Fracturing-Fluid Leakoff Under Dynamic Conditions Part 1: Development of a Realistic Laboratory Testing Procedure

Abstract

Laboratory testing of fluid leakoff enables optimum recommendations for fracturing fluid composition in addition to accurate predictions of relationships between leakoff rates and formation and fracturing-fluid properties. Static fluid-loss measurements, the present standardized testing method, provide inadequate results for comparing fracturing-fluid materials or for understanding the complex mechanisms of viscous fluid invasion, filter-cake formation, and filter-cake erosion. Previous dynamic fluid-loss studies have inadequately addressed the development of proper laboratory methods, which has led to erroneous and conflicting results. This paper (Part 1 in a series) discusses the results of a consortium (1989 to 1994) that was established to help us understand and allow modeling of the fluid-leakoff process in hydraulic-fracturing applications. Part 1 summarizes over 300 laboratory experiments structured to study the effects of cell design, fluid-preconditioning (both high-shear and low-shear regimes), and core thickness on fluid leakoff. The results help define appropriate laboratory equipment and testing procedures for realistic modeling of the filtration process of fracturing fluids. Introduction The rate of fluid leakoff to the formation during a hydraulic fracturing treatment is one of the most critical factors affecting fracture geometry and treatment performance. The filtration rate during a fracturing treatment affects the designed treatment size, optimal proppant schedule, and resulting proppant distribution within the fracture. Excessive fluid loss may result in insufficient fracture geometry, while low leakoff may result in poor proppant distribution within the fracture as a result of long fracture-closure times. For estimates of fluid leakoff rates, field measurements (commonly known as "minifracs") are required before each main treatment to determine estimates of average filtration rates. Accurate analysis of the minifrac data requires a good understanding of the filtration mechanisms only available through laboratory testing, since leakoff may differ significantly with injection volume. Additionally, the ability to use the minifrac data to estimate filtration rates of other fluids or additives and other well conditions (permeability, temperature, and pressure) is presently not feasible, since these variables may dramatically alter the filtration rates. These difficulties require an understanding of filtration mechanisms that can only be determined from laboratory experimentation. Numerous studies have been performed that attempt to develop acceptable laboratory fluid-leakoff databases; however, significant laboratory variations from poor experimental equipment design and procedures have made developing the required relationships to field conditions unattainable. This paper (Part 1) describes extensive laboratory testing for the development of a reproducible and accurate testing procedure. This paper also addresses classical filtration theory as applied in most fracturing simulators, limitations to the classical theory, previous experimental studies, laboratory equipment and procedures, discussion of results, and conclusions. Discussion of results will be separated into sections on cell effects, fluid preconditioning effects, core thickness effects, and recommendations for a standardized fluid-leakoff testing procedure. Later papers in this series will cover the results of shear rate, permeability, pressure, fluid composition, temperature, fluid-loss additives, and fluid-loss modeling. The first three variables are described in detail in SPE 36493, Part 2 of this series of papers. Classical Filtration Theory In the industry fracturing-fluid leakoff is normally modeled with three filtration-resistance coefficients (known as fluid-loss coefficients):the resistance to fluid loss from the filter cake,the resistance to flow in the invasion zone, andcompressibility of the noninvaded zone (Fig. 1, Page 12). The noninvaded and invasion-zone coefficients are normally calculated with the expressions in Eqs. 1 and 2 (Page 2). The invasion-zone expression assumes Newtonian-filtrate properties. P. 805

Fracturing fluid cleanup by controlled release of enzymes from polyelectrolyte complex nanoparticles

“…Classification of polyelectrolytes in terms of their charge, Koets and Kosmella [36] .... 19 Figure 10 Simplified illustration of the surface and zeta potential for a charged suspension drop dispersed in high (saline water) and low (fresh water) electrolyte concentration aqueous solution [37] .. [28,43] Figure 19 Tertiary structure of proteins [28,43] Quaternary structure of proteins [28,43] Schematic picture for catalytic reaction of enzymes [28,43] Figure 25 Comparing the viscosity reduction obtained by a commercial enzyme and a hightemperature enzyme (Tayal et al) [58] Vitthal [61,62] Figure 38 Schematic of a disassembled 175 mL static fluid loss cell presented by Asadi et al [69] See page 64 (Marpaung et al) [80] . (Barati et al) [7] ... pore volume injected [93] .…”

“…[30,62] For example a one inch core which generates reasonable results for medium permeability values causes increase in the slope of fluid loss curve at early times for low permeability cores resulting in negative calculated spurt volumes. McGowen and Vitthal [62] , stating that C w should not be changing significantly with core permeability, published the following equation to calculate the minimum core thickness required for a fluid loss test.…”

“…[62] API recommended practices (RP) 39 [68] was used extensively as a standard procedure for running McGowen and Vitthal [61,62] Hassler sleeve static fluid loss cell Asadi et al [69] provided a standard procedure for measuring fluid loss of fluids used in stimulation and gravel packing under static conditions. After giving guidelines on similarity of procedure for preparing the base fluid, pH, type of containers and mixers, time of mixing, and additives added to the fluid in order to reach repeatability, they presented two typical static fluid loss apparatus with 175 mL and 500 mL capacities capable of using filter paper, synthetic or natural core as porous medium.…”

“…Roodhart, [70] Navarette et al [59] and McGowen et al [61,62] demonstrated three stages for filter-cake buildup during a fracturing job when fracturing fluid flows perpendicular to the direction in which filter cake forms:…”

“…[61] Roodhart [70] and McGowen et al [61,62] Figure 39 Schematic picture of dynamic fluid loss process, Vitthal et al [61] Glenn at al. [30] classified the laboratory models designed by different researchers to measure the fluid loss dynamically into four groups:…”