Protection of Underground Waters
Potable water can be found in the Holocen sediments of the Mura River gravel to a depth of 20 to 30 m. The depth of the Pg-10 and Pg-11A wells is approximately 3,500 m. Operations at the Petišovci field are designed to comply with strict environmental protection measures. The wells are designed with at least 3 steel casings, surface, intermediate and production, cemented in place (see ‘Safe Gas Extraction Procedures’). These casings and cement sheets prevent any fluids or gases from flowing between casing strings, or from unintentionally coming into contact with other reservoirs, especially the shallow water bearing rock layers (aquifers).
The protective casings are inspected by a well inspector and they are pressure tested for leaks. This method of isolating the producing zones and the well bores from aquifers has been studied for many years. The results of these studies were taken into consideration by the Geological Survey of Slovenia when preparing the Environmental Impact Assessment for the Petišovci project.
Hydraulic stimulation activities are carried out at depths below 2,500 m. Between this and the aquifer there are many thick layers of impermeable shale preventing contact between the stimulation fluid and the aquifer.
The fluids used in drilling and stimulation return up the production tubing (flowback) and are collected in tailing ponds beside the well. An authorised wastewater treatment plant disposes of this water in compliance with the relevant government regulations.
To emphasise the difference between operations in sandstone at Petišovci and those in shale in the USA, the Geological Survey of Slovenia produced the table below, in the Environmental Impact Assessment, which compares the two processes.
|Reservoir type||Shale||Sand (with tight gas)|
|Number of wells in one gas field||3,000-10,000||15-20|
|Number of stimulation processes per well||30-50||4-6*|
|Stimulation fluid – amount per stimulation||5,000-20,000 m3||100-500 m3|
|Fracture length||300-500 m||70-100 m|
|Geological reservoir formation depth||150-2,500 m||2,200-3,500 m|
|Depth of aquifer with drinking water||150-400 m||Less than 20 m|
|Depth of aquifer with warm (geothermal) water||Less than 1,200 m|
|Thickness of impermeable shale formations above the gas-bearing sandstone at Petišovci||80-200 m|
Table 1: Comparison between the US and Petišovci hydrocarbon deposits, number of wells, hydraulic stimulation processes and volume of liquids. The data for the US locations were obtained from the Environmental Protection Agency of the United States of America study (2011), and for Petišovci from Ascent and Geoenergo reports.
*Pg-11A well was hydraulically stimulated 3 times and Pg-10 well was hydraulically stimulated twice, both in 2011.