described by Arrhenius equations especially when combined with Diffusion equation.

We conclude that (a) the diffusion time and the flux rate of water molecules into the shale capillaries can be shortened by heating the fracturing fluid. We may estimate the optimal post frac shut-in time and monitor the advance of frac water displacement into the walls of the main hydraulic fracture. Achieving higher gas production can be realized through faster Capillary Saturation/Activation by heated water and (b) the desorbed gas production follows a pulsing, swarm flow, and other gas flow types.

The practical application of the simple technology suggested in this paper is (1) we propose a cost effective methodology for determining the post frac optimal shut-in time which is much shorter than that of shut-in time when compared with cold water frac fluid, and (2) once this optimal shut-in time is implemented the industry may benefit from realizing quicker and higher gas production from their prospects.

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