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The growing energy demand in the world, as well as the current geopolitical situation, require countries to additional investments in the exploration and production of hydrocarbons from their own sources. This means that companies must develop new fields which have remained undeveloped until now mostly because extremely harsh environment where they are located (deep sea, high temperature, high pressure, heavy oils, etc.). The development of these new fields requires the development and adoption of new technology, among other things, and the development of a temperature-stable drilling fluid system able to fulfil all tasks according to the new technical challenges. Except for the technological challenges, there are also growing concerns related to the influence of the drilling operation on the environment. All of the above encourage the industry to develop new, inexpensive, and environmentally friendly additives which will be able to satisfy all technical and technological requirements and challenges of modern drilling. In the last few years, there has been a growing trend of laboratory research that includes different types of biodegradable waste as a potential additive that can achieve useful properties in mud. In this paper, the influence of mandarin peel powder on the filtration properties of mud after the aging process at elevated temperatures is examined. This eco-friendly additive was added to water-based muds in concentrations of 1% and 2% by volume of water. Laboratory research have shown stable filtration properties of the water-based mud containing mandarin peel powder even after exposing the mud to temperatures higher than 130 °C., Sve veća potražnja za energijom u svijetu, kao i trenutna geopolitička situacija, zahtijevaju od zemalja dodatna ulaganja u istraživanje i proizvodnju ugljikovodika iz vlastitih izvora. To znači da tvrtke moraju razviti nova polja koja su do sada ostala nerazrađena uglavnom zbog izuzetno zahtjevnih uvjeta u podzemlju gdje se ona nalaze (duboko more, visoka temperatura, visoki tlak, teška nafta, itd.). Razvoj ovih novih polja zahtijeva razvoj i usvajanje novih tehnologija, između ostalog, i razvoj temperaturno stabilnog bušotinskih fluida koji mogu ispuniti sve zadatke sukladno tehničkim izazovima koji se pred njih stavljaju. Osim tehnoloških izazova, raste i zabrinutost vezana uz utjecaj bušaćih operacija na okoliš. Sve navedeno potiče industriju na razvoj novih, cjenovno povoljnijih i ekološki prihvatljivih aditiva koji će moći zadovoljiti sve tehničko-tehnološke zahtjeve i izazove suvremenog bušenja. Posljednjih nekoliko godina sve je veći trend laboratorijskih istraživanja koja uključuju različite vrste biorazgradivog otpada kao potencijalnih aditiva kojima se u isplaci mogu postići korisna svojstva. U ovom radu ispituje se utjecaj praha kore mandarine na filtracijska svojstva isplake nakon procesa starenja pri povišenim temperaturama. Ovaj ekološki prihvatljivi aditiva dodavan je u isplake na bazi vode u koncentracijama od 1% i 2% na volumen vode. Ispitivanja su pokazala stabilna filtracijska svojstva isplake na bazi vode koja sadrži prah kore mandarine i nakon izlaganja isplake temperaturama višim od 130 °C.

Deep borehole disposal (DBD) of spent nuclear fuel (SNF) and/or high-level radioactive waste (HLW) is, at the moment, a much-debated topic. Some experts think that DBD is a potentially workable solution for the disposal of SNF and/or HLW, especially for small programs, and some think that it is insufficiently researched and technologically unfeasible concept for which disposal containers have yet to be designed and technology tested in life-size. Although the research process for this concept lasts for more than 50 years and has intensified recently, confirmation that the concept is feasible has not yet been achieved. The original DBD concept is not new, however, it is still one of the least developed and researched concepts. Although, the petroleum industry has an abundance of experience in theoretically similar technology, disposal of SNF and HLW is a much different problem than drilling and production of oil and gas. Thus, it is inappropriate to draw a direct parallel between the technology of drilling the wells for oil and gas production with the technology of making boreholes for SNF and HLW disposal. The main reason for favoring this concept is the idea of disposal at depths of more than three kilometers, with minimal rock destruction and without creating underground facilities resembling mines. Several problems are still not solved: achieving the appropriate borehole diameter at the required depth ; preventing canister jams, canister design, canister installation and borehole-closing technology.

During the drilling of a borehole, large amounts of waste material are generated, especially drilled cuttings and used drilling mud. In recent history, the maximum possible reduction of waste material and disposal methods that are more environmentally friendly and safer from the aspect of human health are among the most important parts of operators’ strategies. The main advantages of injection method are minimizing potentially harmful impacts on groundwater, reducing the required surface area for waste disposal, reducing the negative impact on the air and long-term risks for the entire environment. In this paper, various injection methods will be described in detail, along with the criteria for the selection of the injection zone and the equipment required for the operation.

