Potential environmental impacts at each stage of works
Both nature and intensity of the impacts from shale gas production on particular environmental compartments are direct effects of the technology applied and vary from one stage of work to another. Shale gas production project stages are:
- drilling preliminaries,
- well drilling,
- reservoir stimulation (hydraulic fracturing),
- production preliminaries,
- production (gas production from wells),
- well abandonment and site reclamation.
Each of these stages involves a different kind and range of potential environmental impacts for which different assessment procedures and mitigation measures are applied. It is the specificity of production operations that works performed in each successive stage are contingent on results of the previous stage and the approach to its delivery. Moreover, certain impacts tend to be cumulative with the progress of works and may persist upon discontinuation of operations.
Drilling preliminaries stage
Preliminaries stage comprises activities that are associated with drill site construction.
In early stages of technology development in the USA (second half of the 20th century), the necessity to built a dense network of drilling wells was considered as a major environmental challenge. Significant land areas had to diverted from their intended use and impacts from heavy vehicular traffic between the drill sites were a problem. In Europe, where average population density is twice that of USA (almost fourfold in Poland), the problem would be even more pressing.
Nowadays, the site footprint on the ground level is usually 2 to 4 hectares of land and directional drilling enables gas production from of an area equal to 500 drill sites. Increasingly efficient drilling techniques allow for drilling more than 3 km long production legs, without the need to occupy a similar area on the ground surface. This alleviates the shale gas production nuisances, as reported from the USA in the past.
The stage of preliminaries during which drill site infrastructure is built, is essential in terms of safety of operations. Key phases of that stage are:
- organic top soil conservation,
- site surface isolation,
- building a drainage system and stormwater drains,
- installation of stormwater and process water tanks,
- construction of water intakes and service connection pipes,
- designing waste collection system and gas collection/transmission installations to be used in subsequent stages.
It is common today to line the site floor with impervious sheets and cover it with concrete slabs. Top soil is usually removed and stored in embankments around the site as an additional protective barrier.
Moreover, access roads are often built to accommodate transport of massive equipment used in operations.
Normally, it takes a few months to drill the wells. In the phase of production, multiple (usually a dozen or so) wells are drilled from a single pad in two rows of well slots every few meters apart. All wells and their horizontal legs are drilled using the same drilling rig which is moved around the drill site. Drilling operations are delivered on a non-stop basis. This may be a nuisance (floodlight, noise) to the local residents, if buildings are located in the proximity of the drill site.
Key measures intended to alleviate the nuisances include drill site location as far as possible from residential buildings and use of natural barriers and sound screens.
Shale gas wells are drilled in Poland to depths ranging from 3.0 km to more than 5.0 km. These depths are bigger than average depths of wells drilled in the United States and Canada: Barnett Shale: 2.1–2.7 km, Marcellus Shale: 1.8–2.1 km (Modern Shale Gas Development in the United States: A Primer, U.S. Department of Energy, April 2009). Significant depths to productive formations on one hand extend the time and escalate the cost of drilling operations, but on the other they enhance safety of gas production by establishing a better isolation between the producing formation and freshwater aquifers/ground surface.
Whatever the purpose and depth of drilling, it is essential from the viewpoint of environmental impact reduction to ensure that drilling wells are properly designed and constructed so as to prevent any penetration of drilling fluids and gas to adjacent rocks and above all to the aquifers. While drilling, aquifers are isolated with casing strings that are cemented over their entire length. If the well is properly constructed and cement bond integrity checked, the risk of groundwater contamination with drilling fluids or gas migrating along the vertical well is, in practice, eliminated.
Reservoir stimulation phase is critical as it enables gas production from shale formations. At this stage of technology development, hydraulic fracturing is a commonly applied procedure. It involves sequential injections of a highly pressurized process fluid (the so-called fracturing fluid) with proppant (filling medium) into horizontal leg of the well. The injected fluid induces a network of small fractures in the rock and the proppant (well sorted sand or ceramic granules) prevents fracture collapse so that gas may flow to the borehole.
The fracturing fluid is composed of water (approx. 99.5%) with chemical additives that are intended to optimize the fracturing process, primarily to make the fluid slick, reduce its viscosity and prevent the swelling of hydrophilic clay minerals. Bactericides are also added to kill bacteria that may have been introduced to the well with drilling fluids injected to the well and to prevent corrosion of pipes. Moreover, gelling agents are added to enhance proppant carrying capacity of the fluid and prevent proppant buildups at the well bottom and borehole clogging. As the fracturing procedure is over, a part of the fracturing fluid (flowback water) is recovered at the wellhead and then the gas released from the formation starts to flow to the surface.
