Pressure analysis in hydraulic fracturing is used to understand fracture behavior during and after fluid injection. By analyzing pressure response over time, important parameters such as fracture geometry, fluid leakoff, and formation properties can be estimated.
During injection, pressure typically increases as the fracture initiates and propagates. The pressure required to create the fracture is called the breakdown pressure, which depends on in-situ stresses, rock strength, and pore pressure. Once the fracture is open, the pressure stabilizes and reflects the balance between fluid injection, fracture growth, and leakoff.
A key parameter in pressure analysis is the net pressure, defined as the difference between the fracture pressure and the minimum horizontal stress. Net pressure controls fracture width and propagation, with higher net pressure leading to wider fractures.
After injection stops (shut-in), pressure declines over time. This pressure falloff behavior provides valuable information about the fracture and reservoir. The decline is mainly controlled by fluid leakoff into the formation and fracture closure. Early-time pressure decline is dominated by leakoff, while late-time behavior reflects reservoir properties.
One commonly used method is pressure falloff analysis, where pressure is plotted versus time (often on log-log or square-root-of-time plots). These plots help identify flow regimes and estimate parameters such as leakoff coefficient, fracture area, and permeability.
Another important concept is fracture closure pressure, which represents the stress at which the fracture begins to close. It is typically identified from changes in the slope of the pressure decline curve and is critical for determining in-situ stress conditions.
Pressure analysis is also used in diagnostic fracture injection tests (DFIT), where a small volume of fluid is injected and the pressure decline is monitored. DFIT data provides insights into formation permeability, leakoff behavior, and stress conditions without performing a full hydraulic fracturing treatment.
Overall, pressure analysis is an essential tool for evaluating fracture performance and optimizing hydraulic fracturing design, as it links measurable field data to subsurface fracture behavior.