Tehnologija izrade bušotina, u funkciji pridobivanja nafte i plina, u sve težim uvjetima koji vladaju u podzemlju, predstavlja velik izazov za naftne inženjere koji takve rizične projekte osmišljavaju i provode uz uvažavanje tehnoloških, ekonomskih i okolišnih zahtjeva. Većina naftnih i plinskih bušotina je vertikalna i izrađuje se primjenom klasičnog sastava bušaćih alatki. Međutim, bušenjem koso usmjerenih, a posebno horizontalnih bušotina ostvaruju se brojne prednosti te povećava iscrpak ugljikovodika iz postojećih i novih ležišta pa je njihov broj u stalnom porastu. Do sada su izrađene vertikalne bušotine dubine i preko 12 000 m, kao i usmjerene bušotine kojima je duljina horizontalnog kanala preko 11 000 m. Izrada takvih bušotina omogućena je zahvaljujući napretku tehnologije materijala i radnih fluida, korištenju uronjenih motora, rotirajućih upravljivih sustava i vršnog pogona, te razvoju mjerne opreme koja omogućuje da se tijekom bušenja, u realnom vremenu, dobivaju podaci neophodni za točno upravljanje putanjom kanala bušotine. U radu je prikazana tehnologija izrade vertikalnih i usmjerenih bušotina s osvrtom na izradu horizontalnih bušotina velikog dosega, kao i tehnologija bušenja uz pomoć kolone zaštitnih cijevi i bušaćeg lajnera. Nadalje, prikazani su primjeri primjene novih tehnologija u svijetu i postignuti rezultati. Dat je pregled izrađenih bušotina u Republici Hrvatskoj u periodu od 2006. do 2016. godine.

Drilling wells throughout depleted or low pressure reservoirs requires low density drilling fluids, often with density less than water. Methods to reduce the density of drilling fluids have included mixing-in air or nitrogen. However, problems with these approaches include instability of gas bubbles (bubbles collapse or expand) and increased costs. Recently, the use of micro bubbles named aphrons in drilling, completion and workover fluids has proven success in solving many problems related to low pressure reservoirs such as fluid loss control, formation damage, stabilization of multipressure sequences with one fluid and possible differential sticking. Aphrons represent bubble with uniquely structure stabilized with surfactant. Against conventional micro bubbles, aphrons are more stable in downhole conditions and they are generated using standard mixing equipment. Owing to their properties and overpressure in wellbore aphrons penetrate into low pressure layers and set up inner bridging. Depleted wells which are very expensive to drill underbalanced or with other remediation techniques can now be drilled overbalanced. This paper presents description of aphron structure and stability, aphron bridging mechanism, aphron-based fluid composition and properties, and field experiences in applying aphron-based fluids.

Wellbore instability is one of the main problems that engineers meet during drilling. The causes of wellbore instability are often classified into either mechanical (for example, failure of the rock around the hole because of high stresses, low rock strength, or inappropriate drilling practice) or chemical effects which arise from damaging interaction between the rock, generally shale, and the drilling fluid. Often, field instances of instability are a result of a combination of both chemical and mechanical. This problem might cause serious complication in well and in some case can lead to expensive operational problems. The increasing demand for wellbore stability analyses during the planning stage of a field arise from economic considerations and the increasing use of deviated, extended reach and horizontal wells. This paper presents causes, indicators and diagnosing of wellbore instability as well as the wellbore stresses model.

Differential-pressure pipe sticking occurs when a portion of the drill string becomes embedded in a filter cake that forms on the wall of a permeable formation during drilling. The dominant force is usually associated with a difference in pressures between the hydrostatic pressure of the mud and the pore pressure in the contact area, though adhesion and cohesion may also contribute some resistance to pipe movement. Some level of sticking occurs routinely in drilling operations and differential-pressure-pipe-sticking problems may not be totally prevented. These events only became problematic if the force required to pull the pipe free exceeds the pipe strength. If sticking does occur, several methods can be used for freeing the stuck pipe but the most common approach is to place a small volume of oil, or special spotting fluid in a wellbore annulus to free differentially stuck pipe. Spotting fluid reduces the stuck area and the forces of cohesion between the pipe and cake, and allowing pipe to be pulled free. Differential sticking tendency of different drilling fluids were determined in laboratory using sticking tester as well as influence of spotting fluids on freeing differentially stuck pipe. Results of the testing are presented in the paper.

The petroleum industry is increasingly drilling more technically challenging and difficult wells, inherently expensive. Shale composes roughly 75% of the formations drilled and shale instability is the main cause of over 90% of the borehole instability-related problems. The cause of shale instability can be mechanical and chemical. Mechanical-induced rock instability can occur because the in-situ stress state of equilibrium has been disturbed after drilling. Chemical-induced shale instability is caused by the drilling-fluid/shale interaction, which alters shale mechanical strength as well as the shale pore pressure in the vicinity of the borehole walls. The mechanisms that contribute to this problem include capillary pressure, osmotic pressure, pressure diffusion in the vicinity of the borehole walls, and borehole fluid invasion into the shale when drilling overbalanced. Total prevention of borehole instability is unrealistic but it is possible mitigate the problems of borehole instabilities by adhering to good field practices. These practices include proper mud-weight selection and maintenance, the use of proper hydraulics to control the ECD, proper hole-trajectory selection, and the use of drilling fluid compatible with the formation being drilled. Investigating the swelling characteristics of shale formation is vital in selecting a proper drilling fluid to give maximum inhibition and borehole stability. Swelling of selected shale and clay samples in different drilling fluids were determined in laboratory using Dynamic Linear Swell Meter as well as influence of shale stabilizers on filtration and rheological properties of drilling fluids. Results of the testing are presented in the paper.

