A COMPARISON OF FRACTURE TRANSMISSIVITIES BEFORE AND AFTER HYDROFRACTURING WATER WELLS IN GRANITIC BEDROCK

Groundwater supply is often limited by the available aquifers. In many regions of the world, crystalline bedrock aquifers are the only choice for groundwater supply. This is the case in northern Wisconsin, located in the upper Midwest of the continental U. S. Here, groundwater flow to wells occurs only through fractures in the granitic basement. Many of these wells are hydrofractured to increase their yield. In contrast to hydrofracturing for shale gas, this process is at relatively low pressures and uses potable water as the hydrofracturing fluid. Although hydrofracturing is common and generally increases well yield, the precise mechanism for the increased yields remained unknown.

We obtained stressed and ambient flow logs in two 305 meter deep granitic wells in northern Wisconsin prior to hydrofracturing. From those logs, we were able to determine that nearly all of the groundwater flow to the wells occurred in the upper 100 meters with no measureable contribution below that depth. We also found that less than 10 fractures contributed nearly all of that flow in each well. Following hydrofracturing of the wells by a commercial well driller from the area, we again obtained stressed and ambient flow logs. The transmissivity of the wells increased by factors of 8.5 and 23 times. We found that: 1. The fractures that had contributed flow to the wells increased in transmissivity, 2. No new fractures were formed, and 3. Fractures without measureable flow before hydrofracturing remained without measureable flow.

Hydrofracturing increases yields in granitic wells. However, that increase seems to only occur in fractures where flow was pre-existing and in the upper 100 meters of the well. These observations suggest that efforts to enhance yield in granitic aquifers should be focused on the upper part of the well.