Importance Of Fluid Temperature Calculations In Fracture Treatment Design For High Temperature Wells
Authors: Clark P.E. , Cloud J.E. , Warembourg P.A. , Yunko C.G. 
Published: December 2007

The prediction of fracture fluid heat-up rate is important in treatment design for high temperature wells. There are several methods available to calculate fluid temperature distributions in fractures. Since each method yields a somewhat different perspective, comparisons must be made based on field perspective, comparisons must be made based on field results. Two of the primary calculative procedures will be compared with each other and with data collected on a number of fracturing treatments. The general result is that some fluids perform better than would be expected if only laboratory data and job design calculations are considered.

Since an increase in temperature of frac fluids results in a loss of viscosity, a faster-than-expected heat-up rate might cause one of two problems: first, designing a treatment with fluids that have a high viscosity at bottom-hole static temperature could cause more permeability damage, slower cleanup, and higher cost; and second, at extremely high temperatures, design might indicate that no available fluid would function as desired. This latter situation could result in no attempt to stimulate what might be a responsive well. Therefore, calculation of fluid temperature during a treatment at several points in the fracture, is an extremely important task.

Introduction

The search for oil and gas has extended into deeper and hotter reservoirs. The more extreme conditions encountered in these reservoirs have resulted in increased importance of complete treatment designs to help ensure successful treatments.

Fluid heat-up computations are an integral part of the design calculations. Very serious problems can result from a treatment design that is not based on estimates of the fluid temperature in the fracture. If an extremely viscous fluid is used, poor cleanup can result; or an early screen out can occur due to the use of a fluid with not enough viscosity. Idealy, a frac fluid should have adequate viscosity to carry propping agents during the treatment but not so much propping agents during the treatment but not so much viscosity and stability that rapid clean-up is impaired. To balance these two important needs, a great deal of care must be taken during treatment design.

Heat transfer to the frac fluid from the formation during a fracturing treatment affects the fluid in a number of ways. Probably the most important short tern effect is the loss of viscosity that occurs as the fluid heats up. This viscosity loss can be attributed to two factors:temperature thinning; andgel degradation. For short time periods, temperature thinning is the controlling periods, temperature thinning is the controlling mechanism; the effect of gel degradation normally requires a longer period of time to become a factor.

Gel degradation can be inhibited by the use of stabilizing agents which further limit the effects of gel breakdown during the early stages of the treatment.

Crosslinking, which increases the viscosity, has been used as a method to circumvent the problem of temperature thinning. Crosslinking of water-based polymers can add one or two orders of magnitude to polymers can add one or two orders of magnitude to the initial viscosity of the gel, thereby, greatly increasing the upper temperature limit of the gel. The ability to increase the viscosity without increasing the gelling agent concentration it important for good after-frac clean-up and rapid well turn-around.

COMPUTATIONAL METHODS FOR FRAC FLUID HEAT-UP

Two methods for predicting fluid-temperature distribution in a vertical fracture were investigated for this study. These models were selected because of their respective attempts to correlate critical transient heat transfer effects occuring during the fracturing process. Heat transfer in the fracture is largely governed by conduction and convection from the formation via the fracture face. Fluid which leaks off into the formation tends to block the conductive flow of heat from the formation to the fluid remaining in the fracture.