Differential sticking is, for most drilling organizations, the greatest drilling problem worldwide in terms of time and financial cost. Differential-pressure pipe sticking occurs when a portion of the drill string becomes embedded in a filter cake that forms on the wall of a permeable formation during drilling. The dominant force is usually associated with a difference in pressures between the hydrostatic pressure of the mud and the pore pressure in the contact area, though adhesion and cohesion may also contribute some resistance to pipe movement. Some level of sticking occurs routinely in drilling operations and differential-pressure-pipe-sticking problems may not be totally prevented. These events only became problematic if the force required to pull the pipe free exceeds the strength of the pipe. Differential-pressure pipe sticking occurrence can be mitigated if some precautions are taken such as: maintaining the lowest level of fluid loss, the lowest level of drilled solids in the drilling fluid, and drill string rotation at all times, if possible as well as using the lowest differential pressure and selecting a drilling fluid that will yield smooth filter cake with low coefficient of friction. If sticking does occur, several methods can be used for freeing the stuck pipe but the most common approach is to place a small volume of oil, or special spotting fluid in a wellbore annulus to free differentially stuck pipe. Spotting fluids are designed to free differentially stuck drill strings by dehydrating or breaking up the filter cake, allowing penetration of the spotting fluid and wetting of the drill string. It reduces the stuck area and the forces of cohesion between the pipe and cake, and allowing pipe to be pulled free. Differential sticking tendency of different drilling fluids were determined in laboratory using sticking tester as well as influence of spotting fluids on freeing differentially stuck pipe. Results of the testing are presented in the paper.

Hydraulic fracturing has been very well known well stimulation method since many decades back and continues to prove its worthiness enhancing the well productivity if applied properly. But, without a proper technology update on the design management and wellbore tools involved, limited additional improvement is experienced. Over the last 10 years many technical developments and solutions have been made to better stimulate pay zones containing hydrocarbons. New well completion assemblies, to accompany with formation fracturing technology, have to be more reliable, efficient, profound and cost effective in extreme downhole conditions. Many times, with layered complex reservoirs, more than several individual pay zones are required to be stimulated and completed with specifically designed treatments. Multistage fracturing technology is an effective solution for less costly treatment of such complicated reservoirs. This paper demonstrates technological solutions for multizone treatments and its application to different well conditions (temperature, pressure, open hole, cased hole etc.). Multistage fracturing can be done differently, and well completion and treatment design is done with intention to access the reservoir the best possible way. Systems like Wireline operated systems, Coiled tubing applications, Jointed pipe applications and other, are described and introduced.

During the hydrocarbon production, the formation sand movement has a frequent negative influx to the process because of its hazardous nature. Gravel pack placement is a common well treatment procedure for prevention of formation sand flow to the wellbore. This method is used in petroleum engineering industrial practice since 1950s. Based on it as a conventional technique Frac-Pack method came up as a new modern technology of sand control containing gravel pack placement and hydraulic fracturing all-in-one procedure. In Frac-Pack application, the appropriate viscosified fluid and proppant selection has a key role for achievement of desired fracture height, length and width. Fracture dimensions are controlled by TSO (Tip-Screenout) fracturing procedure. In this paper basic types of proppant are presented as well as resin coated sand, which improves packing of fracture with proppant. Numerous and various Frac-Pack fluids able to resist the high temperature and pressure are described. Downhole Frac-Pack equipment is also presented. Paper also contains a field example of Frac-Pack application performed on one of the Adriatic offshore gas fields, Republic of Croatia.

Casing drilling technology involves drilling and casing a well simultaneously. It eliminates the conventional drill string by using the casing itself as the hydraulic conduit and means of transmitting energy to the bit. Casing drilling has been employed in many countries as an effective method of reducing the overall drilling costs by lessening drilling time and drill string problems encountered during conventional drilling process such as lost circulation, differential sticking, well control, and wellbore instability. This paper describes development, application area, benefits and limitation of casing drilling technology.

Geosteering includes trajectory changes in directional and horizontal drilling where decisions are made in real-time related to data acquired by logging while drilling measurements. Steering is possible by placing resistivity, gamma-ray, density and porosity sensors above the motor or integrating sensors in rotary steerable system. Procedures of geosteering provide precise wellbore positioning and production of oil and gas from thin reservoirs, drilling multi-lateral wells and extended-reach wells at lower cost. Currently used geosteering systems and their application during off-shore horizontal well drilling in Croatia will be presented in the paper.

