We’ve seen that geologists are able to predict the orientation and size of fractures if they have knowledge of the local stress fields and the material properties of the rocks being fractured. One of the most important, and difficult, parts of developing a hydraulic fracturing design is measuring these variables in the real world.
Since hydraulic fracturing depends on exceeding the forces compressing the rocks, a detailed quantification of these forces is needed to assess how much pressure will need to be applied and in which direction fractures can be expected to propagate.
In addition, knowledge of the natural pore pressure within a producing formation is required to predict how much additional pressure will be needed to fracture the rock.
Finally, geologists need to know something about the material properties of the rock being fractured – things like Poisson’s Ratio, Young’s Modulus, composition, porosity, and permeability.
Here’s how they typically obtain the needed information:
- Quantifying the Stress Field: Information about the stress field can be found by studying the local geologic setting and looking at well failure or deformation in existing wells. Geologists are most interested in learning the directions of least and greatest stress.
- Measuring Pore Pressure: Pore pressure measurements should have been obtained during formation evaluation. By looking at the pressure logs, geologists should be able to figure out how much additional pore pressure will be needed to fracture each zone.
- Measuring Rock Properties: Mechanical rock attributes can be measured in the lab using core samples taken during formation evaluation or extrapolated from well logging curves.
Determining Pressure Requirements
Hydraulic fracturing is like trying to fill a leaky water balloon with a long straw. A bizarre metaphor, to be sure, but a useful one.
The balloon represents a fracture. As water is added to the balloon, it expands. This corresponds to what happens when fracturing fluids are pumped into a formation.
If the balloon doesn’t leak, the total volume of space in the balloon (which represents the fracture) is exactly equal to the amount of water (which represents the fracture fluids) added through the straw.
However, if the balloon is leaky, it will only expand if water is added at a faster rate than it leaks out. As the balloon expands and the pressure increases, the water will leak out faster and faster due to the fractures increasing in length and width as they move through the rock.
If water stops being added to the balloon, the pressure within will fall and the balloon will deflate as water leaks out.
Since most rock units have some degree of permeability, a fracture system is like a leaky balloon – some of the fracturing fluid escapes through the sides of the fracture rather than contributing to its volume.
The rate at which fracturing fluids escape through the walls of the fracture is called the leak-off rate. In order to increase the volume of fracturing fluids in the fractures and make them grow, water must constantly be pumped in faster than it escapes.
As the fractures grow, the surface area across which fluids can escape becomes greater, and the required rate of pumping to maintain pressure increases. At some point, the total pumping rate required will overwhelm the pumping capacity of the pressure pumping trucks. The best way to maximize the size of the fractures is to make it harder for liquids to escape.
One way to decrease the escape of fluids for a given fracture size is to increase the viscosity, or resistance to flow, of the fluid. This technique is used frequently in hydraulic fracturing. We’ll talk about it more in the next section.
What about the long straw in our balloon example?
The straw represents the borehole down which the pressurized fluids are pumped. A high pressure applied at the wellhead slowly decreases with distance because of frictional forces between the fluids and the borehole walls. The biggest sources of friction are small openings such as perforations or even the wellbore casing.
This affect means that the pressure measured at a surface gauge may be significantly greater than the pressure actually experienced by a deep rock unit. Operators need to take this into account when designing a fracturing job.
One problem hydraulic fracturing crews often encounter is screening out or sanding off. This occurs when proppant materials clog up narrow openings. The increased friction leads to a spike in pressure at the surface. If the pressure continues to climb, the operation must be shut down so that the openings can be cleared of debris.
Given the complexity of a hydraulic fracturing system, it is a challenge to determine the correct pressure capacity and pumping rate for a hydraulic fracture design. Detailed knowledge of the stress field and the formation permeability are essential.
Once the required pressure and rate have been determined, equations are used to determine how many pump trucks to contract.
Images: “Equation Overlay” by Top Energy Training; “Water Balloon” by sampsyseeds via iStock