The main ingredient in natural gas is methane which is formed when organic material derived from the breakdown of plants and animals is included in sedimentary deposits. Over time, the sedimentary deposits change to rock, and the included organic material gets changed into fossil fuel as either coal, oil, or natural gas. The type of fossil fuel that is developed depends on both the original composition of the organic material and the temperature and pressure that it has been exposed to, deep in the earth.
Shale gas is defined as natural gas from a fine grained rock known as shale. The shale acts as the source, reservoir and seal for the natural gas. Older shale gas wells were usually vertical while more recent wells are primarily horizontal and are stimulated to produce gas by hydraulic fracturing. Only shale formations with certain characteristics will produce gas. The most significant trend in U.S. natural gas production is the rapid rise in production from shale formations. Shale gas deposits are usually classified as “continuous” type gas accumulations often extending throughout large areas. This means once the resource is proven in an area low risk development is possible over a large area.
A key element in the emergence of shale gas production has been the refinement of cost-effective horizontal drilling and hydraulic fracturing technologies. These two processes, along with the implementation of protective environmental management practices, have allowed shale gas development to produce gas from previously unproductive areas.
Technology drives the exploration and production of shale gas which is comprised of either vertical or horizontal wells. In both cases, fresh water aquifers are protected by the use of casing and cement effectively isolating them from the deeper intervals. Most shale gas plays involve horizontal well completions to optimize recovery and well economics. Horizontal drilling has a big advantage over its vertical cousin, in that a large amount of the wellbore can come into contact with the reservoir. This also means that far fewer wells need to be drilled and the wells that are drilled can have many wells drilled from the same well pad site thus reducing surface impact.
The second critical element to the success of shale gas exploration and production is the use of hydraulic fracturing. This process involves the injection of mainly water and sand, or a similar “proppant” into a shale formation to generate fractures or cracks in the target rock formation. The sand or proppant holds the fractures open so that the gas in the shale can flow to the wellbore and thus be produced. The fluid, or fracture fluids, are mainly comprised of water and sand with a small balance consisting of additives that improve the efficiency of the fracture process. The particulars of each hydraulic fracture are based on the specifics and characteristics of the rock formation in question.
Given the importance of water to the fracturing process, it is critical that shale gas developers identify sufficient sources of water before the process begins and plan for the peaceful coexistence with the needs of the community for the same water resources. The volume of water used to stimulate a horizontal well to produce gas, ranges from 2 million to 4 million gallons, however, these amounts are small in comparison to water used for agriculture, electric power generation, and in general represent a small percentage of overall water consumption for an area.
At the completion of the drilling and fracturing process, the water used in the fracture stimulation process is produced back along with natural gas. This water requires management to protect surface and ground water resources, and ideally reduce future demands for fresh water. Stakeholders including local and federal governments and shale gas operators look for ways to “Reduce, Re-use, and Recycle” in order to minimize the impact on the environment and the community. Water treatment technologies have been developed for use on shale gas produced water so that it can be re-used.