Effect of polymer swelling that was considered to be a problem, gives today a possibility, to complete wells that are a problem one for conventional packer setting and sealing. Polymers (rubbers) can swell in hydrocarbons or water based fluids as well. Several ways of application have been accepted. Mainly they are connected with open hole and sand screen completions in unconsolidated formations. The ability of changing the volume to more than several hundred percent, can help to avoid packer failure. When defining swelling packer elements it is important to determine their mechanical properties, swelling speed in various fluids, and stability (ageing tests). To implement home made products it was necessary to provide laboratory testing equipment for such purpose. The developed apparatus enables use of different fluids, temperature change up to known reservoir temperature, and pressure differential that element can withstand.

Formate-based fluids has been successfully used in over hunders HPHT well operations since they introduced in field practice. They have many advantages when compared with conventional HPHT drilling and completion fluids such as: minimal formation damage, maintenance of additve stability at high temperatures, reduced hydraulic flow resistance, low potential for differential sticking, naturally lubricating, low corrosion rates, biodegradability with little risk to the environment ect. Formate-based fluids can be applied in deep slim hole drilling, shale drilling, reservoir drilling, salt drilling and gas hydrate drilling. The laboratory testing was carried out to evaluate the rheological behavior of formate-based fluids as a function of temperature. Formate-based fluids were formulated using potassium formate brine, xanthan polymer, PAC, starch and calcium carbonate. Experimental results show that potassium formate improves the thermal stability of polymers.

Formate-based fluids has been successfully used in over hunders HPHT well operations since they introduced in field practice. They have many advantages when compared with conventional HPHT drilling and completion fluids such as: minimal formation damage, maintenance of additve properties at high temperatures, reduced hydraulic flow resistance, low potential for differential sticking, naturally lubricating, very low corrosion rates, biodegradable and pose little risk to the environment etc. Formate-based fluids can be applied during deep slim hole drilling, shale drilling, reservoir drilling, salt and gas hydrate formations drilling. The laboratory research was carried out to evaluate the rheological behavior of formate-based fluids as a function of temperature. Formate-based fluids were formulated using potassium formate brine, xanthan polymer, PAC, starch and calcium carbonate. Experimental results show that potassium formate improves the thermal stability of polymers.

The introduction of positive-displacement steerable motors in directional drilling practice offers better directional control in comparison with earlier used directional drilling systems and makes directional drilling widely accepted technology. But some disadvantages of the technology as drilling the curved parts of the wellbores in sliding mode and hole cleaning connected problems, demand further development of directional drilling systems. That results in rotary steerable systems development introduced in the practice in the late 1990s. Systems enable continuous rotation of the drillstring during drilling and more efficient transfer of weight to the bit. That results in higher rate of penetration and improved hole cleaning. Application of the rotary steerable system in a hole drilling in the Adriatic Sea area in Croatia as well as characteristics of the system and results of the application will be presented in the paper.

One of technologies that can be used to increase profitability in the development of high cost off-shore operations as well as marginal or partially depleted reservoirs is so called intelligent completion. That implies such completion systems that allows monitoring, control and change of relevant data through production, by the use of remotely controlled and operated systems. Essence of the system is such way of completion that allows change of space and hydrocarbons flow conditions from the layer to the surface, without the need for further workover works. Technical characteristics of the system, with evaluation and a way of economic evaluation are presented in the paper.

Differential sticking is defined as stuck pipe caused by the differential pressure forces from an overbalanced mud column acting on the drillstring against filter cake deposited on a permeable formation. It is influenced by drilling fluid properties and characteristics of rock formations and has major impact on drilling efficiency and well costs respectively. Differential sticking tendency of two drilling fluids were determined in laboratory using sticking tester as well as influence of lubricant and increase of solids content on fluid properties. Results of the testing are presented in the paper.

