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

Abstract

Summary The pressure transient that spreads through the formation during and after a hydraulic fracture treatment pressurizes tone 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 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. Introduction The earth's surface can be uplifted and tilted by the pressure transient associated with a hydraulic fracture treatment and by the fracture itself. The question addressed here is whether the pressure-transient effect gives a significant tilt at the earth's pressure-transient effect gives a significant tilt at the earth's surface. Our results show that in three of the six fracture treatments examined, the pressure-transient effect accounted for the surface tilt, which was previously interpreted by a horizontal fracture. That is, previous interpretations in terms of a horizontal fracture may be questionable. In the other three fracture treatments, however, the theoretically calculated tilt is less than that observed, and in these cases interpretation by a horizontal fracture component may be valid. Fig. 1 shows the difference in surface uplift between horizontal and vertical fractures at depth. Fig. 2 shows the difference in the tiltmeter vector patterns corresponding to Fig. 1 (the direction of the tilt vector is maximum "downslope"). The question addressed here is whether the pressure transient associated with a hydraulic fracture treatment can cause tilts at the earth's surface that resemble, in both pattern and magnitude, those at the top of Fig. 2. If the reservoir has a high diffusivity, a pressure transient associated with a hydraulic fracture treatment will travel relatively far from the fracture (Fig 3). The low compressibilities of liquid reservoirs enhance the diffusivity and pressure-transient effect. The end view in Fig. 4 illustrates the pressure transient moving from the fracture faces, thereby pressurizing and slightly expanding the reservoir. When transmitted to the earth's surface, this expansion results in an uplift in the general area above the fracture. The reservoir uplift will be slightly elongated, corresponding to the pressure transient of Fig. 3. The degree of elongation depends on the pressure transient of Fig. 3. The degree of elongation depends on the length of the extending fracture and on the reservoir diffusivity. Nevertheless, the overall effect at the surface will be a more-or-less radial tilt pattern resembling that of a horizontal fracture (Fig. 2). Under the assumption of uniaxial strain in the reservoir, the sideways effect of the pressure results in a backstress. This backstress increases the horizontal stress in the reservoir and therefore the closure stress acting on the fracture. Both surface uplift and an increase in fracture closure pressure have been seen clearly in two fracture treatments: San Juan No. 3 stimulation of a coal seam in the San Juan basins and the Mounds experimental stimulation of a shallow sandstone. In both cases, however, the surface uplift was interpreted by a horizontal fracture without consideration of the pressure-transient effect.