Advances in hydraulic fracturing and horizontal completions have made shale-gas wells more profitable. Methane leakage is one hazard associated with hydraulic fracturing natural gas. The same techniques of horizontal drilling and multi-stage massive hydraulic fracturing are used. Enhanced geothermal systems can trigger earthquakes as part of hydraulic fracturing. Burisma also provides natural gas well services, including hydraulic fracturing. It is also used as a clay control additive in fluids used for hydraulic fracturing. There is concern over the possible adverse public health implications of hydraulic fracturing activity. Kahn was among the first to foresee unconventional extraction techniques like hydraulic fracturing. The program made a number of advances in hydraulic fracturing of shales. An extensive list of radioactive tracers used in hydraulic fracturing can be found below. There have been many protests directed at hydraulic fracturing. Guar gum and its derivatives account for most of the gelled fracturing fluids. In April 2017, Hogan signed a law banning hydraulic fracturing in Maryland. In 2014, Polis sponsored two ballot measures targeting hydraulic fracturing, Initiatives 88 and 89. One example of long-term repeated natural fracturing is in the effects of seismic activity.