The Lower Huron Shale can take claim as one of the earliest discovered sources of natural gas. As with all the shales though, extracting its reserves has been the challenge. To start, the physical properties of shale are not the same as typical sandstones, limestone, and siltstones that are targeted to produce natural gas. Along with this, the mechanisms for which the hydrocarbons are stored in place and transported are unique as well.
Shale is notorious for having ultra-low permeability (µD) and because of this, the primary storage and transport of hydrocarbons comes through the series of natural fracture networks within the shale. Fracturing the shale then becomes a search for the natural fracture network that is holding the hydrocarbons.
After drilling is completed, the question becomes exactly how do we fracture this well to achieve the best results? There are several ways to stimulate the wells that have been drilled in the Lower Huron. They range from straight nitrogen stimulation to varying quality foam stimulations. These two main methods of fracturing can then be broken down into many different style stimulations by changing rates, foam qualities, sand volumes and nitrogen volumes.
A couple of things to look at when designing a stimulation job is depth, field of play, thickness of the zone, and its gamma ray reading on the log. This paper will discuss fracture styles and which ones seem to perform better in wells given certain characteristics and what might be changed to produce the best results.
The Lower Huron Shale has been a mainstay in the Appalachian Basin in the production of natural gas since the early 1900's. With the history of producing the Lower Huron going back as far it does, it is evident that most fracturing techniques have been used to try and stimulate this zone. As technology advanced and the understanding of how natural gas was stored and transport to the wellbore increased, the practice of stimulating wells became more technical and controlled. These advancements came from natural production, shooting nitroglycerine, fracturing with different based frac fluids and foam fluids. All these practices have led the industry to where we are today in the processes and manners in which the Lower Huron is stimulated and what we consider best practices.
The Lower Huron Shale was formed in the Devonian approximately 350 million years ago. Part of the eastern half of the United States was covered by a shallow sea. Organic-rich material accumulated on the sea floor and was eventually covered. With the covering of the organics came high temperatures and pressures and this in turn helped produce the hydrocarbons. The hydrocarbons present are a direct correlation to the amount of organics that were laid down. These layers of organics fall in between layers of minerals producing what we call shale. The color of shale varies according to the amount of organics that were present in the formation. Typically the darker the shale the more organics that were present therefore the more gas that is in place.
Natural gas is stored in rock pores, as absorbed gas, or in the natural fracture network with in the shale. Shale, unlike conventional reservoirs, presents the problem of having low matrix porosities and extremely low permeability. These two issues are what make shale classified as an unconventional reservoir. Conventional natural gas reservoirs rely on higher porosity and permeability to store and move the hydrocarbons from far a field to the wellbores to be produced.