Horizontal well - 1H was drilled in a mature field in Croatia with an extended open hole interval and a high differential pressure. As a result, differential sticking has occurred when drilling the horizontal section. This was the first well to be drilled in that area with application of a drill-in fluid that included specially sized calcium carbonate bridging agent with a blend of viscosifying and filtration control polymers. To identify the best drill-in fluid composition for the horizontal section, three compositions were evaluated based on fluid-loss control and plugging properties, low-shear rheology (LSRV) and formation damage potential. Introduction Recently, drill-in fluids have become more widespread in their use primarily due to the increase of horizontal and multi-lateral drilling, the increase in open hole completion, and the potential for much higher fluid production after their use. They provide a good drilling performance in horizontal wells and, when combined with open hole completion technique, maximize the well productivity. Formation damage tends to be more significant in a horizontal or in an extended-reach well for a number of reasons1. If the fluid used for drilling and/or well completion is not compatible with the reservoir rock bad formation damage may occur. Removal of damage - if possible at all - may be a costly undertaking. Preventing it by adequate drill-in fluid may be a reasonable solution. The use of such fluid may cause minimum formation damage. That can result in substantially improved production. Most reservoirs are sensitive to any fluids other than those contained in them naturally or similar to them. Contemporary drill-in fluids aim at reaching that. A variety of fluids can be used as drill-in fluids, including water-, oil- and/or synthetic-base fluids. The selection of the most appropriate drill-in fluid depends on the type of formation to be drilled and on the completion method to be applied. Some formations tolerate a wider range of drill-in fluid composition than others do. Lower-permeability sand stones and depleted or unconsolidated sandstone reservoirs do not tolerate fluid and particle invasion without suffering extensive damage. In order to drill horizontal wells adequately, the use of properly designed drilling fluid is crucial for drilling success. Not only does the drilling fluid need to be inhibitive; it must also be capable of laying down an impermeable filter cake to seal off depleted/underpressured intervals. The key how best to apply any drill-in fluid is to match the fluid with the reservoir and the completion design. Reservoir Characteristics Reservoirs A1, A2 and A3 of the studied oil and gas field represent a single hydrodynamic unit. The reservoir rock is sandstone (thin, strongly shally sandstone, laminated with non-permeable shale and sandy shale). Principal properties of the reservoir are summarized in Table 1. Laboratory Tests Drill-in Fluid Evaluation. The key for exploit the advantage offered by a horizontal well and obtain the desired performance was minimizing damage to the formation caused by drilling because of the damage due to filtrate invasion into the formation can be significant. Drill-in Fluid Evaluation. The key for exploit the advantage offered by a horizontal well and obtain the desired performance was minimizing damage to the formation caused by drilling because of the damage due to filtrate invasion into the formation can be significant.

Laser processing of photovoltaic cells enables the manufacturing of high-efficiency cells and offers opportunities to cut down costs. Structuring of selective emitters or laser fired contacts requires precise laser beam shaping. In this paper we identify key design parameters for industrial applications of Gaussian-to-top hat converters. We derive the physical limits for top hats close to the diffraction limited and compare the performance of actual beam shapers to these limits. Additionally we present details and first measurements of our improved solution to scan a 50µm top hat, allowing to structure a 156mm wafer in a few seconds.

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INA-Naftaplin has been utilizing the LO-CAT® process in Croatia for protection of ambient air against H2S pollution at its Gas Treatment Plant Molve III already for seven years. This separation unit, incorporated into the gas treatment plant has been erected exclusively for the protection of ambient air against harmful effect of H2S. The unit is treating H2S+CO2, which is being removed from natural gas by an upstream amine process, under conditions of low H2S content and low pressure of gas. Catalytic oxidation is being used to convert H2S to elementary sulfur, and the emission concentration was decreased from 580 ppm to less than 30 ppm. The practice revealed a series of delicate situations: plugging of internals, solution filtering and achievement of required 60 wt% sulfur concentration, disposal of produced sulfur sludge and others. The Institute for Medical Research Zagreb has performed the ambient air quality monitoring, within the scope of its contract for annual base monitoring. H2S imission values were under 5 μg/m3. The continuous control of working area is achieved by twenty-four H2S sensors, and results obtained to this date have always been below allowable limits. The practical experience has revealed that the LO-CAT® desulfurization unit has fulfiled its purpose and existing environmental criteria till 1997, along with significant cost, primarily for power and chemicals. The actual Croatian Directive on TLV of pollutants in the waste gas requires H2S concentration of 3.5 ppmv max. and RSH concentration of 20 mg/m3 max. Now the GTP Molve and Ethane Recovery Plant have emission of H2S and mercaptane above the permissible limit value, which have to be solved until the year 2004. To meet the strict legal requirements, certain upgrading is to be undertaken. This capital investment requires a large financial expenditure of up to 5 million USD, and up to 1 million USD/year for operating cost for both plants. For process improvement a continuous monitoring system has to be solved also. In that respect, measuring of H2S+RSH emission is in accordance to ASTM D-4084-82 and ASTM D-23 85-81 methods. Periodical control by gas tube detector system could contribute to the most reliable and efficient monitoring system of the two most important natural gas plants in Croatia.