The 2 P's of Economic Reservoirs: Porosity and Permeability
It's amazing to me how many people think that oil and gas exists in large caverns or pools in the subsurface. On the contrary, hydrocarbons exist in rock or more specifically the pore space within the rock. This rock a can be any type of rock as long as this rock possesses porosity. Porosity is defined as the fraction of the void space in a material. There are many types of porosity (primary, secondary, fracture, vuggy, etc.) Porosity varies from 0% to 50% depending on the amount of alteration both physical and chemical the rock had been subject to. Typically, porosity values for the Gulf Coast both onshore and offshore range between 10% to 30% while interior basins in the U.S. have much lower porosity values of 5% to 15%. Many shales have high porosity values reaching 20% to 30%. The problem with shale (and coal) is the lack of interconnectivity between these very small pore spaces or the lack of effective porosity. Effective porosity are pore spaces that are connected to one another.
This interconnectivity is referred to as permeability. Take for example a Styrofoam cup. Styrofoam is very porous (which gives it excellent insulating properties) yet it has no permeability because your drink does lot leak out of it. Permeability is a measure of a porous material (i.e. rock) to transmit fluid. Without permeability you can't get oil or gas in or out of the rock. Typically, as porosity increases permeability increases and vice versa. Although there are exceptions, both porosity and permeability generally decrease with depth.
So how does a shale produce oil and gas to a wellbore if low permeability exists? Or to state it another way. How do we increase the effective porosity of a shale? This is where technology steps in. With the development of hydraulic fracturing, permeability can be produced in a rock that has very little naturally occurring permeability. It's this man-made increase in deliverability of hydrocarbons to the wellbore that makes an economic or an uneconomic well.
The important thing to remember is you must have both porosity and permeability to have a producible reservoir. Porosity is static. Permeability can be enhanced
