process is used for extracting shale gas, but there has been movement toward extraction from oil-bearing shales as market demand increases.
Hydraulic fracturing relies on the sophisticated use of pumps to create a pathway into the rock. Sand or an engineered ceramic material is then placed into the pathway to keep the cracks open after the hydraulic pressure is removed. Mr. Cole stated that the typical dimension of a hydraulic fracture is microns to 0.25-inches wide, 500- to 2,000-feet long, and 20- to 400-feet high, depending on the geography.
Thirty years ago, there was typically 1 well for every 40 acres, which translated to approximately 16 well locations in a square mile, connected by gravel roads. With advances in technologies, there is often now one location per square mile and all the wells are drilled in the direction of the least principal horizontal stress. For example, wells near mountainous regions will run parallel to the mountain. Newer technologies allow engineers to use a steel drill pipe to turn and bend in order to orient the well in any direction. Mr. Cole noted that clustering wells to one surface location has a number of advantages, including reducing the number of trees cut down, reducing traffic, and reducing emissions. Nonetheless, each drilling site will affect the surface geography with wells, roads, and supporting facilities.
Mr. Cole explained that careful well planning is crucial to isolate the fluids in the well and avoid contamination of drinking water. Different companies have varying strategies for water management; for example, Shell captures the water used in the fracturing process and places it into tanks with secondary containment for recycling. Other companies may use lined pits to capture this fluid, and the location is chosen based on knowledge of the depth of the groundwater and other local receptors. When planning a new well location, Shell measures the resistance to an electrical current in order to determine where fresh water is located, so that efforts can be taken to protect this water when building the well.
The first step in drilling is to put in a conductor pipe, which is a structure designed to carry the load, akin to the foundation of a house (see Figure 3-1). Additional steel casing strings, blowout preventers, and other equipment are installed through the surface using a drilling mechanism and are cemented into place. The surface casing string, which consists of steel pipes coupled together with screws, is lowered into a drilled hole that runs the depth of the freshwater layer, to protect the groundwater. Cement is pumped down into this casing to seal it into place. Check valves on the bottom of the casing will help to prevent contamination and flowback and preserve isolation of the groundwater. The casing and the cement that make up the wellbore are tested to meet strict specifications of integrity before the drilling begins. Intermediate casing may be necessary, depending on where the drilling takes place.