No abstract

Crystallinity and melting behavior are directly affected by the presence of a noncrystallizable comonomer. Hydrogenated polybutadiene, HPB, emulates a random ethylene−butene copolymer and provides the basis for comparison to the equilibrium theory of Flory. Melting behavior, density (crystallinity), and SAXS long period were measured for HPB's having 12 to 88 ethyl branches per 1000 backbone C atoms. DSC curves calculated from equilibrium theory are compared to experimental traces. It is shown that the equilibrium melting temperature T m c of infinitely thick crystals, while thermodynamically correct, is inaccessible to experiment. Thickest crystals with observable populations melt at the practical final melting temperature T m f, which is below T m c. The peak melting temperature T m p has no relation to the most populous crystal thickness. Crystallization of molten copolymer chains leads to fewer thick and thin crystals than predicted by theory; the difference is attributed to kinetic factors of secondary nucleation barriers and mass transport. Crystallization at feasible rates is achieved when the melt is at a temperature low enough to undercool a sizable fraction of crystallizable segments. Crystallization prevents the motion of segments required to achieve equilibrium, so solidification proceeds as if the system were quenched, accounting for insensitivity of copolymer morphology to cooling rate. Only the size of the largest crystals which melt at experimental T m f can be established by thermodynamics. There is some evidence that small equilibrium crystallinities are approached in highly branched copolymers.

A combination of 13C and magnetic resonance experiments has been performed on a model ethylene copolymer (hydrogenated polybutadiene) of about 100000 molecular weight and 17 ethyl branches per 1000 total carbons. The fraction of ethyl branches found in the crystal in this 41 % crystalline sample was 0.06 ± 0.02, and the ratio of concentrations between the crystalline and noncrystalline regions was correspondingly about 1:10. For reasons of best integrability, the methyl resonance of the ethyl branches was used to deduce the concentrations in each morphological phase. This same resonance is rather ill-behaved in cross-polarization experiments, so several auxiliary experiments were undertaken to deduce the true concentrations attributed to each phase. The experimental technique utilizes cross-polarization as a probe of proton polarization levels; moreover, the success of the method relies on local proton spin diffusion. Results are discussed in terms of other experimental findings regarding the question of partitioning. Also, these results are used to interpret more accurately the data in a previous report on partitioning in an ethylene-l-butene copolymer having a branch concentration about 7 times smaller. Finally, in an appendix, some supplemental data are presented on the effects of magic-angle spinning on the spin dynamics of the noncrystalline region and the influence of this spinning on the validity of the results deduced from cross-polarization experiments.

The lattice parameters of a series of hydrogenated polybutadiene (HPB) model copolymers is measured as a function of branch content between 0 and 73 ethyl branches per 1000 C atoms. Expansion of the a and b axes nearly ceases for branch contents greater than 20 per 1000 C atoms. The c axis is seen to contract by a small amount with increased branching. The major cause of lattice expansion is limitation of crystal thickness by exclusion of branch points from the lamellar crystals coupled with surface stress on thin lamellae. A small fraction of ethyl branches are incorporated in the crystal; these expand the lattice by an additional amount.

Summary Back production of proppant from hydraulically fractured wells continues to create operational problems for oil and gas producers. As much as 20% of the proppant placed in the fracture can be returned to the surface, necessitating costly and labor-intensive surface-handling procedures. The development of unmanned platform operations remains impossible in some areas because of proppant back production. Flexibility in well turnaround and production strategies also can be very limited. Curable resin-coated proppant (RCP) has provided the most cost-effective and widely used method to control proppant back production. While this is a valuable technique that has served the industry well for many years, it is not universally successful. A new technology has been developed to control proppant back production and to increase flexibility in well turnaround and production strategies. The technology has been used successfully on several hundred hydraulic fracturing treatments. In this technology, a mixture of fibers and proppant is pumped into the fracture to form a pack that is resistant to proppant back production under typical oil/gas production conditions. The proppant/fiber mixture depends on a physical mechanism rather than chemical bonding to increase pack resistance to flowback. There are no minimum closure stress, temperature, or shut-in time requirements associated with the use of this technology, which increases the flexibility available to the operator to optimize well turnaround and production strategy. This paper reviews the laboratory data relevant to the understanding and application of this technology. Studies include proppant pack resistance to flowback in one- and two-phase flow, the effect of cyclic loading, aging phenomena, permeability/conductivity studies, and fluid/breaker interactions. The benefits of the technology are illustrated with field studies. Introduction Proppant back production has been of concern in hydraulic fracturing for more than 20 years. It has received increased attention in recent years as larger fracture widths and the use of higher proppant concentrations have become more prominent. Back production usually is observed when the well is turned around after the hydraulic fracturing treatment. Back production may stop in time, be controlled by limiting the production rate from the well, or continue through the economic life of the well. Back production from hydraulically fractured wells presents operational problems. It often necessitates costly and labor-intensive surface-handling procedures and on-site control of chokes when beaning up the wells. The erosion of well and surface facilities presents a safety hazard. Proppant remaining in the wellbore can shut off production by covering the productive interval. The magnitude of the problem and the range of viable approaches vary from location to location. As much as 20% of the proppant placed in the fracture has been returned to the surface in Alaska, and as much as 10% is not uncommon in the North Sea.7 Back production can yield as much as 100,000 lbm of proppant. In Alaska, many operators "live with" the problem. In the North Sea, the development of offshore, unmanned platforms is often impossible. Continuous proppant production can sometimes be stopped by producing wells on a restricted choke. In some cases, changes in job design (proppant type, proppant size, final concentration of proppant in slurry, final pumping pressure, forced fracture closure, or lengthened fracture shut-in period) can be effective in reducing or eliminating proppant flowback. The most economical and widely used proppant-flowback-control technique is the curable RCP, which has been the single most effective technology available. Sales of curable RCPs exceed 60,000,000 lbm/yr (and could be significantly higher than that). While curable RCP's have had a great deal of success, they are not, nor could they be expected to be, universally applicable. A new technology has been developed to prevent proppant back production and to allow more flexibility in flowback design and production strategy. The technology relies on a physical mechanism - fiber reinforcement - to increase the resistance of the pack to flowback during production of oil or gas. This report presents laboratory data and field examples of the advantages of this new technology. Its limitations are also presented.