On the Development of Shallow Hydraulic Fractures as Viewed Through the Surface Deformation Field: Part 1-Principles
Authors: Evans Keith 
Published: December 2007

Summary
An overview of the surface tiltmeter method of mapping shallow hydraulic fractures is presented. The paper is intended to provide the analytical background to a companion paper (Part 2, see Page 411) in which a number of case histories are discussed from the viewpoint of fracture growth and consolidation characteristics. In this paper we discuss the nature of the observations, outline the theoretical framework within which the data typically are interpreted, and comment on the significance of the various fracture parameters resolved. A summary review of other fracture-mapping technologies is presented also.

Introduction
Recently a considerable research effort has been directed toward the formulation of practical guidelines for predicting and possibly controlling the geometrical characteristics of hydraulic fractures. Substantial advances have been made through theoretical and laboratory studies of the various mechanisms thought to play a role in determining fracture geometry. Yet, the degree to which the results of these idealized simulations apply to commercial-scale in-situ hydraulic fractures remains uncertain. A fundamental problem is presented by the inherent difficulty in "observing" the geometry adopted by fractures formed in situ. This has impeded the collection of a substantial observational database pertaining to the form of commercial-scale hydraulic fractures against which the predictions of the various theories can be tested. The establishment of such an observational framework is an essential step in assimilating the large volume of information now available. During the past few years, the surface tiltmeter technique has been used to monitor ground deformations associated with the formation of a number of shallow [less than 450 m (1,500 ft] hydraulic fractures. From the data it is possible to infer broad-scale features of the geometrical development of the fracture. Quantitative estimates of fracture closure behavior following shut-in also can be obtained. In Part 2 we review the shallow-fracture data collected to date from the viewpoint of fracture growth and consolidation behavior. We begin by presenting a summary review of the fracture mapping technology currently in use or under development within the energy industry. The most detailed descriptions of in-situ hydraulic fractures have been obtained through mineback operations. In these experiments, fractures formed with dyed or tagged fluid subsequently are excavated to expose the decorated surfaces along which fluid penetration of the formation has occurred. Clearly this direct approach to penetration of the formation has occurred. Clearly this direct approach to fracture mapping is rarely possible in commercial operations, and it is usually necessary to deduce fracture geometry indirectly through measurements made either downhole or at the earth's surface. The passive seismic technique has enjoyed considerable success in mapping fractures formed in crystalline rocks, and the method shows practical potential for application to hydrocarbon reservoirs. The underlying principle is to determine the path of fluid penetration into the medium from the hypocentral distribution of microseismic emissions stimulated as a result of increased pore, pressure. A number of methods of sampling the seismic radiation field have been used, including surface-based microseismic arrays, and downhole-based hydrophones or three-component geophones. Systems for detecting the change in surface electrical potential that accompanies fracture formation have been developed although a satisfactory strategy for data interpretation remains to be established. Examples of fracture mapping using active seismic methods, in which an artificial seismic source is used to irradiate the fracture, have been presented by Stewart et al. and Aki et al. Overton has rigorously investigated the potential of oriented downhole superconducting magnetic systems (SQUIDS) potential of oriented downhole superconducting magnetic systems (SQUIDS) for detecting the distribution of magnetic tracer fluid injected into the fracture.
JPT
p. 406