This study attempts to predict and verify possible spawning aggregation sites and times in the Los Roques Archipelago National Park, Venezuela, based on physical reef characteristics and the knowledge of experienced local fishermen. Three possible aggregation sites were selected for monitoring based on satellite images, low-cost bathymetric mapping and interviews with experienced local fishermen. Abundances and sizes of 18 species that are known to form reproductive aggregations were monitored at these sites using underwater visual census for 7 days after each full moon from February to August, 2007. While spawning events were not observed, possible indirect evidence of spawning aggregations was found for Lutjanus analis at Cayo Sal and Boca de Sebastopol, Lutjanus apodus at Cayo Sal, Lutjanus cyanopterus at Cayo Sal and Piedra La Guasa and Epinephelus guttatus at Bajo California and Cayo de Agua. Additionally, indirect evidence was identified for the past existence of a spawning aggregation of Epinephelus striatus in the northern part of the archipelago, which may have been eliminated by overfishing c.15 years ago. Bathymetric mapping showed that the shelf edge at sites monitored in this study was shallower than at spawning aggregation sites in other parts of the Caribbean, and that sites were not proximal to deep water. While this study does not prove the existence or locations of spawning aggregations of reef fishes in the archipelago, it does add insight to a growing understanding of generalities in the relationship between seafloor characteristics and the locations of transient reef-fish spawning aggregations in the Caribbean.
Community structure of sponges (Porifera) in three reefs at Morrocoy National Park, Venezuela and its correspondence with some environmental variables. Sponges have an important ecological role in coral reef ecosystems. However, when compared to other benthic Phyla, it has been little researched. This research was focused in the variability of the community structure of sponges in three locations at Morrocoy National Park (Cayo Sombrero, Playa Mero and Punta Brava) exposed to different environmental conditions (transparency and currents intensity) and affected in different degree of severity by a mass mortality event in 1996. A total of 15 transects (10m long and 1m wide) were evaluated in three strata (between 3 and 15m depth) in each site, where all the individuals were counted by species. Relative abundance by species, diversity and evenness were calculated. Locations showed differences respect turbidity, wave and current intensity. 27 species were found in Morrocoy; Cayo Sombrero (23), Playa Mero (18) and Punta Brava (15). Agelas sceptrum, Amphimedon erina and Niphates erecta were the most common in first location; Niphates erecta and Dysidea etheria in Playa Mero and Dysidea etheria, Niphates erecta and Amphimedon erina in Punta Brava. The species composition showed statistical differences between all three locations; Cayo Sombrero resulted the most diverse and even, followed by Playa Mero and Punta Brava. According to Sorensen Similarity Index results, Cayo Sombrero and Playa Mero were more similar, while Punta Brava resulted the most different. The variability in environmental conditions and the differential mass mortality effects of 1996 in all three reefs, were probably the main causes of the differences between their sponge communities. Nevertheless, we cannot conclude about the weight of these factors. Rev. Biol. Trop. 61 (3): 1229-1241. Epub 2013 September 01.
