Developing coalbed methane (CBM) in China has faced a series of unprecedented technical challenges over the past decade. Apart from the geological constraints of the coal seams that have very low cleat permeabilities, the poor performance of the fracturing fluid system has also been identified as one of the contributing factors leading to subpar gas production rate after well intervention. Innovation in borate crosslinking ability is a critical need for CBM fracturing advancement.

With the objective of enhancing the functionality of guar-based fracturing fluid for this application, a self-buffered liquid hybrid borate-based crosslinker has been developed in the laboratory. Unlike conventional borate crosslinker behaviour, instant crosslinking occurred typically when polymer loading was significantly low. Controllable crosslinking was achieved by impeded dissolution of borate feature, which, in turn, allowed sufficient delay time in polymer crosslinking.

This breakthrough in the ultralow guar (ULG) fracturing fluid system was viable without compromising its viscosity stability. The polymer loading of the crosslinked fluid was further decreased to 11-lbm and the fluid still demonstrated sufficient shear recovery in laboratory rheological testing. In field application, this fluid causes less formation damage and provides better flowback efficiency in the coalbed formation.

The benefit of the ULG system has been validated by several field applications. In all, 8100 m3 of ULG crosslinked fluid has been applied in coal seam hydraulic fracturing across 22 wells, with total of 31 stages, including both horizontal and vertical wells in the Hancheng area, Ordos Basin. A field implementation case history will be discussed in this paper.

The novel crosslinker is capable of overcoming the inadequacy of conventional borate crosslinker used in guar-based fracturing fluid by addressing the performance needs for fracturing CBM wells.

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