The identification of fracture barrier is important for optimizing horizontal well drilling, hydraulic fracturing, and protecting fresh aquifer from contamination. The word "brittleness" has been a prevalent descriptor in unconventional shale reservoir characterization, but there is no universal agreement regarding its definition. Here a new definition of mineralogical brittleness is proposed and verified with two independent methods of defining brittleness. Formation with higher brittleness is considered as good fracturing candidate. However, this viewpoint is not reasonable because brittleness does not indicate rock strength. For instance, fracture barrier between upper and lower Barnett can be dolomitic limestone with higher brittleness. A new fracability index is introduced to overcome the shortcoming of brittleness by integrating both brittleness and energy dissipation during hydraulic fracturing. This fracability index considers that a good formation for hydraulic fracturing is not only of high brittleness, but also requires less energy to create a new fracture surface. Therefore, the formation with lower fracability index is considered as a fracture barrier, while with higher fracability is considered as better fracturing candidate.Logging data from one well of Barnett shale is applied (1) to verify the principle of new brittleness and fracability index model; (2) and to demonstrate the process of screening hydraulic fracturing candidates employing fracability index model.

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The identification of the fracture barrier is important for optimizing horizontal-well drilling, hydraulic fracturing, and protecting fresh aquifer from contamination. The word "brittleness" has been a prevalent descriptor in unconventional-shale-reservoir characterization, but there is no universal agreement regarding its definition. Here, a new definition of mineralogical brittleness is proposed and verified with two independent methods of defining brittleness. Formation with higher brittleness is considered as a good fracturing candidate. However, this viewpoint is not reasonable because brittleness does not indicate rock strength. For instance, the fracture barrier between upper and lower Barnett can be dolomitic limestone with higher brittleness. A new fracability index (FI) is introduced to overcome the shortcoming of brittleness by integrating both brittleness and energy dissipation during hydraulic fracturing. This FI considers that a good fracturing candidate is not only of high brittleness, but also requires less energy to create a new fracture surface. Therefore, the formation with lower FI is considered as a bad fracturing candidate, whereas that with higher fracability is considered as a better target. Logging data from one well in the Barnett shale are applied (1) to verify the principle of the new brittleness definition and FI model and (2) to demonstrate the process of screening hydraulic-fracturing candidates with the FI model.

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Brittleness has been used as one of the important descriptors for the characterization of unconventional shale reservoirs. The degree of brittleness in shale reservoirs is determined upon the basis of its mineralogical composition, which can be obtained from mineralogical logging tools (such as ECS™, FLeX™, GEM™, Litho Scanner™), or XRD tests in the laboratory. Generally, measurements of mineralogical brittleness are obtained from physical sources and lead to relatively reliable interpretation results. However, mineralogical logging is expensive and not commonly available in the shale play. Alternatively, brittleness can also be calculated from dynamic Young's modulus and Poisson's ratio, but the absence of shear slowness in some wells restricts its wide application. Internal friction angle based brittleness can give similar interpretation results as the preceding two methods, but its accuracy depends highly on the quality of correlations. It is observed that the curves of the three different brittlenesses demonstrate similar shapes. Therefore, we have attempted to build correlations between mineralogical brittleness and porosity or sonic compressional slowness for typical shale plays (Woodford, Barnett, and Eagle Ford shale), and have proven their validity with the data obtained from wells not included in the development of correlations. Applications of these findings include: (1) enabling the possibility of evaluating brittleness in plays lacking mineralogical and shear sonic loggings, thus reducing the quantity of laboratory testing, (2) inspiring operators to develop in-house correlations of brittleness for shale gas plays, and (3) investigating similar correlations in emerging unconventional oil plays, such as Granite Wash.

The drag-reduction (DR) behavior of xanthan gum, partially hydrolyzed polyacrylamide, guar gum, and hydroxyethyl cellulose at various polymer solutions has been experimentally investigated with a full-scale coiled-tubing (CT) test facility that consists of 1-, 1 1 ⁄2-, and 2 3 ⁄8-in. CT reels. Data analysis revealed that the tubing diameter, curvature ratio, and polymer concentration are important factors affecting the DR in CT. The modified DR envelope is a useful tool for evaluating the DR performance of polymer solutions in CT. IntroductionCT has found many applications in the petroleum industry, including drilling, cementing, wellbore cleanout, and hydraulic fracturing. 1 However, the excessive friction-pressure loss because of the relatively small tubing diameter and because of the tubing curvature often limits the maximum obtainable fluid-injection rates. A recent experimental investigation 2 indicates that the frictional loss in CT is significantly higher than in straight tubing.One way to increase the injection rate and reduce pumping costs is to use drag-reducing additives (or drag reducers). Typical drilling and completion fluids, usually polymer solutions, have been found to exhibit some drag-reducing property. Therefore, it is of practical importance to investigate the DR properties of these solutions in CT.Frictional pressure in turbulent pipe flow can be drastically reduced by adding small quantities of certain long-chain polymers to the solvent, such as water. This phenomenon is called DR. Credit is generally given to Toms 3 for being the first to observe the phenomenon; therefore, DR is also called the Toms effect.Several references 4-10 from petroleum literature indicate the importance and potential applications of DR to this industry. Savins 4 reported pipe flow tests using a number of synthetic and natural polymeric materials and three tubing sizes, and factors affecting the drag ratios were studied. He also compared the test data with Dodge-Metzner 11 friction-factor correlation and observed the "diameter effect."There have been several extensive reviews on DR, such as Lumley, 12 Hoyt, 13,14 Virk, 15 and Berman. 16 The paper by Virk 15 addresses the DR fundamentals of dilute solutions of linear, random-coiling macromolecules in turbulent pipe flow. It covers broad areas of DR studies, including gross flow, mean velocity profile, turbulence structure, and mechanisms. Virk 15 proposed the concept of a maximum DR asymptote and the DR envelope, which has proven to be a useful tool for evaluating the DR performance of polymer solutions.The additional difficulty in studying DR behavior in CT is caused by the different flow fields in CT flow. Because of the effect of centrifugal forces, a secondary flow of vortical form occurs in the CT cross section. Dean 17,18 pioneered the theoretical study of Newtonian fluid flow in curved pipes. A similarity parameter, later called the Dean number [defined as N DN ‫ס‬N Re (a/ R) 0.5 ] was introduced to characterize flow in curved pipes. In 1929, White 19 conduct...