A reservoir numerical simulation study was done to model a polymer gel treatment pilot test without mechanical isolation, that was performed to improve the injection profiles and the sweep efficiency in a water injector well of an hexagonal pattern in the Lagomar field located in the Maracaibo Lake in western Venezuela. The gel treatment pilot test was performed in the C-4 reservoir, a layered highly heterogeneous Eocene sandstone. In this reservoir water injection for secondary have been performed for over tree decades. High water cuts have forced management to shut in several producing wells. A reservoir characterization analysis that included PLT logs, core analyses, and geomechanics, identified the presence of induced fractures as a cause of low sweep efficiency. Based on this reservoir study, a polymer gel treatment was designed for an injection well located in the center of a hexagonal well pattern. A commercial gel polymer technology (MARCITsm) was selected to modify the injection profile in this pilot test. The results from the test injecting 3700 barrels of gel in the water injector well were very promising, showing an average reduction of 30% in water cut and 26,850 barrels of incremental oil five months after the treatment. A reservoir simulation study was developed using STARS, a K-value compositional reservoir simulator with gel modelling capabilities. The model reproduces reasonably the performance observed in the field pilot test. The simulation study is a first approach to developing a predictive tool to assist in the planning and design of future non selective gel treatments in water injector wells, applicable in the Lagomar field and analogous water injection projects. Introduction In recent years, the produced water associated with oil production operations in Venezuela, has reached levels over 140 million barrels per year. These volumes of produced water come from oil fields that produce under natural active aquifers and from more than 180 water injection projects. Water management costs are continuously increasing and uneven water injection profiles leaves large amounts of oil behind the waterfronts. A growing use of chemical systems to control water and improve recovery from reservoirs with high water cut has been documented in the SPE literature. Polumer gel is the most common water shutoff technology in producer wells. The application of the gel technology in injection wells to modify the water injection profile has also been evaluated resulting in improved oil recovery in several fields 1,2. In Venezuela gel treatment technology has been studied and applied in many producer wells, and in a few cases in water injector wells, with the objective to modify the water profiles and to increase the recovery factor. To evaluate the technical feasibility of the gel technology in water injector wells in Venezuelan reservoirs, a pilot test was planned and performed in an injector well to improve sweep efficiency in a hexagonal well pattern located in the Lagomar field in the Maracaibo Lake.1 This paper describes a numerical simulation study based on the available data from the Lagomar Pilot Test. The objective of the numerical simulation study is to develop a predictive tool to assist in the design of gel treatments in water injector wells. More specifically, we wish to predict the incremental recovery that can be expected from a specific gel treatment, and the magnitude of the reduction in the water cut.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractA gel formulated for High Pressure High Temperature (HPHT) Reservoirs was evaluated for conformance treatments to reduce water production and to improve injection profiles in Eastern Venezuelan fields. The gel formulation was based on the permeability ratio reduction capacity obtained from core tests. Field tests were carried out in HPHT wells that exhibited high water cut and low oil rates. These tests showed a strong reduction in the water production and increasing oil rates, verifying the effectiveness of the formulated gel.A laboratory study under field conditions was conducted to determine the chemical parameters to model the gel injection. The gelation kinetic and visco-elastic effects were determined using dynamic rheology, which shows the time dependence of dynamic mechanical measurements using an elasticity theory to relate the storage modulus (G') of the gel with the crosslink number density. The obtained reaction is a third order function of phenol-formaldehyde crosslinkers concentration and 3/2 order polymer concentration. The adsorption of the polymer system was evaluated on Berea core and matched to a Langmuir Isotherm. Coreflood simulations were done to calibrate the chemical model to laboratory data.A mechanistic field simulation study was developed with the STARS simulator using laboratory data. A range of variation of critical parameters was evaluated in the simulator in order to understand the gelation process in the reservoir. The model was used to predict the reservoir response under different gel treatment scenarios as: treatment in injector well, treatment in the producer well, and simultaneous treatment in both injector and producer wells. Idealized and field type simulation results showed a water cut reduction from 90 to 30%, matching the trends observed in wells at different locations and the gel treatment effects in typical Eastern Venezuelan reservoirs.
A floristic analysis war conducted of the secondary vegetation derived from slash and burn agriculture in a montane rain forest region at Santa Cruz Tepetotutla, located in the Northern Oaxaca Range, Mexico. The analysis of the studied chronosequence is based on a collection of 2 668 specimens encountered in 60 parallel 0.01 ha belt transects (25 X 4 m), distributed in 18 second-growth stands with ages ranging between 5 and ca. 100 years. A total of 499 species were distinguished, which are distributed in 223 genera and 104 families (including 38 secondary vegetation species collected outside of the transects), among which the following growth forms are represented: trees, shrubs, herbs, herbaceous and woody climbers, palms, ferns, and epiphytes. Only 28 morphospecies were not determined to any taxonomic level. Floristic richness did not decrease nor increased significantly with stand age. In contrast, changing trends, albeit non significant, were observed for different life forms, as arboreal species gradually replaced herbaceous ones, whereas palms and tree ferns only appeared in stands of intermediate age and their abundances increased thereof. The results of this study suggest that a considerable proportion of the regional floristic diversity occurs in the secondary vegetation. The abandonment of traditional agricultural methods for modern but usually inadequate, productive systems threatens this floristic potential, because it affects characteristics of the system fundamental for the maintenance of species, such as stand age and the area of the primary vegetation matrix in which these stands are embedded.
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