This article by Dan A. Billman appears in the April 2021 issue of The PIOGA Press.

Since the Marcellus Shale play started 10-plus years ago, we have been using the terms unconventional and conventional incorrectly. And “we” includes the state, the public, the press and much of our own industry. First, let us notice that the title of this article includes the word “reservoirs,” not “wells,” not “production,” not “gas” and not a lot of other possible, relevant nouns.

Unconventional reservoirs are defined as those reservoirs (rock formations) that have low permeabilities and/or contain product that is very viscous (thick, like molasses) (Cander, 2012). Permeability accounts for the ability for a fluid to move through the rock. In the case of our Marcellus Shale, we are talking about moving natural gas through the rock, or how pore spaces are connected. Natural gas is not a viscous substance, so viscosity must not be the issue. If we were discussing heavy oil deposits or tar sands, those would be unconventional reservoirs based on the viscosity of the fluid in the rock. So, if our Marcellus Shale is an unconventional reservoir (and it is), it must be a low permeability rock formation (and it is). Permeabilities in the Marcellus Shale range from 130 to 2,000 microdarcies (Zagorski, et al., 2012) to nanodarcy permeability. A darcy is a measure of permeability. A microdarcy is 0.000001 of a darcy. It is really small! So, an unconventional well typically needs a permeability enhancement, during completion of the well. We will address completions a bit later.

From: Cander, H., 2012, What are conventional resources? A simple definition using Viscosity and Permeability, AAPG Annual Convention and Exhibition, April 22-25, 2012

So, where does horizontal/lateral drilling fit in? It is only a drilling technique. Over the last 15 to 20 years the industry has been perfecting the technique, which allows for much more of a reservoir to be encountered by the borehole. If drilling vertically in Washington County, an operator might encounter +/- 75 feet of Marcellus. But if drilling horizontally, they could encounter 5,000 to 10,000 feet, or longer, by drilling laterally through the Marcellus Shale. Although lateral drilling is an expensive process, it is a process that is incredibly impactful to the economics of a well, as the borehole will encounter thousands of feet of reservoir. Although the perfection of horizontal drilling is only a recent phenomenon, it is not “unconventional” and is not used only for unconventional reservoirs. Already in Pennsylvania, numerous lateral wells have been drilled through conventional reservoirs.

So, what about hydraulic fracturing? Hydraulic fracturing is a completion technique that can be utilized in both vertical and horizontal wells. There are numerous completion techniques, but for the purposes of this discussion, we are going to address hydraulic fracturing. Typically, conventional reservoirs do not need hydraulic fracturing, as the permeability is large enough that gas can flow freely from the reservoir. Numerous wells in Pennsylvania are “free-flowing” wells that did not require hydraulic fracture treatments. However, the vast majority of our legacy, vertical wells were indeed hydraulically fractured and are producing from unconventional reservoirs. The Medina/Whirlpool Sandstones in northwestern Pennsylvania are tight (low permeability) gas sandstones and require hydraulic fracturing to enhance the permeability, to allow the well to produce natural gas or oil in economic quantities. Pennsylvania’s legacy Upper Devonian wells throughout the southwestern and central parts of the state are unconventional reservoirs, too. Most Upper Devonian sandstone wells also require a hydraulic fracture treatment for economic production. Certainly, the hydraulic fracturing of a vertical tight gas sandstone (like the Upper Devonian or the Medina Sandstones) require a different fracture treatment than a lateral, thousands of feet long within the Marcellus Shale well. Thousands of feet of Marcellus Shale require significantly more water, sand (proppant to hold created fractures open) and chemicals used in pumping the “frac job.” The scale of the hydraulic fracture treatment has certainly increased from the vertical well to the horizontal well. Both the Marcellus Shale and the Medina Sandstone reservoirs are unconventional, as the permeability needs to be enhanced to obtain economic production.

Both horizontal drilling and hydraulic fracturing have seen incredible technological advances over the last 10 to 20 years. And as incredible as these advances have been, they are not the basis of an “unconventional well.” An “unconventional well” is defined by the rock formation encountered, not the drilling or completion techniques used for that encounter.

In the future, it is assumed we will see many more conventional reservoirs drilled horizontally. Commonly high permeability reservoirs (conventional reservoirs) are compartmentalized, meaning the natural gas or oil resides in separate compartments within the reservoir. And if one can drill to connect those compartments, a highly economic well may be the result. A conventional reservoir, drilled with horizontal drilling, is a “conventional well.” Again, the reservoir (rock formation) dictates that classification.

Ultimately, why does this matter? It matters because words mean things. Words are what the state uses to regulate our oil and natural gas industry. Words are used by the media to inform (or misinform) the populous. Government and the media, along with a lot of others, typically refer to the processes of horizontal/lateral drilling and high-volume hydraulic fracturing as an “unconventional well”―and this classification is incorrect. It is the rock formation that dictates the “unconventional” or “conventional” classification, and not how the well is drilled and completed.

Dan Billman, P.G./C.P.G., is president of Billman Geologic Consultants, Inc. in Houston, Pennsylvania.

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