Although climate change has been modifying grassland ecosystems for a long time, few studies on grassland ecosystems have focused on large-scale responses to climate change. Hence, grassland net primary productivity (NPP) from 1981 to 2010, as well as its variations in China, North America, Europe and Australia, was assessed and compared using a synthetic model in this study. Subsequently, the correlations between the NPP of each grassland type and climate factors were evaluated to reveal the responses of grassland ecosystems to climate change. The results showed that North America, which has the largest area of grassland ecosystems, exhibits maximum grassland NPP of 4225.30 AE 215.43 Tg DW year À1 , whereas Europe, which has the least area of grassland ecosystems among the four regions, exhibits minimum grassland NPP of 928.95 AE 24.68 Tg DW year À1 . Grassland NPP presented an increasing trend in China and Australia, but decreasing in Europe and North America from 1981 to 2010. In addition, grassland NPP is positively correlated with mean annual precipitation, but demonstrates notable differences with mean annual temperature. In conclusion, climate change has a significant role in explaining the spatiotemporal patterns of and the variations in grassland NPP in the four regions.

Oil reserves of the thin heavy oil reservoirs are estimated to be over 400 billion barrels. The recovery factor of waterflooding in these reservoirs is as low as 10−20% due to the high oil viscosity and correspondingly unfavorable mobility ratio. In addition, the commonly used thermal recovery methods are also unsuitable in such formations due to the significant heat loss to the adjacent formations. Thus, it is urgent to find an efficient and economic enhanced oil recovery (EOR) method to maximize the recovery factors in the thin heavy oil reservoirs. In this study, the feasibility of polymer-enhanced foam (PEF) flooding for the thin heavy oil reservoirs is investigated using the micromodel and core-flood experiments. The micromodel experiments show that foam quality has significant effects on the resistance factor and heavy oil recovery of the PEF flooding, where the displacement front of a low-quality foam case is more even than that of a high-quality foam case and surfactant−polymer (SP) flooding case. Core-flood tests further reveal that there is an optimal slug size under the experimental conditions, and the heavy oil recovery of PEF flooding is 23.9% higher than that of SP flooding when using the same slug size. Finally, a field-scale reservoir simulation is conducted, and the results show that after initial water-flooding in thin heavy oil reservoirs, the recovery factor achieved by the PEF flooding is 11.7% higher than that of the SP flooding and 21.4% higher than that of the continuous water-flooding process.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractPolymer flooding is one of the most important EOR technologies currently producing substantial oil increment in China onshore oilfields. The application of polymer flooding in the offshore environment is more complex and face to many technical challenges due to the absence of the fresh water source and limited space on the platform. Compared to regular anion polyacrylamide mostly used in onshore oilfields, hydrophobically associating water-soluble polymer (HAWP) could keep relatively high viscosity in the high salinity brine. A kind of HAWP was introduced in the first pilot test of polymer flooding conducted in SZ36-1 oilfield of Bohai Bay, China. The single injector pilot test was applied successfully, obtaining 2,5000m 3 incremental oil in the corresponding production well and water cut decreased from 95% to 54%. It indicates the technical effectivity of HAWP used as a driving agent in the high salinity heavy oilfield. A scale up test has been conducted in the well group and the same polymer has been employed. The corresponding wells have showed a good response and produced 12,000m 3 incremental oil up to the Feb. 2007. Meanwhile, the produced fluid containing trace of HAWP has been collected for evaluation of its influence on the offshore production system. The tests showed the feasibility of HAWP for offshore polymer flooding and provided experience for its further application.

Surfactant Polymer flooding (SP flooding) has drawn more attention than Alkaline Surfactant Polymer flooding (ASP flooding) in China due to the difficulty of demulsification and the scale formation problem from the alkali in ASP. Traditionally, the primary requirement for the surfactant in SP flooding is the lowest interfacial tension (IFT). However, core flooding tests performed in a series of heterogeneous models have demonstrated there exist optimized surfactant IFT and polymer viscosity for SP flooding, which can maximize the oil recovery of a heterogeneous formation. The optimized surfactant IFT has a low value but not the lowest possible value, and polymer viscosity is at an appropriately high value, but not the highest possible value. This paper summarizes and analyzes more than 40 core flooding test results and provides possible mechanisms for the results.

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