The Three Legged Stool – An Unconventional Gas Analogy
If you have ever sat on a three legged stool you will know how precarious it is to balance. Consider the challenge should one of those legs be removed. The development of technology that has unlocked the natural gas potential of fine grained rocks such as shale and sandstones shares a similar structure. Threekey technologies form the foundation upon which much of the unconventional resource industry sits. Horizontal drilling, multi-stage fracturing and micro-seismic modeling give industry legs.
The first of these three, and arguably the most important, is horizontal drilling. As exploration and development programs expand to include an unconventional focus, the need to enable the wellbore to access more of the potential oil or gas bearing zone grows simultaneously. Drilling a horizontal well allows more of the reservoir to be intersected; essentially creating a very large “pay” zone that has been intersected by a single wellbore. The horizontal portion, or “leg”, of the wellbore provides a platform through which complex multi-stage fracture stimulations can be deployed. Horizontal drilling for unconventional shale gas was first conducted in 2003 in the Texas Fort Worth Basin by Mitchell Energy.
Today, nearly 60 per cent of all wells are drilled horizontally in Canada and United States. While conventional reservoirs typically display properties which allow oil or gas to flow easily to the wellbore, the same cannot be said for unconventional reservoirs. Commonly, these rocks are “tight” with very low permeability (a measure of the ability for fluids or gases to flow in the rock) which will not allow sufficient flow of hydrocarbons without some form of fracture stimulation. The process of transmitting pressure by fluid or gas to create cracks or to open existing cracks in hydrocarbon bearing rocks many thousands of feet underground is known as hydraulic fracturing; the second leg of the stool. The purpose of hydraulic fracturing an oil or gas reservoir is to enable the oil or gas to flow more easily from the formation to the wellbore. Used extensively in unconventional gas stimulation today, this process was first developed in the 1950’s in Oklahoma. The technological evolution during the last 60 years has been so significant that most oil and gas wells drilled today are hydraulically fracture stimulated in some manner. Engineering principles are well understood and industry has a strong track record of safe development practices demonstrated in hundreds of thousands of wells drilled throughout North America. Technological improvements have enabled multi-stage fracture stimulations, in horizontal wellbores, to create economic reservoirs in tight oil and gas bearing rocks which were previously unprofitable.
Multi-stage fracturing, the latest evolution in hydraulic fracturing, involves the segmentation of the horizontal leg of the wellbore. Each stage is isolated using either plugs or packers so that fracture energy, applied to the wellbore from the surface fracturing equipment, is concentrated within each stage. The result is the creation of extensive fracture patterns which allow the oil or gas to flow more easily to the wellbore. Stimulation procedures are applied to each stage individually.
The final leg of the stool monitors stimulation. While not as impressive as the other two, in terms of equipment, this technology is equally important. During fracture stimulation operations, it is important to know where the fractures are being created in the reservoir. Monitoring of the fracturing process in real time can be accomplished using a variety of techniques. Pressure responses and micro-seismic monitoring are two such techniques. Measuring micro-seismic events that are occurring as the fracture stimulation takes place provides industry professionals with visual evidence that fractures are being developed both vertically and horizontally. Because these micro-seismic events are measured in real time, immediate adjustments can be made during the operation to ensure that the fractures created stay within the zone that has production potential.
Once completed, the micro-seismic model can be used to define the limit and reach of fracture stimulations in each wellbore and allow for optimal field development. Industry will continue to advance technologies that will broaden unconventional resource opportunities and, ultimately, improve productivity and recovery potential. All three legs of unconventional technology will be required as each provides a different measure of support for the development of Canada’s unconventional gas resources.