In recent period, researchers have started conducting laboratory research to determine whether different types of biodegradable food waste can be used as additives in water-based muds (drilling fluids). Correctly designed composition of the mud is extremely important for the successful construction of the wellbore, since mud is complex fluid whose composition and properties affect the efficiency of drilling process. Therefore, any new type of additive must be researched in detail in relation to all the necessary properties that must be obtained during drilling operations. Wasted drilling mud and associated waste generated during drilling operations represents one of largest waste volumes accumulated during exploration and production projects in oil and gas industry, so this kind of research can have considerably positive results for the environment. Among the analyzed types of biodegradable waste, there is some data on the use of mandarin peel for these purposes, and due to the considerable annual production of this type of fruit in Croatia, it was decided that detailed laboratory research will be carried out on this type of biodegradable waste. By adding dried, grinded and sieved mandarin peel powder, in the water-based drilling mud, it was tested how this additive affects the mud filtration properties, rheological properties and the wellbore stability. Drilling muds that have a certain concentration of mandarin peel powder in their composition were compared with base mud which consists of water, bentonite and sodium hydroxide (NaOH). In addition to studying the influence of the concentration of mandarin peel powder, the influence of different particle sizes divided into two groups, particles smaller than 0.1 mm and particles between 0.10 mm and 0.16 mm in size, was also researched. The selected concentrations in the mud samples varied from 0.5% to 2% by the volume of water, since it was concluded during preliminary laboratory test phase that a concentration higher than 2% leads to a significant increase in plastic viscosity of the tested muds. On the basis of the conducted laboratory tests and the analysis of the results, useful insights were obtained on the effect of mandarin peel on the observed mud properties. Results show that mandarin peel have positive effect on mud filtration, since every mud sample with added mandarin peel powder showed decrease of filtrate volume. An exceptionally positive influence of mandarin peel powder was also observed in data obtained after swelling of pellets that have a clay component in their composition, when pellet is in contact with mud to which this eco-friendly additive is added. Laboratory tests showed significant swelling reduction, and this correlates well with results of filtration measurements which is essential to II prove the improvement of the wellbore stability. The rheological properties of mud samples that are containing mandarin peel were not decreased but also did not significantly increase observed properties. Rheological parameters show increase and remain within acceptable limits up to concentration of 1.5% of mandarin peel powder by volume of water. Because of this data set, the recommended concentration of this additive should be kept in the range of 1.0% to 1.5%, and at those concentrations observed additive has a significant positive effect on the filtration properties and wellbore stability as well as maintaining stable rheological properties at the same time. Considering the particle size used for laboratory research of mentioned mud properties, it can be concluded that slightly better results were obtained with larger particles between 0.10 mm and 0.16 mm but both sizes provide satisfactory water-based drilling mud properties. Also, by conducting laboratory tests of temperature stability, it can be concluded that water-based drilling mud with added mandarin peel powder is stable even after exposing the water-based drilling mud to temperatures up to 133 °C., Mnogi komercijalno dostupni aditivi spadaju u kategoriju nerazgradivih i ekološki opasnih materijala te postoji potreba za pronalaženjem i stvaranjem novih ekološki prihvatljivih aditiva koji osiguravaju postizanje svih potrebnih svojstava isplake kao i komercijalno dostupni aditivi, ali uz minimalan utjecaj na okoliš. Stoga se posljednjih nekoliko godina aktivno provode mnoga istraživanja o mogućnostima primjene otpada od hrane i drugih vrsta biorazgradivog otpada u pripremi isplaka na bazi vode. Budući da su preliminarna ispitivanja pokazala pozitivan utjecaj praha kore mandarine na različita svojstva isplake na bazi vode, te uzevši u obzir razinu godišnje proizvodnje mandarina u Republici Hrvatskoj (40.580 tona u 2021. godini), korisno je detaljnije istražiti može li se otpad koji nastaje konzumacijom ovog voća iskoristiti u naftnoj industriji, odnosno za pripremu bušotinskih fluida. Ciljevi i hipoteze Ciljevi ovog rada su utvrditi primjenjivost koncepta kružne ekonomije u tehnologiji bušenja korištenjem otpadnog materijala (kore mandarine) u pripremi isplake te odabrati koncentraciju i veličinu čestica praha kore mandarine čija će primjena poboljšati reološka i filtacijska svojstva isplake, smanjiti bubrenje glinovitih stijena i posljedično povećati stabilnost kanala bušotine. Ciljevi su bazirani na tri hipoteze: (i) dodavanjem praha kore mandarine u isplaku na bazi vode poboljšat će se njena filtracijska i reološka svojstva, (ii) dodavanjem praha kore mandarine smanjit će se bubrenje glinovitih stijena i poboljšati stabilnost kanala bušotine i (iii) prah kore mandarine smanjit će toksičnost isplake. Znanstveni doprinos Rezultati ovog istraživanja donose uvid u odabir koncentracije i veličine čestica praha kore mandarine s ciljem optimiranja svojstava isplake i povećanja stabilnosti kanala bušotine u različitim bušotinskim uvjetima. Na temelju provedenih ispitivanja moguće je pojasniti mehanizam temeljem kojeg čestice praha kore mandarine djeluju na svojstva isplake i povećanje stabilnosti kanala bušotine te potvrditi primjenjivosti koncepta kružne ekonomije u