Numerical Simulation Of Hydraulic Fracturing Treatments With Low-Viscosity Fluids
Authors: Settari A. , Bachman R.C. , Morrison D.C. 
Published: March 2010

Numerical simulation of hydraulic fracturing treatments with low-viscosity fluids is discussed in this paper. Field experience indicates that some fluids of this type (e.g. liquid CO2) may be preferable in sensitive reservoirs where there is a risk of formation damage with conventional treatments. Simulation of the process, which is characterized by low viscosity and temperature effects on fluid PVT requires consideration of turbulence and its effect on fracture geometry, leak-off and proppant transport. The new model for low-viscosity fluids described here was successfully used to match CO2 frac field data with very short closure limes. Simulation results indicate effective propping of the fracture. The model is applicable to other low-viscosity situations.

Introduction

This paper presents engineering analysis of the fracturing process with low-viscosity fluids using numerical simulation, and the application of the model developed for field cases. The largest application has been the liquid CO2 process, with over 450 wells fractured to date. Field experience indicates that liquid CO2is the preferred fracturing fluid in formations where other fracturing fluids result in formation damage due to clay sensitivity, emulsions, relative permeability effects or gel residues. CO2as a frac fluid is usually Don-damaging to the formation and, in oil reservoirs, may have the additional benefit of dissolving in the oil and further improving the well clean-up. On the other hand, all low-viscosity fluids are limited in achieving fracture width and therefore the potential for fracture conductivity.

Another concern often raised with low-viscosity design is the excessive settlement and poor propping efficiency. However, we will show that this concern results from the use of the conventional fracturing model with the physics for viscous fluids which is not applicable here.

Conventional fracturing fluids rely on high viscosity for proppant transport and creation of large fracture width to maximize propped conductivity. Consequently, laminar flow concepts are used for pressure drop as well as proppant transport calculations. In contrast, low-viscosity fluids can be in turbulent flow (e.g. liquid CO2viscosity during pumping is less than one centipoises, and Reynolds numbers of 20 000 are typical). To account for the turbulence, concepts of equilibrium velocity and lateral slip velocity must be incorporated in the proppant transport model. Calculation of turbulent pressure drop in the fracture is required to obtain realistic fracture geometry. Also, the model must account for thermal effects during pumping and possible phase change to gas during closure.

Several case histories were investigated to determine the deficiencies of the conventional approach and to validate the new features of the model developed here. Results of two of these are described in this paper. Because the case histories are for the liquid CO2process, the description of the modeling is presented in terms of this fluid, However, the concepts are equally applicable to other types of treatments such as using ungelled water as a carrying fluid or for conditions in which CO2or N2 would be in the gaseous state during pumping.

The theory presented in this paper is also important for modelling fracture geometry for waterflood-induced fractures and the model has been verified in this application.