Fracture Length and Aperture

How big are hydraulic fractures? Generally, companies don't know the exact length and aperture (width) of fractures, but can estimate these properties based on the expected rock strength and the force applied by the high pressures. A lot of people are concerned with the lack of accuracy of these calculations concerns many people, especially within the concern of water contamination. Some people wonder how petroleum companies can be certain that they are not contaminating ground water when they don't even know how extensive the fractures are.

Comparison of hydraulic fractures in horizontal and vertical wells

Recently, a study was performed by C.E. Renshaw and J.C. Park to better understand how mechanical interactions of formations effect fracture size. From my understanding of the paper, hydraulic fracturing is very difficult to model because of the complexity of the variables. The fracture size depends on the type of rock that is being fractured, the number of fractures being produced, the pressure causing the fracture, as well mechanical interaction. Overall, the study warns that common estimates of fracture propagation lengths may not always be good approximations and that universal scaling laws are not are not always applicable.

Generally, hydraulic fracturing is performed in many stages, to ensure that the entire producing interval receives stimulation. It was determined through this study by Renshaw and Park that the growth rate exponent (a variable in a correlation proposed in the study) is larger when just single fractures are produced than when multiple fractures are made. This is important information for the industry, because it supports that completing a well in multiple stages will allow for the best stimulation and ultimately better production. It was also determined that when multiple fractures interact, the impact can be both positive and negative. This can have a positive interaction that will increase the lengths and apertures or decrease the lengths and apertures in a negative interaction. 

The uncertainty and amount of estimation required in this study is much like many other aspects of the oil and gas industry. Since the majority of the work is done below the Earth's surface, measurements and calculations are rarely more than an educated estimate. With increasing regulations of hydraulic fracturing, will new technology soon be required to trace and measure the extent of hydraulic fractures?

-Kelsi