Productive zones are within the well are then isolated for fracturing where water and chemicals are injected under high pressure into the wells to fracture the rock. ‘Proppants’, usually sand or ceramics, in the injected water solution hold the fracture crack open to prevent their ‘healing’ and allow the continued release of natural gas. This gas is in two forms: ‘free gas’ which is released first and ‘adsorbed gas’ on the surface of organic matter, which is released when the pressure in the well drops.
The solution injected into the well also contains a very small quantity of additives such as gelling agents to cause the rock to crack, biocides to kill contaminating micro-organisms and surfactants to sterilise the well. Additives are also use to increase the efficiency of the process. Typically, these additives comprise of around 0.5% of the total injection volume. The composition of additives used depends upon the conditions of the well such as pressure, temperature and also the quantity of proppant used.
Total estimates for its operations an average of 30 ‘fracs’ are performed for each 1,000 metre well and each ‘frac’ uses 300 m3 of water, 30 tonnes of sand and 0.5% additives in the solution mixture. Therefore, the process is very water intensive, which is a big issue for water-stressed states where gas shale plays are located such as Texas. This water needs to be extracted from aquifers or trucked in to the site on access roads.
The initial production (IP) from the wells is high but tails off rapidly compared to conventional wells. This rate of decline typically follows a hyperbolic curve, as shown below for the Marcellus shale play. Therefore, a larger number of wells and repeated high-pressure fracturing is needed to maintain production compared to a conventional well. The recovery factor for unconventional wells is usually in the 5% to 40% range due to low permeability compared to above 90% for conventional wells.