području istraživanja ugljikovodika. Metode i postupci U svrhu definiranja utjecaja praha kore mandarine na odabrana fizikalna svojstva isplake na bazi vode provedena su detaljna laboratorijska ispitivanja prema standardima API 13A i 13B-1. Sukladno navedenim standardima za istraživanje utjecaja ovog biorazgradivog aditiva na bubrenje stijene korišteni su umjetno stvoreni uzorci stijene (peleti), a njihovo bubrenje ispitano je uređajem za mjerenje bubrenja (engl. Dynamic Linear Swell Meter). Laboratorijsko ispitivanje filtracije kroz keramičke diskove različite propusnosti u uvjetima povišenog tlaka i temperature provedeno je na uređaju za određivanje sposobnosti isplake da stvori premoštenje na licu formacije (propusnom mediju) (engl. Permeability Plugging Tester (PPT)). Primjenom skenirajućeg elektronskog mikroskopa (SEM) dobivene su SEM snimke površine isplačnog obloga kako bi se utvrdio mehanizam temeljem kojeg čestice praha kore mandarine čepe pore i tako smanjuju vrijednosti filtracije. Reološka svojstva (plastična viskoznost, naprezanje pri pokretanju i čvrstoća gelova) određena su Fann viskozimetrom model 35A. Rezultati i zaključci Kako bi se što bolje istražio utjecaj praha kore mandarine na filtracijska i reološka svojstva te smanjenje bubrenja umjetnih uzoraka glinovitih stijena (peleta), za usporedbu je odabrana isplaka na bazi bentonita (osnovna isplaka). Rezultati dobiveni laboratorijskim ispitivanjima osnovne isplake uspoređeni su s rezultatima ispitivanja osam isplaka na bazi vode u čijem se sastavu nalazila različita koncentracija praha kore mandarine različitih veličina čestica. Od toga su četiri isplake (oznake A1, A2, A3 i A4) sadržavale prah kore mandarine s česticama manjim od 0,10 mm, dok su preostale četiri isplake (oznake B1, B2, B3 i B4) sadržavale prah kore mandarine s veličinama čestica između 0,10 i 0,16 mm. Prah kore mandarine dodan je u sastav isplake u četiri različite koncentracije: 0,5% (oznake A1 i B1), 1% (oznake A2 i B2), 1,5% (oznake A3 i B3) i 2% (oznake A4 i B4) na volumen vode korištene za pripremu isplake. Laboratorijskim ispitivanjima utvrđeno je da prah kore mandarine značajno smanjuje API filtraciju isplake već pri koncentraciji praha kore mandarine od 0,5%, ali je dodatkom 2% praha kore mandarine volumen filtrata smanjen čak za 44% u odnosu na osnovnu isplaku. U uvjetima povišenog tlaka i temperature (34,5 bar i 88 °C) dobiveni su još bolji rezultati, odnosno zabilježeno je smanjenje volumena filtrata za čak 61,5% (isplaka B4) u odnosu na osnovnu isplaku. Vrijedno saznanje vezano za filtracijska svojstva je da prah kore mandarine uspijeva dodatno smanjiti filtraciju isplake na bazi vode koja sadrži PAC R (komercijalni aditiv za smanjenje filtracije) do čak 46,2%. Metodom 16- V satnog starenja isplake na povišenim temperaturama (75 °C, 100 °C, 125 °C, 133 °C, 142 °C i 150 °C) ispitivana je i temperaturna stabilnost isplake na bazi vode u koje je dodan prah kore mandarine te je dokazano da ovaj materijal ima pozitivan utjecaj na filtracijska svojstva isplake i nakon njenog dugotrajnog izlaganja temperaturama do 133 °C. Vrijednosti reoloških svojstava isplaka na bazi vode koje, u različitim koncentracijama i veličinama čestica, sadrže prah kore mandarine nisu smanjene, ali nisu niti značajno povećane što znači da dodavanje ovog materijala ne narušava navedena svojstva isplake. Općenito, dodavanjem praha kore mandarine (obje ispitivane skupine veličina čestica) vrijednosti reoloških svojstava ispitivanih isplaka se povećavaju, ali ostaju u prihvatljivim granicama do koncentracije od 1,5% praha kore mandarine na volumen vode. Pri koncentraciji od 2% praha kore mandarine (veličine čestica između 0,10 i 0,16 mm) značajno se povećavaju vrijednosti reoloških parametara. Korištenje isplake na bazi vode s 2% praha kore mandarine u bušotinskim uvjetima rezultiralo bi povećanim gubitkom tlaka pa se može zaključiti da je prihvatljiva koncentracija praha kore mandarine do maksimalno 1,5% na volumen vode za testirane veličine čestica. Iznimno pozitivan utjecaj praha kore mandarine uočen je i kod ispitivanja bubrenja umjetnih uzoraka stijene (peleta) koje u svom sastavu imaju glinovitu komponentu. Smanjenje bubrenja peleta u isplakama koje sadrže prah kore mandarine, u usporedbi s bubrenjem u osnovnoj isplaci, bilo je vidljivo već nakon dodavanja 0,5% ovog aditiva na volumen vode za pripremu isplake. Međutim, najbolji rezultati, odnosno najmanje bubrenje peleta, zabilježeno je u isplakama na bazi vode koje su sadržavale 2% praha kore mandarine. Smanjenje bubrenja peleta nakon 24-satnog ispitivanja bilo je do 45% u sobnim uvjetima, a do 48,6% na temperaturi od 90 °C. Jednako kao i za provedena ispitivanja filtracijskih svojstava, još bolji rezultati zabilježeni su u uvjetima povišene temperature u odnosu na sobne uvjete, a čestice veličine između 0,10 mm i 0,16 mm imaju nešto bolji učinak na smanjenje bubrenja, tako da ovi rezultati dobro koreliraju s rezultatima mjerenja filtracije što je bitno za procjenu poboljšanja stabilnosti kanala bušotine. Usporedbom zbirnih rezultata ispitivanja utjecaja praha kore mandarine na sva promatrana svojstva isplake na bazi vode, može se zaključiti da su nešto bolji rezultati dobiveni s većim česticama praha kore mandarine (između 0,10 mm i 0,16 mm), iako se s obje veličine dobivaju zadovoljavajuća svojstva ispitivanih isplaka. Dodavanjem praha kore mandarine u koncentraciji od 1,0% do 1,5% na volumen vode za pripremu isplake, bez obzira na veličinu čestica, dobiveni su iznimno pozitivni rezultati vezani uz filtracijska svojstva i stabilnost kanala bušotine. Navedena koncentracija može se smatrati optimalnom za pripremu VI isplake na bazi vode koja će se koristiti za izradu kvalitetnog kanala bušotine. Međutim, osim očitog pozitivnog utjecaja na filtracijska i reološka svojstva isplake te povećanje stabilnosti kanala bušotine, primjenom praha kore mandarine u isplakama na bazi vode doprinosi se i konceptu kružne ekonomije u tehnologiji bušenja obzirom da iz otpadnog materijala nastaje koristan aditiv za dizajn isplake.

