Identifying the conditions that could lead to failures.

The substantial increase in gas production from hydraulic fracturing has been accompanied by controversy about the additional effects of stimulation to the subsurface beyond the confines of the tight, hydrocarbon-bearing formation. Generally speaking, the concerns are that reservoir stimulation creates significant environmental hazards via the fracturing of the overburden of reservoirs (and creating fast permeability pathways). This could result in contamination of potable ground water resources by escaping hydrocarbons and other reservoir fluids that ascend through the subsurface.

Many concerns about hydraulic fracturing center on potential risks to drinking water resources, although other issues have been raised. In response to public concern, Congress directed the United States Environmental Protection Agency (EPA) to conduct research to examine the relationship between hydraulic fracturing and drinking water resources. The purpose of this modeling project is to support EPA’s reports to Congress by providing credible simulations of possible failure scenarios, and identifying the range of conditions under which they might be expected to occur.

This project will investigate the possible mechanisms of stimulation-induced overburden-failure that could lead to such upward migration of hydrocarbons, and the conditions under which such catastrophic scenarios are possible.

Possible Failure Scenarios

There are five possible mechanisms for the upward migration of contaminants following stimulation.

Failure of the well completion during stimulation because of inadequate/inappropriate design and/or weak cement. In this case, the cement around the well is the weak link, and is fractured during the stimulation process, but the overburden is not fractured. Improper cementing and well completion can create a high-permeability pathway around the wellbore, through which contaminants can move upward.

Fracturing of the overburden because of inadequate design of the stimulation operation, with the resulting fractures reaching abandoned unplugged wells in conventional reservoirs. These aging wells intersect and communicate with ground water aquifers, and inadequate or failing completions/cement can create pathways for contaminants to reach the potable ground water resources.

Fracturing of the overburden because of inadequate design of the stimulation operation, with the resulting fractures reaching ground water resources, or even permeable formations that communicate with shallower ground water-bearing strata.

Induced fractures move upward and reach ground water resources after intercepting conventional hydrocarbon reservoirs, which may create an additional source.

Sealed/dormant fractures and faults are activated by the hydrofracturing operation, creating pathways for upward migration of hydrocarbons and other contaminants.

This project will develop models that can be applied to proposed applications of hydraulic fracturing technology to determine the range of conditions that can lead to these types of failures. Each potential failure scenario is simulated by a separate model. The purpose of these models is to take generalized information on hydraulic fracturing operations, and provide non-site-specific insight into potential failure scenarios. The models are appropriate for their intended purpose, as they can simulate the flow and transport of gas, water and dissolved contaminants concurrently in a fractured and porous reservoir.

Database Development

The EPA proposes to collect all available data (geological, reservoir, geophysical, geochemical, flow, geomechanical, type of environmental problem, extent of contamination, etc.) in cases/instances of reported problems related to stimulation operations in the United States and/or Canada. It will attempt to identify cases related to all five types of geomechanical failure discussed in this proposal, and will try to make the database as wide as possible. This task will require extensive interaction with federal and/or state environmental protection agencies; these will provide all the available data. Insights obtained will provide a basis for identifying and concentrating on the most likely/frequent/problematic failure mechanisms and the resulting ground water contamination hazards.

Preliminary Evaluations

A preliminary evaluation, assessment and ranking of the various possible or potential ground water contamination mechanisms will be executed. It will be based on:

• the analysis of the data originating from the database development,
  
  
• the evaluation of the physical likelihood of occurrence of the various failure and contamination scenarios,
  
  
• on literature review, and
  
  
• discussions with people involved in field operations of hydrofracturing.

The result of these efforts will help to develop an initial understanding of the relative likelihood of occurrence and the corresponding contamination hazard of the various mechanisms under consideration, which would allow directing the subsequent research effort in the direction of the greatest potential impact.

Geomechanical Wellbore Failure

Investigations will focus on the flow and geomechanics processes involved in the failure of the well completion (mainly cementing) during the stimulation process. Such a failure can allow escaping hydrocarbons to ascend through the damaged annular space around the wellbore to reach vulnerable ground water resources. This task aims to determine:

• the well design properties, variables and conditions that can lead to such failures when exposed to the pressures that develop during stimulation operations,
  
  
• the possible length of the failed zone along the wellbore, and the likelihood that the fractured cement can extend over long distances from the stimulation point, and
  
  
• the short- and long-term environmental effects of such failure of the near-wellbore zone during hydrocarbon production and after a well shut-down.

Abandoned Unplugged Wells

Researchers will focus on the flow and geomechanics processes involved during the stimulation processes, and aim to determine:

• the geologic properties and conditions that can lead to development of such limited penetration fractures,
  
  
• the stimulation operations that can cause such fractures,
  
  
• the extent to which the stimulation-induced fracture can reach into the overburden, and
  
  
• the short- and long-term consequences of the creation of such a fracture system into the overburden during hydrocarbon production and after a well shut-down, including the extent of the contaminant migration and its spread into the ground water.

Consequences of Induced Fractures

The flow and geomechanics processes involved during the stimulation processes will be examined in order to determine:

• the geologic properties and conditions that can lead to induced fractures reaching ground water resources,
  
  
• the stimulation operations that can cause extensive fracturing of the overburden,
  
  
• the extent to which the stimulation-induced fracture can reach into the overburden, and the short- and long-term consequences of the creation of such an unwanted extensive fracture system into the overburden during hydrocarbon production and after a well shut-down, including the extent of the contaminant migration and its spread into the ground water.

Conventional Reservoir Interception

Experts will investigate the results of stimulation-induced fractures intercepting conventional reservoirs (overlying or underlying). Emphasis will be the estimation of the likelihood of presence of such conventional reservoirs in the vicinity of tight ones, and the possibility of a priori identification of such conventional reservoirs by means of geophysical surveys.

Native Faults and Fractures

The flow and geomechanics processes involved in the response of native fractures and faults to the stimulation process will be studied. The aim is to determine:

• the geomechanical conditions (both in terms of system properties and stimulation practices) under which the displacement of the subsurface during stimulation is a reversible process (with the system returning to its original state after the end of stimulation),
  
  
• the effect of displacement on the native fracture and fault aperture and permeability,
  
  
• the short- and long-term transport effect of communication of induced fractures with native fractures and faults (in terms of the reach into the overburden) during hydrocarbon production and after a well shut-down, and
  
  
• the short- and long-term ground water pollution that escaping reservoir fluids can cause under this scenario.

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This article is excerpted from “Analysis of Environmental Hazards Related to Hydrofracturing,” by the U.S. Environmental Protection Agency with support from Lawrence Berkeley National Laboratory.