Gas hydrates are crystalline solids formed by water and light molecules when a specific thermodynamic condition of high pressure and low temperature is attained. The formation of such structures can plug the production line, causing a shutdown with expensive consequences. In fact, besides waxy deposition, gas hydrate formation is among the hugest problems in flow assurance faced by oil companies. High concentrations of CO2 have been reported in the Brazilian presalt oil wells, with large potential to form hydrates, but to our knowledge, this kind of scenario has not been the subject of a deep rheological study. Here, we conduct a sequence of tests, using a high-pressure rheometer system, to take into account the effects of the water fraction and shear rate on the hydrate formation. We also investigate the ability of reconstruction of the hydrates and its memory effect. The main tests are displayed in terms of viscosity over time. By doing so, the hydrate formation is indicated by a viscosity jump.

This paper studies the loss of efficiency of polymeric drag reducers induced by high Reynolds number flows in tubes. The overall pressure was fixed and the apparatus was built so as to minimize the polymer degradation. We used three kinds of polymers: two flexible and one rigid. We conducted our tests to take into account the drag reduction (DR) for a wide range of concentrations of each polymer. The main results are displayed for the DR as a function of the number of passes through the apparatus. The mechanism of the loss of efficiency for the Xanthan Gum (XG) solutions (the rigid one) seems to be completely different from that observed for Poly (ethylene oxide) (PEO) and Polyacrylamide (PAM) (the flexible materials). While the PEO and PAM mechanically degrade by the action of the turbulent flow, the XG seems to remain intact, even after many passes through the pipe flow apparatus. From the practical point of view, it is worth noting that the PAM solutions are clearly more efficient than the PEO and XG. Another practical point that deserves attention is concerned with the asymptotic drag reduction found for XG. Although its maximum DR was significantly smaller than that found for PEO, the final value for both polymers were quite the same, which is obviously related to the intensified mechanical molecule scission in the PEO solutions. Our results for the relative drag reduction (the current value of DR divided by its maximum obtained at the first pass) was quite well fitted by the decay function proposed in our previous paper [A. S. Pereira and E. J. Soares, “Polymer degradation of dilute solutions in turbulent drag reducing flows in a cylindrical double gap rheometer device,” J. Non-Newtonian Fluid Mech. 179, 9–22 (2012)], in which a rotating apparatus was used. This strongly suggests that the physical mechanism that governs the degradation phenomenon is independent of the geometry. We also used a degradation model for PEO proposed by Vonlanthen and Monkewitz [“Grid turbulence in dilute polymer solution: Peo in water,” J. Fluid Mech. 730, 76–98 (2013)] to fit our data of relative drag reduction for PEO and PAM.

Gas hydrate formation is a huge flow assurance problem in offshore production of oil and gas. However, there have been some reported cases in oil-dominated systems where the hydrates do not form, even though the high-pressure and low-temperature environments induce favorable thermodynamic conditions. The reason for this unexpected result seems to be related to the presence of natural chemical compounds in crude oils that prevent the hydrates’ nucleation and agglomeration. Because the number of works in this specific topic are scarce, in the present work, we study the role played by saturates (hydrocarbon compounds) and asphaltenes (heterocyclic compounds), which are commonly present in crude oil, on hydrates that are formed from CO2 molecules in water–CO2–oil systems. Our tests were carried out in an assembly composed of a rotational rheometer with a magnetic pressure cell, which was connected to a high-pressure system. Our main results are displayed in terms of viscosity as a function of time at constant shear rate, pressure, and temperature. In this kind of experiment, hydrate formation is associated with a jump of viscosity. Our data suggest that asphaltenes retard the CO2 hydrate nucleation and formation in the crude oils studied in this work.

No abstract

No abstract