Prediction of Proppant Transport From Rheological Data

Abstract

Summary To ensure that a fracturing treatment will be successful, fracturing gels are formulated and viscosity parameters are measured. Steady-shear viscosity cannot measure the elastic component of viscoelastic fluids. Dynamic-oscillatory shear can provide measurements of elastic properties, but such measurements are made in the absence of any particles. A third technique, a slurry viscometer (SV), can incorporate particles and measure proppant-transport characteristics of the fluid. This paper compares all three measurement techniques for verification of transport prediction. Steady-shear viscosities of gelled fracturing fluids were measured using couette viscometers. Viscoelastic characteristics of the same fluids were measured with a dynamic-oscillatory viscometer to determine G' and G" moduli and crossover frequency. Proppant particles were added to these same fluids, and properties were measured with the SV to determine elastic transport and viscous transport times. Comparison of the three data sets shows that the elastic modulus, G', and crossover frequency have a high correlation with elastic transport times measured with the SV. Steady-shear viscosities greater than 10 sec-1 do not correlate directly with transport times. For years, dynamic-oscillatory measurements have been thought to predict particle-transport phenomena, but any correlations generated could be incomplete because particles are excluded from the measurements. The SV does include particles in the fracturing gel and can be used to verify trends of proppant transport indicated by G' and the crossover frequency. Introduction The intent of a fracturing treatment is to create a fracture, pump a slurry of proppant and fluid into a subterranean formation, and transport proppant into the fracture. The majority of fracturing treatments employ a viscoelastic fluid to enable proppant transport. Viscous fluids with varying degrees of elasticity include polymer solutions, crosslinked-polymer gels, foams, emulsions, and surfactant gels. It is common practice to measure the viscosity of such fluids with couette viscometers for both design and for quality-assurance/quality-control (QA/QC) purposes (Cameron and Prud'homme 1989; RP 13M/ISO 13503-1 2004). Service companies may also include gel breakers in test fluids for the purpose of estimating a time to reach minimum viscosity for transporting proppants. The elastic character of a fluid is an important component of viscoelastic fluids and their ability to transport proppant (Harris and Walters 2000; Geol and Shah 2001). Elastic character is not reflected in typical steady-shear measurements. A dynamic-oscillatory rheometer can generate signals to separately identify the elastic property. The elastic response is given by the G' storage modulus, and the viscous response is reflected in the G" loss modulus. The relative magnitudes of G' and G" vs. oscillation frequency produce a crossover frequency that can be used to infer an ability to support and transport particles, even though particles are not a part of the measurement. A third type measurement can be made with an SV (Harris et al. 2005; Walters et al. 2004). The inner and outer cylinders of a typical couette viscometer were replaced with a stator and outer cylinder, each having flags attached. These flags bypassed each other during rotation of the outer cylinder, and the cyclic shear induces a torque on the stator. Sufficient clearance between the rotating and static flags allowed proppant particles in the test fluid. Placement of the flags near the bottom of the SV make it sensitive to the larger torque values as proppant settles during the experiment. The torque value can be interpreted for both elastic and viscous properties. The SV gives an indication of slurry viscosity in the wellbore. However, the primary function of the device is to help determine the suspension properties of the fluid when it is in the fracture. This paper will examine these three methods of measuring fracturing-fluid properties. Steady-shear viscosities measured at shear rates greater than 10 sec-1 provide no direct indication of proppant transport, and the choice of minimum viscosity is arbitrary, on the basis of field experience with the fluid. The dynamic-oscillatory technique provides a method of estimating elasticity, and though proppant is not included in the test fluid, the crossover frequency can correlate with settling measurements. The SV apparatus enables a dynamic measurement of settling in fracturing-fluid slurry.

Formation Damage due to Drilling and Fracturing Fluids and Its Solution for Tight Naturally Fractured Sandstone Reservoirs

“…During the fracturing, a large volume of proppants is carried into the reservoir by the fracturing fluid, and this is aimed at creating a complex and highly conductive fracture network [15,16]. Gel is typically used as the fracturing fluid to maximize the proppant-carrying capacity because of its excellent viscosity and elasticity [17][18][19][20][21][22]. However, gel residuals can block fractures and pores at fracture faces, thus impeding the flow of hydrocarbon [23][24][25].…”

Study on Fluid-Rock Interaction and Reuse of Flowback Fluid for Gel Fracturing in Desert Area

“…Since the elasticity of fracturing fluid may enhance its ability to transport proppants [18,44,45], these divalent metal ions need to be shielded when the flowback fluid is recycled and reused.…”

“…To maximize the proppant-carrying capacity of the fracturing fluid, cross-linked gel is typically used [4,[14][15][16]; this is attributed to its excellent viscosity and elasticity that can prevent proppants settling [17][18][19]. However, fracturing fluid can interact with formation minerals and the connate water therein, resulting in high contents of suspended and dissolved solids in the flowback fluid [20,21].…”

Proppant Transport of Fracturing Gels is Influenced by Proppant Type

“…Base gel fluids were mixed in a blender with a buffer, salt, surfactant, and either guar or xanthan polymer in water (Harris et al 2005, Harris 2008, Walters et al 2004. A concentration of 9 lb/gal of 20/40 mesh proppant was added at moderately high speed with an overhead stirrer.…”