Exploration around the salt bodies has always been challenging for well planning and construction due to associated geological complexity and well-planning uncertainties. With the placement of wells in a deep and complex offshore salt environment, the wellbore stability becomes an all-important element for meeting well construction objectives. The well construction risks associated with instabilities encountered around salt structures can be high to the extent where they drastically influence the well design and delivery costs. To facilitate the planning and drilling of wells in such a challenging and expensive environment, the in-situ conditions around salt bodies must be accurately characterized. In turn, that will enable accurate quantification of formation pressure and fracture gradients, a crucial input for casing design. The standalone approach to well architecture design is often preferred during the planning stage due to its simplicity. However, such an approach may become inadequate once the influence of salt structures on the well design is properly understood. The salt-induced stress perturbations and local geomechanical instabilities found around salt bodies, if underestimated, may incur substantial well construction costs and consequently jeopardize the overall economics of a prospect. Hence, one shall adhere to a holistic approach toward the well architecture design by considering the multiple factors including, but not limited to, geomechanical instabilities, abnormal pore pressures, areas of accelerated creep, and other trajectory-specific drilling problems. In other words, leveraging a multidimensional approach to well architecture design, such that all specific risks peculiar to the offshore salt environment are well understood and evaluated, shall enable operators to capture additional value from the well construction while improving returns, decreasing risks, and enhancing operational safety in Deepwater salt plays., Složenost geoloških struktura u podzemlju i nesigurnosti povezane s istraživanjem ležišta ugljikovodika u blizini solnih naslaga dodatno usložnjavaju proces projektiranja i izrade bušotina u odobalju. Pri projektiranju bušotina u dubljim i složenijim dijelovima odobalja, stabilnost kanala bušotine postaje vrlo važan element u postizanju ciljeva izrade istih. Stupanj nestabilnosti kanala odobalnih bušotina povezanih s bušenjem naslaga soli može utjecati na projektiranje i tehničku izvedbu tako da ugrožava ekonomičnost bušotine. Stoga je važno odrediti deformacije i stanje naprezanja unutar i u neposrednoj blizini naslaga soli, a s ciljem lakšeg planiranja i što efikasnije izrade bušotine. Precizna karakterizacija spomenutih veličina omogućuje točnu procjenu pornog tlaka i gradijenta loma, ključnih parametara za projektiranje zacjevljenja kanala bušotine. S ciljem pojednostavljenja projekta izrade bušotine, utjecaj perturbacija naprezanja i lokalnih geomehaničkih nestabilnosti uzrokovanih dugotrajnim puzanjem soli često je umanjen tijekom procesa planiranja. Posljedično, necjelovit pristup projektiranju može prouzročiti dodatne troškove izrade bušotine u odobalju te, u određenim slučajevima, dovesti u pitanje ekonomsku opravdanost investicije u razradu polja. Stoga je, zbog višestrukog utjecaja solnih struktura na tehničku izvedbu kanala bušotine, vrlo važno holistički pristupiti projektiranju putanje bušotine. Sveobuhvatnim pristupom moguće je konvencionalan pristup planiranju proširiti razmatranjem utjecaja geomehaničkih nestabilnosti, lokalnih perturbacija pornog tlaka, rizika vezanih uz puzanje soli i ostalih problema koji izravno utječu na projektiranje putanje kanala bušotine. Naime, sagledavanjem svih rizika svojstvenih naslagama soli u odobalju, naftno-plinskim kompanijama omogućeno je stvoriti dodanu vrijednost kroz ubrzani povrat ulaganja, smanjenje rizika te povećanje operativne sigurnosti.

