Coalbed methane reservoir stimulation using guar-based fracturing fluid: A review

As a greener and efficient energy source, the development and utilization of coalbed methane (CBM) can not only increase the energy supply for the State, but also reduce the greenhouse gas (GHG) emission by replacing other carbon-intense energy sources, such as oil and coal. CBM reservoirs are known as low to ultra-low permeability reservoir and thus the fracturing stimulation is commonly required for commercial gas production from coal seams. This article reviews the main components, rheology, friction pressure, and proppant transport characteristics of the guar-based fracturing fluid, and its field applications. Meanwhile, both advantage and disadvantage for CBM fracturing treatment were comprehensively analyzed. Guar-based fracturing fluid is composed of guar gel, and various additives, mainly crosslinker and breaker. As a complex mixture, the effectiveness of guar-based fracturing fluid is not only closely related to the concentration of various chemical additives but also influenced by fluid-coal interactions at the in situ reservoir conditions. The gel residual due to low flowback rate can potentially damage the formation and hinder the effectiveness of gas production. The formation damages include impacts on gas adsorption, diffusion, and transport in CBM reservoirs. Coal matrix has a strong adsorption capacity for guar-based fluids and is likely to have a sorption induced matrix swelling and reduce the effective permeability. Hence, it is necessary to develop more efficient breakers to increase flowback of guar-based fracturing fluid. The long-term engineering practice shows that the fracturing effect of guar-based fracturing fluid is quite different in field applications at different operation sites. Therefore, the mechanisms of the impact of guar gel on the methane flow in coal should be further studied, and it is important to determine the applicability and improving performance of the guar-based fracturing fluid for site specified application based on the reservoir pressure, temperature, hydrological environment, structural geology and other unique reservoir properties.