Subterranean coal seams contain enormous amounts of natural gas, primarily in the form of methane. Coal seams also contain a network of cleats that typically contain water. The bulk of the gas is adsorbed onto the coal matrix. The recovery of the methane gas from the formation generally involves drilling a well, hydraulically fracturing the formation and producing formation water (dewatering) from the coal seams thereby reducing the formation pressure. Upon reduction of formation pressure, methane gas desorbs and flows through the cleat network to the wellbore.
The recovery of methane gas from coal seams is different from the recovery of gas from conventional formations such as sandstone or limestone type formations. The lithology and mineralogy of the coal are substantially different. Also, the gas in coal seams is adsorbed on the matrix of the coal while the gas occupies the interstices of the sandstone or limestone formations. Generally, the desorbed gas seeps through the cleat networks in coal seams to the created fractures and then flows to the wellbore. The permeability of the coal seam is determined to a great extent by the cleat network. Compared to conventional sandstone or limestone, coal is much more soft and brittle. Therefore, during drilling, fracturing or other well operations, substantial amounts of coal fines are generated. Coupled with generally low formation pressure, the coal fines can readily penetrate into cleats causing great damage to the permeability of the coal seam leading to reduced gas production.
Water-based drilling fluids are widely used in drilling wells in coal seams. Among them, straight water or brine is commonly used, due mainly to their low cost. During the drilling operation, large amounts of coal particles, ranging from fines to pebbles, are generated. Drilling fluid, such as water or brine, is circulated through the wellbore and transports the particles out of the wellbore. However, during the operation there are substantial amounts of particles, which fail to be transported out of the well by water or brines. Significant amounts of fines will penetrate into cleats reducing formation permeability. It is difficult to transport large pebbles out of the wellbore by water or brines. The remaining large pebbles in the wellbore reduce the drilling rate.
After the wells are completed and fractured, it is not unusual for there to be large amounts of coal particles, ranging from fines to pebbles, left in the wellbore. These coal particles impede the dewatering and gas production and thereby need to be removed from the wellbores. Water or brines are often circulated through the wellbore at a high flow rate to carry the particles out. It is known that turbulent flow condition plays an important role. Foaming surfactants are sometimes added to generate foamed fluid to facilitate the particle removal. Despite its wide applications, this technique faces several challenges. One of them is its ineffectiveness in removing debris, especially those large pebbles in horizontal wells.
In order to produce methane gas from coal seams economically, the coal formation normally needs to be hydraulic fractured. In a hydraulic fracturing operation, a fracturing fluid is injected through a wellbore into a subterranean formation at a pressure sufficient to initiate fractures to increase gas production. Frequently, particulates, called proppants, are carried into the fractures as slurry by the fracturing fluid. Proppants include sand, ceramic particles, glass spheres, bauxite (aluminum oxide), and the like. Among them, sand is by far the most commonly used proppant. The most commonly used fracturing fluids for coal seams are aqueous fluids, including water, brines, or water containing polymers or viscoelastic surfactants. At the last stage of the fracturing treatment, fracturing fluid is flowed back to surface and proppants are left in the fractures to prevent them from closing back after pressure is released. The proppant-filled fractures provide highly conductive channels that allow gas to flow to the wellbore more efficiently. The conductivity of the proppant packs formed after proppant settles in the fractures plays a dominant role in increasing oil and gas production.