Hydraulic fracturing treatments in unconventional (coalbed and carboniferous shale) natural gas wells have been successful in stimulating gas production. However, the wells often do not perform up to potential following the treatments for various reasons. Some of the factors that can contribute to the relatively poor performance include low coal or shale permeability, complex cleating and natural fracture networks, gas content and adsorption characteristics, water content, and fracturing fluid interaction with formation surfaces. Fracturing fluid damage in coal formations has been shown to cause significant reductions in permeability, and whole gel leakoff into the cleat networks can further impair production. The same damage mechanisms can impair the production of carboniferous shale formations.
To avoid the damage associated with polymer-based fluids, a coal/carboniferous shale-compatible solids-free (CCSF) fluid was developed for unconventional natural gas well applications. The CCSF fluid is a nitrogen-foamed fluid that was designed to be environmentally friendly to minimize risk to ground water that could be contacted during fracturing treatments.
This study investigates the effects of various hydraulic fracturing fluids including conventional polymer-based fluids, water containing friction reducers, and a coal/carboniferous shale-compatible solids-free fluid on coal and shale pack cleanup. Laboratory coal pack cleanup tests demonstrate a significant reduction in retained permeability with polymer-based fluids. In addition, a high cleanup factor (a parameter introduced to provide a measure of the pressure and time required for cleanup) is required to establish only marginal flow conditions with brine injected in the production direction. Even water containing low concentrations of friction reducer may result in less than 50% retained permeability. The CCSF fluid provides 70 to 100% retained permeability and a low cleanup factor for effective coal pack cleanup with coal samples from seven coal basins in North America. Similar improvements were observed with carboniferous shale pack cleanup tests.
Field case histories in coal and carboniferous shale formations are consistent with laboratory observations and demonstrate a dependence of fracture cleanup on retained permeability and the cleanup factor measured in the laboratory. Production results demonstrate a 30 to 60% increase in production with the CCSF fluid compared to conventional fracturing fluids in coal and carboniferous shale basins in the United States.
Unconventional natural gas reservoirs include coalbed natural gas (CBNG) and carboniferous gas shales (GS). The commercial extraction of methane from subsurface coal seams and fractured carboniferous shales through rotary-drilled wellbores has entered its third decade. Worldwide estimates of unconventional natural gas reserves from CBNG and GS range from 3,500 to 9,500 Tcf, and anywhere from 1,000–3,000 Tcf likely exist in North America alone (Figure 1). From these estimates, it is easy to postulate that CBNG and GS could be a significant source of clean-burning energy.
There are two main differences between conventional sandstone formations and unconventional CBNG and GS formations. First, unlike conventional sandstone formations, where natural gas can be trapped and stored in the pore spaces within the matrix, CBNG reserves are adsorbed onto the coal surfaces. That is, coal does not require a structural or stratigraphic trap to store natural gas. The gas remains adsorbed in the coal surfaces unless the surrounding pressure is decreased below a critical point. As a result, the potential natural gas storage capacity of coal can be as much as five times higher than that possible in sandstone formations, especially when the formation pressure is below 2,000 psi.1,2