Two studies have been published recently that give recommendations on safe distances between induced hydraulic fractures and shallow groundwater. They both build on the 2011 publication of comprehensive data on hydraulic fracture height growth from major US shale gas plays (Fisher and Warpinski, 2011); they also combine this with hydrogeologic transport modeling (A) and the study of natural analogs (B).

A large data base on hydraulic fracture high growth in unconventional reservoirs was also published by Maxwell (2011), (pdf-file), who showed that fracture hight growth is limited to less than 100 m on average (300 m max.), in line with Fisher and Warpinski, 2011.

(A) Impact assessment of fracturing operations on shallow groundwater

As a follow-up to the extended summary version of "Risikostudie Fracking" (english document: "Hydrofracking Risk Assessment"), which was published in April 2012, detailed expert reports on different aspects of the risk study are now available. The report "Assessment of the impact of fracturing operations on shallow groundwater" (available only in German) addresses, among others, the risk of fracturing fluids migrating from the induced fractures to shallow groundwater horizons.

The report states that the risk of uncontrolled migration of fracturing fluids in groundwater horizons is low, as long as cap-rocks are intact and as long as potential migration pathways, such as permeable fault zones, play a negligible role in the transportation of matter.

The experts conclude that in the presence of basically intact cap-rock formations without a large permeable fault zone, a minimum distance of about 1000 m between land surface and perforated production liner is sufficient to ensure that groundwater quality is not affected by hydraulic fracturing operations. This distance is based upon consideration of

• a maximum vertical fracture length of 500 m (Fisher and Warpinski, 2011)
• a maximum fluid transportation distance of 200 m (based upon a 50 m transportation distance that was simulated in a model of the Bad Laer case, plus remobilisation of 50 m by a second hydraulic fracturing operation in a nearby well, multiplied by a safety factor of two).

The uppermost 300 m is not considered as cap-rock, due to enhanced permeabilities caused by natural tension release.

The experts emphasize however, that before any drilling is performed, local geologic and hydrogeologic conditions have to be thoroughly investigated in order to detect and evaluate potential migration pathways for fracturing fluids.

(B) Hydraulic fractures: How far can they go?

There is minimal probability of hydraulic fractures, such as those produced in fracturing operations for shale gas, contaminating shallow aquifers, according to a study that was published earlier this year in the journal Marine and Petroleum Geology.

The research team led by scientists at Durham University, U.K., suggests that "a minimum distance of 600 meters should be maintained between the fracture zone and an aquifer. The maximum upward propagation of a stimulated hydraulic fracture that has been recorded is 588 meters in the U.S.A. Nevertheless, data presented in the study suggest that their probability of extending beyond 350 meters is around 1%."