Uplifts and Tilts at Earth's Surface Induced by Pressure Transients From Hydraulic Fractures

Abstract

Summary The pressure transient that spreads through the formation during and after a hydraulic fracture treatment pressurizes the formation and induces a certain "swelling." This swelling and the accompanying uplift at the earth's surface can be estimated with poroelastic theory. The uplift may produce a significant signal in tiltmeter devices placed at the surface. Six fracturing treatments in liquid-saturated reservoirs, all with tiltmeters to diagnose the fractures, were analyzed. In five cases, routine tiltmeter analysis postulated a horizontal fracture component (in the remaining case, a near-vertical fracture of dip 70 to 85° probably could also be interpreted by a vertical fracture with a subsidiary horizontal component). In three treatments, however, theoretical tilts calculated from the pressure transient can account for the magnitude of the tiltmeter signals, which had previously been explained by a horizontal fracture component. This result is important because the formation swelling and surface uplift associated with the pressure transient from a hydraulic fracture treatment are not usually deconvolved from the raw tiltmeter signals before their interpretation in terms of hydraulic fracture geometry. In some cases, this may be necessary. Otherwise, the shape/size of inferred horizontal and vertical fracture components may be in error; i.e., neglect of the pressure-transient effect may cause overestimation of the size of the horizontal fracture component. At worst, neglecting the pressure-transient effect may result in an inferred horizontal fracture component where one does not actually exist.