Surface waters and shale gas exploration/production

What is the potential effect of shale gas exploration/production on the status of surface waters?

Surface waters play a key role in the ecology of aquatic and water-depended ecosystems. They are recharged mainly by groundwater inflow and runoffs from the ground surface. In Poland, unconventional gas exploration and appraisal areas are located in regions that have diversified hydrographic conditions. The proximity of rivers, lakes and man-made water reservoirs involves potential risks of surface water pollution.

Surface water pollution may be caused by water runoff from the drill site area. Moreover, surface water may be used for reservoir stimulation purposes, potentially threatening the hydrological system of the drainage area involved. All of these aspects must be considered at drilling location selection stage.

A review of shale gas exploration stages has indicated potential impacts on surface waters that are presented below.

 

Protection of surface water quality during shale gas exploration and production operations

Shale gas exploration, appraisal and production operations may potentially involve impacts on surface water quality on the following stages:

  • well drilling,
  • reservoir stimulation (e.g. hydraulic fracturing),
  • gas production,
  • well abandonment and site reclamation.

 

Well drilling stage

Gas-bearing shales occur in Poland at average depths ranging from 3 to 5 km, in areas of different geology, diverse hydrogeological and hydrographic conditions. Drilling well location in an area of varied hydrographic network means that the safety of surface waters will depend primarily on drill site sealing and properly made stormwater drainage system. Moreover, due to the depth of shale occurrence, the wells have to pass through local aquifers that may be in hydraulic contact with the surface waters.

Therefore, drilling operations may cause surface water pollution as a result of:

  • penetration of pollutants from site surface to top aquifers that are in hydraulic contact with the surface waters (e.g. oil substances from operated vehicles and machinery) – if the site has not been sealed properly,
  • runoff water carrying loads of pollutants (e.g. following a heavy rainfall or due to inadequate system of drill site drainage),
  • migration of process fluids or gas to top aquifers that are in hydraulic contact with the surface waters – caused by improper well construction,
  • other emergencies (e.g. a rig failure involving spills of process fluids and their penetration to   to top aquifers that are in hydraulic contact with the surface waters).

At this stage, the risk of surface water pollution is effectively eliminated by:

  • Adequate investigation of the local geology and hydrogeological conditions (including seismic surveys) by the investor so as to enable good well design and construction that guarantee groundwater integrity,
  • isolation of aquifers with casing string cemented over the entire length. 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,
  • adequate investigation of hydrogeological conditions, including the determination of the contact and potential groundwater impacts on the surface waters (e.g. groundwater flow directions, contact of aquifers with the surface waters),
  • permanent supervision by competent District Mining Office and environmental compliance inspections by national authorities (PIOŚ, WIOŚ, GIOŚ, RDOŚ).

 

Reservoir stimulation – hydraulic fracturing stage

Reservoir stimulation procedures enable shale gas production. Fracturing involves sequential injections of a highly pressurized process fluid (the so-called fracturing fluid) with proppant (filling medium) into the horizontal leg of the well. The fracturing fluid is composed of water (approx. 99.5%) with chemical additives (For more information see Groundwater, Reservoir stimulation Stage – Hydraulic Fracturing). (Fig. 2.)

Reservoir stimulation procedures involve potential surface water pollution risks that may arise out of:

  • penetration of pollutants from site surface to top aquifers that are in hydraulic contact with the surface waters (e.g. fracturing fluid spills, leaking flowback water pits),
  • fracturing fluid or gas migration – if escape routes are formed in the local faults so that top aquifers in hydraulic contact with the surface waters are contaminated,
  • other emergencies (e.g. equipment failures involving process fluid spills in the well and on the site surface).

These risks can be 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 (Fig. 3).

At this stage potential risks to surface waters are:

  • the duration of the production period. With time casing pipes may corrode and cement bonds fail. In that case pollutants may penetrate to the aquifers and migrate in the aquifer which is hydraulic contact with the surface waters,
  • other emergencies (e.g. equipment failures involving penetration of pollutants to top aquifers that are in hydraulic contact with the surface waters).

At this stage, potential risks will be 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 production 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: Etap likwidacji otworu i rekultywacji terenu

The purpose of the well abandonment stage is to restore the original intended use of the site area. If not delivered properly, well abandonment operations may involve changes in the local water system from filling of drainage ditches or natural watercourses. Moreover, potential pollution of surface waters by runoff water flowing from the production site should be considered.

At the stage of well abandonment the risks 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 the aquifers and the ground surface may occur following the 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 & Małgorzata Woźnicka

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