Abstract

Knowing the width of a hydraulic fracture behind casing can be useful in evaluating both reservoir performance and fracture design methods. This paper presents a method to obtain the widths of hydraulic fractures behind casing using radioactive, isotope-traced proppants. A tool-specific relationship between the gamma ray flux detected in a wellbore and the fracture width was developed using Monte Carlo simulation of gamma ray transport around a wellbore. This method provides fracture width estimates with a vertical resolution of about one foot. The method has been successfully used in the field and compares favorably with other methods for evaluating fracture widths.

Introduction

Hydraulic fracturing is an important stimulation process for many petroleum reservoirs. Unfortunately, many hydraulic fracturing treatments fail to provide the optimum stimulation because they are not properly designed for the existing in-situ conditions. These failures have been very costly to the petroleum production industry.

Two approaches are being used to improve hydraulic fracture design: improved computer modeling and improved wellbore diagnostic techniques. Complex 3-dimensional computer models have been developed that include in-situ petrophysical properties and in-situ stresses. These models can provide estimates for the created and propped fracture length, height, and width, as well as local proppant concentrations (lbm/ft2) and fracture conductivity. Because of the large number of assumptions required with these models, they are of uncertain value if they are not properly validated. New wellbore diagnostic techniques have been developed that use multiple radioactive tracers which can provide some of this needed data.1–6 The use of radioactive tracers to measure fracture height is well documented.5–6

A critical parameter needed for optimizing hydraulic fracturing treatments is the local propped fracture width. To measure the fracture width behind casing requires a sensing technique that can "see" through the casing. One such technique is to use a radioactive-traced proppant and measure the resulting gamma ray flux in the wellbore. Radioactive tracers have been utilized for over 35 years to evaluate hydraulic fracture effectiveness. The development of the Zero Wash tracer particle has eliminated tracer wash-off and residue in the wellbore, enhancing the confidence that any gamma ray signal comes from the propped fractures. This intermediate strength particle is resistant to abrasion loss and crushing and can be tailored to be used in hydraulic fracturing, acidizing, cementing, gravel and frac packing, as well as a wide range of other applications.7

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