Surveying
Starting off, building a well for a shale deposit looks a lot like building a regular well. Geologists typically start by surveying a site to find the right place to drill. It’s impossible to see underground, especially in the deep ocean, so other methods must be used to visualize what’s below the surface. On land, special trucks equipped with a vibrating pad can send pulses through soil and rock and then interpret the echoes that return. Sometimes, geologists may use explosives in small amounts to create pulses for them to read. It’s a bit like reading an x-ray in the way that certain types of materials (like shale filled with oil) return a signature that experts recognize.
After a site has been identified as a potential site, the company will usually drill a small exploratory well to get a better idea of what exactly is down there, and how deep it lies, in order to decide if there’s likely to be enough oil or gas produced to provide a healthy return on their investment. If things continue to look good, they’ll proceed to drilling the actual well.
What is Shale?
Most new well sites in the U.S. are located in shale plays. Shale is a very fine, sedimentary rock, which is formed where mud and sand have been deposited in layers and compacted over time. The resulting stone easily breaks into thin layers along the bedding planes. The same tendency can be found in slate, which is formed from shale. You may have seen pallets of decorative slate at your local garden center where it’s found in large, thin pieces and used to create stone walls, paving materials, or even roofing.
Some types of shale are formed from sediment that is mixed with organic material. Over thousands of years, that organic material, when exposed to intense heat and pressure, is changed to oil and natural gas, which remains trapped within the compacted sediment. Because the spaces where the oil and gas are located aren’t interconnected, there is no way for it to flow. This is why conventional wells are completely unsuited to extracting shale or tight oil.
When a shale oil or gas play is developed, the well must be “stimulated” so that the trapped oil and gas is able to escape the matrix of tiny pores and flow into the wellbore to be pumped to the surface. Often, several types of stimulation may be used together, depending on specific site characteristics, but the most common type of stimulation is hydraulic fracturing (fracking).
Hydraulic Fracturing
Hydraulic fracturing, in concept, isn’t a complex process. At its most basic, a large volume of fluid is pumped underground at extremely high pressure, causing the surrounding stone to crack, or fracture. These cracks form channels where trapped oil and gas can flow and collect for pumping to the surface. But by itself, hydraulic fracturing and other well stimulation techniques wouldn’t have made shale oil plays productive enough to develop.
Horizontal Drilling
In conventional wells, oil is tapped in large underground reservoirs that may span several hundred miles, where a relatively few vertical wells can be drilled, and the pooled oil is sucked up without too much fuss. That’s not the case in tight oil. Since the oil must first be collected efficiently in large volumes, it’s necessary for each well to cover a wide area; basic vertical wells just can’t do it.
Horizontal drilling is the other key technology that has made shale oil economically feasible. In this process, a typical vertical well is drilled in stages to the shale layer, which could be 10,000 or more feet deep. Once the appropriate depth is reached, the drill head is gradually turned approximately 90 degrees and drilling then continues horizontally along the shale layer, potentially for several miles, vastly expanding the well’s catchment area. The entire drilling process for a single well may take two months or more.
Fracking the Well
Once drilling is complete, steel pipes (casings) are dropped into the well and cement is poured into the space between the pipes and the rock to prevent breakage and leaks that could contaminate the local environment.
In the fracturing process itself, a large volume of specially formulated liquid containing proppants, slickwater, and other chemicals is pumped into the well and along the horizontal arm at pressures that may exceed 9,000 psi. This extreme pressure creates cracks and fissures or opens existing ones along the arm of the well. Sand, clay, or other proppants in the fracking fluid keep the new fissures open, while slickwater lowers the viscosity of the oil and additional chemicals allow the newly freed oil and gas to flow readily into the pipe. The fracking step itself will normally take from 3-5 days to complete, after which pumping can commence and the extracted oil transported to a refinery. A completed well may reliably produce oil and gas for decades after a single frack, even in shale plays which were previously considered inaccessible.
In some cases, the production of oil may slow or may even stop while analysis indicates that more oil is present. In these cases, well owners may choose to re-stimulate the well(s) by re-fracking or by using substances such as acid or gas instead of (or in addition to) typical fracking fluids.
