What potential effect shale gas exploration/production may have on the status of groundwater?
Groundwater is a key environmental compartment that may be affected by shale gas exploration and appraisal projects which are currently underway in Poland. The key issues focus on water requirements and potential contamination risks at drilling and stimulation (hydraulic fracturing) stages. Oil and gas exploration and production operations give rise to a number of questions on groundwater use and protection capabilities. Most of them are answered in this section.
Groundwater quality protection during gas exploration and production operations.
Considering the approach to delivery of operations that are associated with exploration and production of unconventional hydrocarbons (shale gas), as well as existing available and commonly applied technologies, potential risks to the groundwater are expected to occur in the following stages:
- well drilling,
- reservoir stimulation (hydraulic fracturing),
- gas production,
- well abandonment and site reclamation.
Drilling stage
Gas-bearing shales occur In Poland at depths ranging from 3 to 5 km, in areas of different geology and hydrogeological conditions. The maximum depth of freshwater aquifer occurrence in Poland is estimated at approx. 300 m below the ground surface. The depth may vary, depending on both regional and local factors. Fore example, in northeast Poland aquifers occur at depths ranging from 200 to 300 m below the ground, whilst in southeast Poland (Lublin and Zamośc) the depth does not exceed 160 m below the ground.
Moreover, mineral and thermal waters occur in the existing shale gas exploration areas. The depths to mineral waters range from 200 m below the ground (near Torun) to over 500 m below the ground level (for example north and northeast of Lublin). Thermal waters occur at a depth of approx. 2500 m below the ground level (Hydrogeologia regionalna Polski, vol. II, PGI, 2007).
Due to the depth of gas-bearing shale occurrence, drilling wells have to pass through local aquifers that frequently are the only source of water supply to the local residents.
Drilling operations may potentially contaminate the aquifers as a result of:
- drilling fluid or gas migration – in the event of inadequate well construction,
- penetration of contaminants from the site surface (e.g. spills of oil substances from operated vehicles and machinery) – if the ground is not properly protected,
- emergency events (e.g. a drilling rig failure and related drilling fluid spills).
At this stage, the risk of groundwater contamination is effectively eliminated by:
- proper investigation of geological structure and hydrogeological conditions by the investor (including seismic surveying). This makes it possible to design and construct a well that is safe to the groundwater (Fig. 1.). Seismic surveys do not represent a risk to groundwater quality, provided that they are performed according to applicable procedures,
- isolation of aquifers with casing strings cemented over the entire length (Fig. 2.). Cement bonds are tested for integrity under the supervision of District Mining Office,
- 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, chemicals and waste storage areas, as well as by appropriate drainage system for conveying stormwater out of the drill site,
- permanent supervision by competent District Mining Office and environmental compliance inspections by national authorities (PIOŚ, WIOŚ, GIOŚ, RDOŚ).
The depth to gas-bearing shale rocks helps to eliminate the risk of groundwater contamination. A thick overburden of impervious rocks isolates freshwater aquifers that occur at depths of up to 300 m from the ground level (Fig. 3).
Reservoir stimulation (hydraulic fracturing) stage
Hydraulic fracturing is a technical procedure which establishes a network of induced fractures in the shale formation. Interconnected fractures form a path of gas migration to the borehole and then to the surface. Normally, the range of hydraulically induced fractures is approx. 100 m from the borehole.
Fracturing (Fig. 4) 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 fracturing fluid is composed of water (approx. 99.5%) with chemical additives (that are intended to make the fluid slick, reduce its viscosity and prevent the swelling of hydrophilic clay minerals), bactericides and gelling agents to prevent proppant buildup at the well bottom.
Potential groundwater contamination risks from reservoir stimulation are associated with:
- fracturing fluid or gas migration – if escape routes are formed in the local faults,
- other than expected fracture propagation – uncontrolled migration of gas or fracturing fluids,
- penetration of pollutants from site surface (e.g. fracturing fluid spills, leaking flowback water pits),
- other emergencies (e.g. process fluid spills from defective equipment).
At this stage, the risk of groundwater contamination is effectively eliminated by:
- seismic surveys made by the investor prior to drilling operations. A proper interpretation of seismic survey results and an analysis of preexisting geological data in practice eliminates the risk of well location in a faulted area,
- a thick natural overburden of impervious rocks (3 -4 km as a minimum),
- 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, chemicals and waste storage areas, as well as by appropriate drainage system for conveying stormwater out of the drill site,
- permanent supervision by competent District Mining Office and environmental compliance inspections by national authorities (PIOŚ, WIOŚ, GIOŚ, RDOŚ).
Gas production stage
Gas may be produced for several decades from several tens of horizontal wells drilled out from a single site.
At this stage potential risks to groundwater are:
- the duration of the production period. With time casing pipes may corrode and cement bonds fail. In that case penetration to the aquifers is possible,
- potential penetration of pollutants from the site surface,
- emergencies (e.g. equipment failures).
The risks are effectively eliminated by:
- permanent monitoring of casing and cement bond integrity throughout the stage of production,
- sealing with impervious sheets and concrete slabs the ground in immediate proximity of the well, as well as by appropriate drainage system for conveying stormwater out of the site.
Well abandonment and 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.
If not delivered properly, well abandonment operations may involve:
- emissions of reservoir gas and groundwater contamination with gas.
At this stage the risk can be mitigated by:
- delivery of well abandonment operations in compliance with applicable regulations (e.g. placement of cement plugs or mechanical barriers to isolate reservoir gas from aquifers), based on abandonment program approved by the competent authority (District Mining Office),
- permanent monitoring of well tightness throughout the well abandonment operations.
Exceptionally, well leakage and migration of reservoir gas to aquifers and the ground surface may occur following well abandonment. In that case the investor should prepare a recovery plan and cooperate with competent authority so as to minimize environmental impacts and restore integrity of the abandoned well.
authors: Magdalena Nidental i Małgorzata Woźnicka
Spis literatury:
- B. Paczyński, A.Sadurski red. Hydrogeologia regionalna Polski tom II, PIG-PIB Warszawa 2007
