Improved Methods To Determine Hydraulic Fracture Height
Authors: Dobkins Terrell A. 
Published: December 2007

Temperature surveys are generally the most reliable method for estimating fracture height at the wellbore. However, there are times when the surveys are inconclusive. Several methods were used successfully to supplement postfracture surveys for a better estimate of fracture height. These methods included (1) laboratory and field measurements of thermal conductivity, (2) computer simulation of temperature surveys, (3) post fracture gamma ray logs to locate radioactive proppant, (4) radial differential temperature (RDT) log, and (5) noise log.

Introduction

In the last 5 years, massive hydraulic fracturing (MHF) has become a vital part of the oil and gas industry. Formations with microdarcy permeability now can be produced because of MHF treatments. Often the success of a treatment depends on whether or not the fracture height used for the design is the same as the actual fracture height obtained. It has become necessary to predict accurately fracture heights from offset well treatments. Therefore, the determination of fracture height has gained an unprecedented importance.Temperature logs have been used for many years to estimate wellbore fracture height created by hydraulic fracturing treatments. Agnew in 1965 presented the theory and the interpretation method of postfracture temperature surveys. This remains the most reliable fracture height determination method available. However, the surveys are sometimes difficult to interpret. The interpretation problems usually are caused by unusual temperature behavior adjacent to the treatment zone. Specifically, one of the unusual temperature behavior patterns on a postfracture survey is a warm anomaly of 10 to 80 degrees F (-12 to 26 degrees C) above the treatment zone. This anomaly may extend as far as several hundred feet (about 60 m) above the intended zone. An example of this is shown in Fig. 1. Historically, the fracture top usually has been estimated as the bottom of the anomaly. This interpretation, however, may not always be correct.The problem of the warm anomaly is widespread geographically. Examples have come from east and south Texas, Oklahoma, Colorado, and southern Wyoming. The knowledge of fracture height is so important and the problem so widespread that a research effort was begun to provide a better understanding of postfracture temperature surveys.Two basic approaches were used. First, laboratory measurements, computer simulations, and prefracture temperature surveys were used to study the effects of thermal conductivity variation. Second, three production logging techniques were used to substantiate the interpretation of temperature logs. The logs used were the RDT, the noise log, and the gamma ray log to detect radioactive proppant. All of these techniques added to the understanding of postfracture temperature behavior. The overall result was better evaluation of fracture height from temperature surveys. An important consideration is that all methods currently available for measuring fracture height have a small radius of investigation about 2 ft (0.6 m). Currently, there is no way to determine the fracture height deep in the formation.

Theory

The thermal conductivity variations in the earth are important in interpreting postfracture temperature surveys.

JPT

P. 719^