An Assessment of Risk of Migration of Hydrocarbons or Fracturing Fluids to Fresh Water Aquifers: Wattenberg Field, CO

Abstract

The United States National Science Foundation, engaging 29 researchers at nine institutions, has funded a Sustainability Research Network (SRN) focused on natural gas development. The mission of this Sustainability Research Network is to provide a logical, science-based framework for evaluating the environmental, economic, and social trade-offs between development of natural gas resources and protection of water and air resources and to convey the results of these evaluations to the public in a way that improves the development of policies and regulations governing natural gas and oil development.Currently, there are a wide range of estimates of the probability of shallow aquifer contamination. There are a series of independent events that must occur to allow hydrocarbon migration and estimates were made of these probabilities. An analysis of data from drilling in the Wattenberg field, CO was made to quantify the probability of contamination.It has been determined that there are five events that must each independently happen to allow the migration of fracturing fluids, and there are three events that must occur independently for the migration of hydrocarbons. The lower number of independent events, which must arise for hydrocarbon migration to occur, explains the infrequent, but well publicized natural gas migrations in poorly constructed wellbores, and the lack of such publicized events of hydraulic fracturing fluid contamination, which was confirmed by our analysis.The significance of these results is to help quantify the risks associated with natural gas development, as related to the contamination of surface aquifers. These results will help shape the discussion of the risks of natural gas development and will assist in identifying areas of improved well construction and hydraulic fracturing practices to minimize risk.

Groundwater methane in relation to oil and gas development and shallow coal seams in the Denver-Julesburg Basin of Colorado

“…This result is likely due to the fact that, since 1993, surface casings are required by COGCC regulations to extend at least 50 ft (15 m) below the deepest potable aquifer. The majority (89%) of horizontal wells also have production casings that are either fully cemented or cemented above the shallowest hydrocarbonbearing formation (20). This evidence supports the growing consensus that wellbore barrier failure, not the process of high-volume hydraulic fracturing itself, is the main thermogenic stray gas migration pathway (1,3,9,10,20,27,29,30).…”

“…Wellbore designs in the DJ Basin have evolved with the history of COGCC regulations and operator practices. Wells with the highest probability of failure have "short" surface casings, a legacy of an earlier (pre-1993) regulatory era when surface casings were not set deep enough to protect aquifers not in use at the time (20,33) (Fig. 5).…”

“…5). These wells also have uncemented sections of production casing in which hydrostatic pressure of fluid in the wellbore annulus is the only barrier to vertical migration of gas originating from intermediate-depth formations (20) (Fig. 5).…”

“…Based on remedial cementing records and bradenhead pressure data, Fleckenstein et al (20) identified 388 of 11,617 wells with short surface casings and uncemented intermediate sections in Wattenberg field with "possible barrier failures." Although there were far fewer (n = 42) water wells with thermogenic stray gas, it seems probable that wellbore barrier failures could have contributed to unresolved cases of thermogenic stray gas migration even though specific wellbores were never identified.…”

“…over which oil and gas wellbore construction evolved from vertical wells with uncemented sections to horizontal wells with fully cased and cemented vertical sections (20). We obtained data from the COGCC in October 2014, and after rigorous data screening and quality control protocols (SI Appendix), we examined the distribution and origins of groundwater methane in relation to oil and gas development in the DJ Basin.…”