Abstract

In tight gas reservoirs, gas well production after hydraulic fracturing (HF) is often greatly impaired, through various mechanisms by invasion of fracturing fluid (FF) into the matrix and fracture and poor cleanup efficiency. In the last four decades, fracture face damage and fracture conductivity impairment has been considered as the main sources of such poor performance albeit with conflicting reports on the relative significance of these two effects. Furthermore, there are still several important issues, which have been either overlooked or not properly addressed in this process.

The scope of this study is to investigate the performance of cleanup efficiency in gas and gas condensate reservoir during HF. A single-well model was constructed using ECLIPSE300 compositional numerical simulator to conduct a comprehensive sensitivity study on pertinent parameters. The effect of FF depth of invasion and its negative impact on the fracture permeability (kf), matrix permeability and that of capillary pressure (Pcm), pressure drawdown, initial water saturation (Swi), FF mobility and hysteresis on the gas production from two tight rocks were investigated.

We have been able to reproduce the simulation results reported by previous investigators. However, our results demonstrated that when a more realistic FF depth of invasion into the fracture and matrix, as proposed here, was simulated the reported severe reduction in the gas production, during some of HF operations, was observed in our simulation study. Pcm increased the negative impact of FF invasion into the formation over that of the reduction in kf. If Pcm of the invaded zone was reduced using surfactants, back flow of FF improved, which impaired the out flow of gas for sometime. The negative impact of permeability reduction of the matrix invaded zone is more pronounced for the case with Swi. The improved mobility of FF and presence of condensate improved the FF recovery. These results, which verify the real causes of an ineffective cleanup, can be used for an improved planning of HF operations.

Introduction

It is well documented that hydraulic fracturing, although generally a successful practice, sometimes does not respond as expected. Ineffective fracture cleanup is one of the main reasons put forward to explain this underperformance. Fracturing fluid (FF) is one of the most important components of hydraulic fracturing treatments. These fluids create fracture and transport the proppants, which in turn prevent the closure of the fracture after treatment. Fluid cleanup after treatment is meant to remove the fracture fluid from the fracture and the matrix.

Fracturing fluid (FF) impairs the gas production through various mechanisms by invasion into the matrix and fracture. Over past four decades numerous studies had been conducted to investigate the effect of fracturing fluid on the well performance of hydraulically fractured wells. The laboratory tests performed by Cooke (1973, and 1975) demonstrated the effect of fracturing fluid residue (FFR) and reservoir environment (closure pressure & reservoir temperature) on the fracture conductivity during cleanup process. He developed a theoretical model, based on the volume of FFR after it degrades in the fracture and decreases the fracture porosity, to calculate fracture permeability (kf) reduction. He also measured the reduction of fracture porosity as a function of closure pressure and reservoir temperature and presented the results in the form of a chart. These latter results can also be used to calculate further fracture permeability reduction due to these effects using the same equation developed for calculation of kf reduction due to FFR.

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