A high
flowback rate of hydraulic fracturing fluid (HFF) yields
a high gas production rate in a tight sand gas reservoir. This idea
is well followed when it comes to the hydraulic fracturing of a shale
gas well. Therefore, numerous studies have focused on increasing the
flowback rate of HFF in shale gas reservoirs to mitigate the water
blocking damage. Yet, only a small portion of the injected fluid (less
than 20%) could be recovered during flowback operation. Moreover,
there is no good correlation between the gas production rate and flowback
rate for shale gas wells. Surprisingly, a phenomenon that a low flowback
rate of HFF usually yields a high gas production rate is found recently
by field data studies. In this case, the authors investigated the
reasons for this abnormal phenomenon and believed that reducing the
flowback rate of HFF in a shale gas reservoir could be a new strategy
for dealing with the injected HFF. Therefore, a new concept called
zero flowback rate (ZFR) of HFF is put forward to serve as an unconventional
way to manage the high volume of HFF. ZFR of HFF is a concept compared
with the flowback rate of the fracturing fluid for a conventional
reservoir. It is worth noting that zero in the ZFR concept does not
stand for the absolute number 0. It means that reducing the flowback
rate of HFF is the goal of the ZFR strategy. To analyze the feasibility
of realizing ZFR, the geological conditions of gas shale and engineering
conditions of hydraulic fracturing were evaluated by comparing the
relative studies. It showed that gas shale has the potential to imbibe
most of the HFF and retain the imbibed fluid without contaminating
the groundwater. ZFR can be realized by extending the shut-in time
or adjusting the properties of HFF. Compared with the conventional
idea of increasing the flowback rate of HFF, ZFR is of great significance.
It does not only recover the relative gas permeability of the shale
reservoir by redistributing the liquid phase from the fractures into
the shale matrix but also enhances it by creating new microfractures.
ZFR is cost-saving and environmentally friendly by dealing with a
reduced volume of flowback HFF. ZFR has a high potential of becoming
a viable strategy for the development of shale gas reservoirs.
The permeability jail refers to a specific water saturation range in a tight gas reservoir, where almost no gas or water phase can flow effectively. In the process of drilling and fracturing, water saturation rises and falls into the permeability jail. To reduce or avoid falling into the permeability jail in the recovery process, a method for measuring gas–water relative permeability of tight sandstone is established here that considers salt sensitivity, gas slippage effect, stress sensitivity, and high bound water saturation. Then, the permeability jail range was determined to provide guidance and suggestions for field application. Considering a typical tight sandstone as an example, the proposed method was used to expand the measurement range of gas–water relative permeability and observe the permeability jail range, laying an experimental foundation for accurately determining the permeability jail range in a given formation. The Byrnes model can preliminarily predict the permeability jail range with accurate bound water saturation and residual gas saturation. When the permeability jail phenomenon occurs in the core, the larger the permeability is, the smaller the permeability jail range will be; and the larger the porosity is, the smaller the permeability jail range will be. When the permeability jail phenomenon occurs in the tight sandstone reservoir, the damage to the reservoir due to external fluid and solid phased particles should be strictly controlled. The damage is stronger, the permeability and porosity decline, and the permeability jail range is wider. Other gases or solvents can be used as fracturing fluids to minimize formation damage.
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