How Instantaneous are Instantaneous Shut-In Pressures?

Abstract

Abstract Instantaneous shut-in pressure is a term commonly employed, but often used indiscriminately. Shut-in pressure is conventionally regarded as a static pressure is conventionally regarded as a static pressure-equilibrium condition existing between a slightly pressure-equilibrium condition existing between a slightly open fracture and the in-situ stress regime. Several techniques for determination of this pressure level, from pressure-time records, have been presented. This report reviews the various definitions and mechanisms for shut-in pressures and critically evaluates the general validity of this concept as a stress parameter indicator. It also outlines the factors parameter indicator. It also outlines the factors which may affect the positive identification of a distinct shut-in (i.e., leakoff, nonuniform fracture azimuth, presence and distribution of proppant and fracture length). The report also provides an alternate interpretation technique, allowing for discrimination between effects of this nature and the existing in-situ stress field. The alternate technique is corroborated by precise laboratory testing and by back analyses of precise laboratory testing and by back analyses of various field situations, including micro-hydraulic fracturing and hydraulic fracturing treatments. Introduction In 1957, Hubbert and Willis stated, "Once a fracture has been started, the fluid penetrates the parting of the rocks and pressure is applied to the parting of the rocks and pressure is applied to the walls of the fracture. This reduces the stress concentration that previously existed in the vicinity of the wellbore and the pressure, alpha p, required to hold the fracture open in the case of a nonpenetrating fluid is then equal to the component of the undistorted stress field normal to the plane of the fracture. A pressure only slightly greater than this will extend the fracture indefinitely, providing it can be transmitted to the leading edge. "The minimum down-the-hole injection pressure required to hold open and extend a fracture is therefore slightly in excess of the original undistorted regional stress normal to the plane of the fracture." Extending this concept, Kehle suggested that "…if the well is shut in immediately upon completion of the fracturing treatment, the pressure will quickly stabilize to a value just slightly greater than the far-field stress acting perpendicularly to the fracture plane. If this stabilization pressure should be greater than the perpendicular regional stress, a simple calculation reveals that stress concentrations of such great magnitudes exist at the ends of the crack that it would certainly propagate. This would increase the volume of the fracture, and, because no additional fluid is supplied to the system, the pressure would decrease. The final stable pressure would pressure would decrease. The final stable pressure would be, in fact, that pressure sufficient to maintain the fracture but insufficient to cause further propagation, and it would be approximately equal to the propagation, and it would be approximately equal to the perpendicular far-field stress." perpendicular far-field stress." Adhering to these principles, shut-in pressures have been routinely used to predict in-situ stress regimes. Unfortunately, the situation is somewhat more complicated. For appropriate use of pressure-time data for the prediction of stresses pressure-time data for the prediction of stresses influencing fracturing and production, it is important to appreciate the controlling factors. It is further important to interpret post-pumping pressure records accurately. The accuracy of interpretation is particularly important for containment considerations. particularly important for containment considerations. The flippant nature of the title of this paper highlights the difficulties in stress level determination from pressure-decay curves and further indicates the importance of precise nomenclature. TERMINOLOGY AND DEFINITIONS Shut-In Pressure (Refer to Figure 1) Simplistically, shut-in pressure is the pressure, as a function of time, after the termination of pumping.