Field Investigation of Heat Transfer in Hydraulic Fractures and the Effect of Heat Transfer on Fracturing Fluid Design
Authors: Craig David P. , Brown Ted D. , Ely John W. 
Published: December 2007

Abstract
Fracturing fluid temperature is a key variable in the design of hydraulic fracturing treatments and the formulation of fracturing fluids. Heat transfer in a hydraulic fracture dictates the fluid formulation and the concentration of chemical "breakers" used to degrade the fluid and maximize proppant pack conductivity. This paper contains the results of an investigation of heat transfer in hydraulic fractures, and documents the recording of bottomhole temperature during Piceance Basin Mesaverde fracturing treatments and during immediate flow back ("forced closure"). Bottomhole temperature was measured with a gauge set in a perforated interval, and the data shows minimal "cool down" of fracturing fluids, i.e., flow back temperatures increased rapidly to near bottomhole static temperature. Computer simulations are also presented and a fracturing fluid design methodology is suggested which balances fluid rheological requirements with degradation requirements, for maximizing proppant pack conductivity.

Introduction
The rate of heat transfer to hydraulic fracturing fluids is a key variable in the viscous properties and degradation of typical water based fluids. Fracturing fluids are formulated based on (1) chemical compatibility with the formation and reservoir fluids and (2) the desired rheological properties. Rheological and degradation properties of aqueous fracturing fluids are a function of chemical composition, temperature, time, and shear history. In general, fracturing fluid rheological properties and degradation can be controlled by altering the chemical composition for a given temperature and shear history.
Concentrations of polymer, crosslinkers, buffers, and breakers can be formulated in a delicate balance to deliver a fluid that will maintain sufficient viscosity to transport proppant during fracture growth and support proppant during fracture closure. When temperature estimates are incorrect, the fluid formulation can result in either a fluid that degrades too quickly, thus proppant transport is jeopardized, or a fluid that degrades slowly, which can lead to poor fracture fluid cleanup or, in the worst case, severe proppant pack conductivity damage.
In an effort to confirm the design philosophy used in Piceance Basin Mesaverde treatments and to examine heat transfer in hydraulic fracturing treatments, bottomhole pressure and temperature gauges were set in perforated Mesaverde intervals to record during a treatment and during forced closure. The pressure and temperature data recorded were then compared with the predictions of a 3-D fracture simulator and used to history match the treatment.

Background
Prior to the development of mathematical models for predicting fracturing fluid temperature versus location in a fracture, fracture designs and fluid formulations were based on assuming the fluid heated instantaneously to static bottomhole temperature. At the time and with the the level of chemical technology available, the ability to design treatments and formulate fluids for deep, hot wells was limited. At the same time, a body of evidence was being collected to suggest formation "cool down" occurred during fracture treatments as cool fluid was injected into hot reservoir rock.
Temperature surveys run after fracture treatments indicated "significant temperature anomalies" in the stimulated zones. When cool fluids were injected, a cooled interval would be indicated from temperature surveys run several hours after the treatment. When a large enough temperature difference existed between injected fluid and formation temperatures and no "backflow" or flow back occurred, an estimate of fracture height could be made.
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