Multi-stage hydraulic fracturing, made sequentially in all wells drilled out on the site, is a complex and technologically advanced procedure that involves potential risks to the natural environment.
The noise, gas and dust emissions from the operation of high-performance generator sets, engines and force pumps are a short-term nuisance that can be minimized using the same measures as at the drilling stage.
The risk of potential penetration of contaminants into the soil and top aquifers is effectively minimized by sealing with impervious sheets and concrete slabs the ground in immediate proximity of the well, fracturing fluid preparation and injection areas, as well as fuel, chemical and waste storage areas. Moreover, a stormwater drainage system is in place (drains, drainage ditches and pits).
Potential fracturing fluid or gas penetration to the groundwater from the horizontal leg of the well must be considered in the analysis of environmental risks that are associated with hydraulic fracturing procedures. This may happen in the event of uncontrolled reaction of the rock mass, for example fluid escape routes formed in wide-area dislocation zones. However, considering a significant thickness of shale rock formations (in excess of 3 km) and average range of induced fracture propagation (approx. 100 m) and the presence of thick impervious formations in the overburden that act as effective sealants, it does not seem possible for the pollutants to penetrate this way into the aquifers that occur at depths to 300 m below the ground surface.
In theory, an intervention into the rock mass that involves injection of huge volumes of fracturing fluid may trigger relocation of rocks, especially in vulnerable areas, such as those with preexisting faults or tectonic zones. These movements may manifest themselves on the surface by perceptible vibrations or seismic shocks. Seismic vibrations may be induced by fracturing fluid injections, too. This happened in Blackpool (UK), where earthquakes of 1.5 and 2.3 magnitude were reported after hydraulic fracturing operations. It should be emphasized, however, that the risk of hydraulic fracturing-induced earthquake is lower comparing to other industrial activities, such as mining, geothermal energy or conventional oil and gas production. Existing active seismic areas are particularly at risk. As the entire Poland is considered as tectonically stable area, the risk of any seismic activity induced by hydraulic fracturing operations is minimal.
This does not mean, however, that the risk can be neglected. Seismic monitoring should be established at successive stages of stimulation operations and carried out until seismic response of of the rock mass is established in particular areas of production.
Gas production preliminaries
The stage of gas production preliminaries includes demobilization of reservoir stimulation equipment, waste management and construction of gas recovery and transmission facilities. This stage marks the transition from field testing to field development.
Production tests are carried out in the wells to establish gas flow rates at the stage of production. Christmas trees are installed for gas recovery from the well and transmission facilities are built. Water tanks and other no longer needed site facilities are removed. Transportation of significant amounts of equipment involve a heavier vehicular traffic which is a potential nuisance to the local residents.
Unconventional gas production with a dozen or so horizontal wells may be continued for several decades. This stage does not differ from similar stages of conventional gas production (at present, gas is being produced from 199 out of 285 proven conventional gas reservoirs – The Report on Mineral Resources in Poland, 2012).
In the production phase, site footprint is much smaller than in previous stages and is normally limited to the immediate proximity of the Christmas tree. Moreover, the site houses tanks for production fluids that are recovered along with gas and, in some instances, manifolds to accumulate the gas before it is injected to the transmission network. The remaining portion of the site must be reclaimed so as to restore the original intended use.
Reservoir water may be recovered from the well along with gas at the stage of gas production. Reservoir fluids that are produced must be properly stored and handled. Potential migration of gas in the near-well (skin) zone, especially as casing and cement bonds fail with time, is a serious issue, often raised by environmental organizations. Since this may represent a major environmental risk, casing and cement integrity should be monitored throughout the phase of production.
Site reclamation stage
Well abandonment and site reclamation is the final stage when gas production is discontinued. Its purpose is to restore the original intended use of the site area. Works are delivered in the same way as in any other extractive operations. It is important to assess potential changes in soil geochemistry, the degree of top soil degradation due to the loss of organic substance and subsoil compaction as a result of prolonged exposure to loads from drill site facilities.
authors: Małgorzata Woźnicka & Monika Konieczyńska