International Scientific Journals
of Scientific Technical Union of Mechanical Engineering "Industry 4.0"

Natural gas in Republic of Croatia is being produced from three main regions: Sjeverni Jadran, Duboka Podravina and Međimurje. The natural gas produced from Međimurje region has been taken as an example for modelling the dehydration process because, compared to natural gas from Duboka Podravina region, it contains higher amount of acid gases. The associated gas with much smaller content of acid gases, currently being flared at region site called Istočna Hrvatska, has been taken as a second example. The comparison of these two cases has shown the effect of acid gases to the dehydration process efficiency. In both cases, absorption by triethylene glycol is chosen. Due to a large discrepancy in acid gas content, water content difference observation is much easier. Modelling of the dehydration process has been made in Aspen Hysys software.

Drilling mud is a working fluid which consists of a water and different added additives and is used during drilling of wells. By circulating through the wellbore, it ensures continuous cuttings removal, required pressure to stop the influx of reservoir fluid in wellbore, lubrication of drilling tools and maintaining wellbore stability while drilling through various types of formations. Since the well is often drilled through water-soluble rocks, which are mostly consisted of clay minerals, the penetration of the filtrate (water) into such rocks can lead to their destabilization, so the industry is considering the application of new types of additives because conventional additives, due to their size, can not enter in pores of rocks which has very low permeability, plug them and thus reduce further penetration of the filtrate. In recent years, scientists have paid great attention to the testing of nanoparticles (most tests are carried out with SiO2 nanoparticles) for drilling through low permeability rocks. Nanoparticles are particles whose diameters range from 1 to 100 nanometers so they can enter into the pores and create a high-quality mud cake, thereby reducing the filtrate invasion and increasing the wellbore stability. This paper describes a laboratory test carried out with SiO2 nanoparticles which average particle size was 20 and 60 nm added in concentrations of 1 and 3 wt% to water-based muds. Special emphasis was put on rheological and filtration properties. By
optimizing the concentration and size of the used nanoparticles, the rheological and filtration properties of the used drilling muds were improved.

Primary oil production represents the process of production only by exploiting the natural reservoir energy (reservoir pressure), i.e. eruption phase. This mode of production lasts until the reservoir pressure drops, so that it is no longer capable of pushing the oil to the surface. Oil wells which can’t erupt, must apply one of the methods of artificial lifting. The artificial lift system of oil production by using downhole sucker-rod pumps with piston rods is the oldest system for lifting fluids from wellbore to the surface. It is also the most widespread artificial lift system in practice in the world (over 80%). The basic principle of the operation of a downhole sucker-rod pumps is based on the transfer of the drive energy from the surface to the pump by piston rods. In principle, downhole sucker-rod pumps with piston rods can be applied in oil wells from depth of 500 to 2500 m while enabling production ranging from 1 to 100 and more m3/d of liquid. This system is suitable for wells with a smaller gas factor and consolidated production layers. The advantage of downhole sucker-rod pumps is their efficiency and economy because initial capital investments per well are lower. The disadvantages of downhole sucker-rod pumps are higher maintenance costs, i.e. more frequent workovers. This paper analysis the most common causes for well workover that produce oil with downhole sucker-rod pumps and recommendations in order to reduce their number.