Cleanup efficiency of hydraulically fractured vertical and multiple fractured horizontal wells

Abstract

The initiation and propagation of fractures in unconventional tight and ultra-tight reservoirs are achieved through the injection of high volumes of fracturing fluid, FF. Several field experiences have shown that ineffective FF cleanup can significantly impair gas production. In this thesis, results of 109 different sets of vertical well (VW) and multiple fractured horizontal well (MFHW) numerical simulations, each consisting of 4096 or 1000 runs depending on two different sampling approaches, are presented studying the impact of pertinent parameters during FF injection (stimulating), soaking (shut-in) and production periods for a total of 384,394 runs. In these sets I studied, the impact of a combination of various shut-in time, matrix permeability range, applied drawdown pressure and injected FF volume. For the MFHW case, the impact of fractures spacing and horizontal length were also investigated. In each set, twelve pertinent parameters related to fracture and matrix relative permeability and matrix capillary pressure have been varied. Gas production loss (GPL) and produced FF (PFF), as the response terms, have been calculated based on two separate response surface statistical models. In each set, the correlation between parameters and GPL and PFF have then been established and compared with a base reference set and other similar sets but with one different variable. An extensive evaluation of capillary pressure (Pc) correlations available for tight and ultra-tight formations was performed to investigate the reliability of available Pc correlation models. Additionally, a comprehensive investigation was conducted on the unconventional relative permeability (Kr) with jail effect in unconventional formations. A practically attractive approach has been adopted to successfully model the weak and strong permeability jail effects to express unconventional Kr models.