Hydraulic Fracture Diagnosis Using Chemical Tracers
Authors: Gardien C.J. , Pope G.A. , Hill A.D. 
Published: December 2007

Abstract
In this paper, the use of a single-well tracer test for hydraulic fracture diagnosis is explored. Existing methods for hydraulic fracture diagnosis (e.g. well logging and pressure transient testing) often do not provide accurate or unique solutions, so improved methods are needed. The principle behind the use of tracers for hydraulic fracture diagnosis is the dominance of the fracture on the flow field in a hydraulically fractured reservoir and the strong influence of fracture parameters on the flow field. This will be reflected in the tracer response curves of a single-well tracer test in a fractured well, making the tracer test diagnostic of the fracture and its parameters.
Several kinds of tracers (conservative, sorbing, reactive and partitioning) have been tested for their diagnostic value by numerical modeling. The major factors of influence on the tracer response have been identified. These factors are combined in a ratio, the tracer influence ratio, which can be used for the estimation of the product of fracture length and fracture height. The tracer behavior is dispersion dominated, which implies that the tracer dispersivity must be known for the tracer test interpretation, or it could be estimated from another tracer test in the same well or another well in the formation.
Extensive sensitivity analyses have indicated that the tracer test is only mildly sensitive to a large number of variables which is beneficial for the purpose of hydraulic fracture diagnosis. A conservative tracer appears to be the best candidate for hydraulic fracture diagnosis using a single-well test since it has the largest diagnostic value when the tracer test is optimized.

Introduction
A significant number of oil and gas wells are hydraulically fractured to enhance hydrocarbon production. Several methods for the diagnosis of hydraulic fractures are available in order to estimate fracture dimensions. Knowledge of hydraulic fracture parameters is necessary for evaluation of the fracture treatment and treatment design and in order to be able to predict the production potential of the fractured well. The existing methods for hydraulic fracture diagnosis (e.g. logging, pressure transient testing) have several drawbacks, mainly involving the accuracy and the uniqueness of the estimates (Davis et al., 1995; Gidley et al., 1989). This leaves considerable room for improvement of hydraulic fracture diagnosis. This research explores the potential of the application of existing tracer technology in order to improve hydraulic fracture diagnosis.
Tracers are markers of fluids, either the fluid they are introduced with or fluids existing in the reservoir. Research has led to the identification of suitable chemical tracers for both liquids and gases.
Tracers have been used in the oil field for several decades. The main purpose of a tracer test is reservoir characterization including the identification of trends of fluid movement, layering, flow barriers and saturations (Zemel, 1995). In a single-well test, the tracer is introduced and produced through the same well. The result of the test is the measured tracer concentration in the produced fluid, which can be plotted versus either time or produced volume. A single-well tracer test is mainly used for the estimation of residual oil saturation, although other variables can be estimated with this technique as well e.g. heterogeneity (Descant et al., 1989) and wettability (Ferreira et al., 1992).
Tracer research at the University of Texas has led to the development of a numerical model, UTCHEM, used to design and evaluate tracer tests (Allison et al., 1991; Ferreira et al., 1992; Maroongroge et al., 1995). This model is a multiphase, multicomponent, three-dimensional finite-difference chemical compositional reservoir simulator. This paper extends these studies to include the use of tracers for the diagnosis of hydraulic fractures.
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