A new algorithm for fast absolute permeability calculation was developed and implemented into computed code. The algorithm uses binarized digital micro-CT rock sample images as input data and estimates sample permeability by analysing pore geometry., Razvijen je, i implementiran u računalni kod, novi algoritam za brzi proračun apsolutne poroznosti. Ulazni podatci uključuju digitalne binarizirane mikro-tomografske slike uzorka stijene. Geometrijskom analizom mreže pornog sustava, izračunata je apsolutna propusnost uzorka.

In regards with the global increase in energy demand, especially regarding oil and gas, the industry brings up new innovations daily in order to satisfy the increasing demand. One of the solutions in oil and gas production could be the hydraulic concentric tubular pump, which is invented and tested at the Montanuniversität Leoben, Austria. For a better understanding of this artificial lift system, three scientific works are presented dedicated to this system. Based on them theoretical cases were made and an economic comparison has been made between this system and the sucker rod pumps, which are the most common in the oil industry. After the calculations in set cases, it may be concluded that in general this innovation requires less energy in comparison with the most common artificial lift system, which can postpone the economic limit in oil and gas production and in the end increase the ultimate recovery factor of a reservoir., S obzirom na globalni porast potražnje za energijom, prije svega nafte i plina, industrija dolazi svakodnevno do novih inovacija u svrhu zadovoljavanja sve veće potražnje. Jedno od rješenja u proizvodnji nafte i plina može biti i hidraulička pumpa u bušotini s koncentričnim opremanjem tubinga, koja je proizvedena i testirana u Sveučilištu u Leobenu u Austriji. Za bolje razumijevanje samog proizvodnog sustava, u ovom radu predstavljena su tri znanstvena rada posvećena ovom sustavu. Na temelju njih su postavljeni teoretski slučajevi gdje se provela ekonomska usporedba između ovog proizvodnog sustava i dubinskih sisaljki s klipnim šipkama koje se najčešće koriste u naftnoj industriji. Nakon izračuna u postavljenim slučajevima, može se doći do zaključka kako općenito takva inovacija zahtijeva manje energije u odnosu na najčešću proizvodnu opremu, što može dovesti do pomicanja granice ekonomičnosti u proizvodnji nafte te konačno do povećanog iscrpka ležišta.

In order to reach more complex reservoir and increase ultimate recovery, engineers are searching for new technologies. One of these is intelligent completion which provides system monitoring, fluid production or injection control, and optimization. Operator can make decisions about managing completion based on real-time data coming from the downhole sensors. In addition, machine learning is becoming more popular in the oil industry. It finds application in automatization of processes and reducing time and error in decision making process. The aim of the thesis is to couple intelligent completion with machine learning (neural network) on the real example-gas well. The goal is to see if neural network can predict optimal interval control valve sizes for specific scenarios., Inženjeri kontinuirano istražuju nove tehnologije kako bi razradili kompleksnija ležišta i povećali njihov ukupni iscrpak ležišta. Jedan od načina je i inteligentno opremanje koje pruža mogućnost daljinskog nadgledanja i kontrole cjelokupnog procesa pridobivanja ugljikovodika, te optimizaciju cijelog procesa. U takvom sustavu operator donosi odluke na temelju podataka koji dolaze u stvarnom vremenu sa senzora postavljenih u bušotinu. Nadalje, strojno učenje (engl. machine learning) postaje sve popularnije i u naftnoj industriji. Primjenjuje se u automatizaciji procesa kako bi se smanjilo vrijeme i greške prilikom donošenja odluka. Cilj ovoga rada je spojiti inteligentno opremanje sa neuronskom mrežom na stvarnom primjeru plinske bušotine. Uz to, cilj je vidjeti može li neuronska mreža predvidjeti optimalne veličine intervalnog kontrolnog ventila za različite slučajeve.

The aim of this thesis was to present a general overview of Paradigm Sysdrill 10 well planning software, with a comparison of torque and drag results calculated by the Sysdrill 10 Torque and Drag module for different BHA at the same depth. The data regarding a geothermal well drilling were given by the drilling company Cougar that operates in Turkey. Geothermal well drilling was presented in the thesis to show the complexity of such operations, particularly in directional drilling, and the benefits that such software has on it, in terms of planning and monitoring directional wells., Cilj ovog rada bio je prikazati opće značajke Paradigm Sysdrill 10 sustava za projektiranje bušotina, uz konkretnu usporedbu vrijednosti torzije i natega za različite BHA na istoj dubini duž kanala bušotine. Podaci vezani za geotermalnu bušotinu dobiveni su na korištenje od strane kompanije za usmjereno bušenje Cougar, čije se djelatnosti odvijaju u Turskoj. Objašnjena je izrada geotermalnih bušotina u cilju prikaza složenosti tih postupaka, s naglaskom na usmjereno bušenje i kako sustav poput Sysdrill 10 moţe pomoći u projektiranju i praćenju usmjerenih bušotina.