@RiazHaq and
@niaz have a lot of knowledge on this topic.
This basically involves horizontal drilling. Easiest way to understand is that standard oil & gas reservoirs are like a big porous rocks filled with hydrocarbons located 4,000 ft or more deep. When you vertically bore into it; because the reservoir is already under pressure from the ground above it; crude oil & gas flows out. When reservoirs are nearing exhaustion, you re-inject gas into the reservoir and strip it. Some Jurassic formations can be as much as 20,000 ft. underground.
Tight gas & Shale gas /oil reservoir is similar but the permeability of the rock is much less. Thus you can’t get hydrocarbons trapped into the rock by simply drilling vertically into it. Instead you drill a few vertical bores into the reservoir; then you drill a hole on the side and horizontally into the formation. When you pumped water mixed with some special stripping agents thru the horizontal bore and apply pressure, reservoir is crushed and hydrocarbons are squeezed out thru the vertical bores. Something similar to squeezing last remnants of tooth paste by pressing down hard on the sides of the tube. Additionally, most tight/shale reservoirs are sandstone instead of limestone.
It is the horizontal drilling technology that is currently monopoly of the US companies.
One must understand that porosity & permeability are two different things. Porosity refers more to the amount of space within the rock where oil/gas, water etc. can accumulate. Permeability is an indication of the ability for fluids (gas or liquid) to flow through rock.
Permeability is normally measured in milli Darcy (mD). Henry Darcy was a French Engineer who invented the method. 1 Darcy is roughly equal to 10 to the power of -12 meter square. In simple terms, a normal hydrocarbon bearing rock has a permeability 1000 mD (10 to the power of -8 cm2). Tight gas rock permeability is about 0.1 mD and Shale reservoir is at 0.001 mD (10 to the power of -14 cm2)
This is a new technology and my own knowledge is limited, thus regret inability to explain it without using technical terms.