Impact of de-ionized water on changes in porosity and permeability of shales mineralogy due to clay-swelling

Experimental study of true triaxial high pressure subcritical water impact fracturing

A new fluid alternative to slick water for fracturing shale gas can reduce the waste of water resources and improve the extraction efficiency, enabling volumetric fracturing. For the new fracturing technique, the experiments of different release pressures under pre-injection and for pre-injection were conducted using a self-designed true triaxial experimental system, and the pressure pulse curves were plotted to analyze the fracturing principle. The experimental results showed that: (1) the pressure rise curve in the reactor can be divided into five stages: initial reaction, linear pressure rise, rate slowdown, instantaneous pressure release, and residual pressure stages; (2) Pre-filling fracturing requires a smaller expansion ratio, weaker pressure degradation, resulting in better fracturing effect; (3) The increase in the initial fracture length leads to an increase in the pressure required to extend the fracture, and high-pressure subcritical water impact fracturing achieved fracture extension at a lower fluid pressure; (4) The fractal dimension has a strong linear relationship with fracture complexity, which is a new option when evaluating the fracturing effect. Volumetric fracturing allows for the creation of more tiny trenches that increase reservoir permeability, leading to better recovery of the reservoir's energy resources.

Prediction of gas production rate from shale gas reservoirs using a micro-macro analysis

Shale gas has become one of the important contributors to the global energy supply. The declining pattern of the gas production rate with time from an unconventional gas reservoir is due to the depletion of shale gas stored in the nanovoids of the shale formation. However, there are only limited ways to predict the variation of the gas production rate with time from an unconventional gas reservoir. This is due to the multiple transport mechanisms of gas in nano-scale pores and changes in shale gas permeability with pressures in nano-scale pores, which is impacted by the pore structure of the shale. In this study, the permeability-pressure (K-p) relationship for different shales (Eagle Ford, Haynesville, Longmaxi and Opalinus) were determined using an equivalent anisotropic pore network model (PNM). This PNM has REV-scale shale gas flow in randomly generated nanovoids and their connection in the shale matrix, and the multiphase flow of shale gas including viscous flow, slip flow and Knudsen diffusion. These predicted K-p correlations were then used in a finite element model (FEM) to predict the variation of the gas production rate with time (flux-time curves) at the macroscale. The simulation results show that the flux-time curves can be simplified to two linear segments in logarithmic coordinates, which are influenced by the fracture length and initial gas pressure. The predicted results using the PNM-FEM were validated by comparing them with the reported field test data. The method described in this study can be used to upscale the gas transport process from micro- to macroscale, which can provide a predictive tool for the gas production in shales.