TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractEffective fracture length is the portion of the propped fracture that cleans up after hydraulic fracturing procedure and contributes to well productivity. Studies indicate that this effective length is often less than 10% of the total propped fracture length. A large portion of our fracture stimulation dollars are wasted! This paper presents a comparative well study performed in the Cement field in south central Oklahoma. Stimulation of the Springer Sands using hydraulic fracturing with conventional low polymer fluids was compared with the use of low molecular weight polymer fracturing fluid. The depth of the three Springer Sands (Cunningham, Britt, and Boatwright) ranges from 12,500 feet to 15,500 feet and have an average permeability range of 0.1 to 5.0 md. This evaluation includes several components.Well production history matching and pressure analyses are used to determine effective fracture length. Results of these analyses are compared with calculated values based on laboratory generated cleanup data for the two fluid systems. Flowback rate, pressure, accumulated volume, viscosity, and polymer content were collected following the fracture stimulation treatments.The fluid systems compared in this study are a conventional low polymer system with gel breakers and a new, low molecular weight polymer system that requires no breakers. Both fluids use borate cross-linking chemistry. The low molecular weight fluid system creates transient, high molecular weight polymer chains at higher pH conditions. After exposure to the formation minerals, the pH drops and it reverts to a clean, nearly Newtonian, low viscosity fluid that causes little conductivity damage.The results of this study show that the use of low molecular weight fracturing fluid provides significant improvements in the effective fracture length over conventional low polymer fracturing fluids. Simple engineering tools have also been developed to evaluate both fluid and proppant selection and job design to achieve improved well performance. It also demonstrated that improved recovery of the fracturing fluid can be achieved at excellent rates without the use of conventional gel breakers.

Effective proppant placement during hydraulic fracturing is essential to obtain maximum stimulation effectiveness. Understanding proppant placement requires the understanding of the time and space dependent dynamics of proppant motion in fluids, which include the phenomena of proppant transport, bridging, settling, and resuspension. This paper proposes a laboratory test method that can be used to investigate any aspect of proppant dynamics in a variety of channel configurations and fracturing fluids. 3D printing technology is used to rapidly manufacture channel flow devices of various dimensions. After a 3D printer is available, such manufacturing is extremely inexpensive with rapid turn-around times. These channels, in conjunction with laboratory scale pumps and blenders, are used to investigate proppant transport and bridging, settling, and resuspension in various fracturing fluids. Several different channel configurations, ranging from uniform width to uniform tapered, are used to investigate the dynamics of small and large diameter proppants with fluids ranging from water to linear gels. The results from these experiments are compared with numerical models for validation, and in some cases, calibration of model inputs, that can ultimately lead to improved fracturing treatment design and understanding. In addition, the paper provides a comparison to existing data (Patankar et al. 2002) to validate settling and resuspension models.

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.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe poorer-than-expected performance of some fracturing treatments has been an issue for decades. Considerable effort has been devoted to improved modeling of fracturing treatments so that improved expectations can be provided. Fracturing fluids have been modified to enhance cleanup. Proppant conductivity studies have lead to a better understanding of fracture performance. Yet, there are still many treatments, particularly in low-permeability gas wells, that defy efforts to clean up quickly and to produce at the expected rates. This paper revisits the question of whether fracture face damage is an issue in the subsequent performance of a gas well. It will be demonstrated that the landmark paper by Holditch 1 has been misquoted for 25 years. A numerical simulator has recently been written that has reproduced the earlier work, but also expands on it by demonstrating the physical mechanisms by which fracture face damage can reduce gas production and accelerate water production. The simulator includes relative permeability curves for both gas and water, and capillary pressure functions. The role of Laplace pressure, or capillary pressure, will be highlighted in the explanation of how fracture face damage can cause significant loss in well productivity. In addition, the role of relative permeability to gas will be highlighted as to how it ultimately leads to decreased gas production and increased water production when the fracture face is damaged.

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