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

At the start of exploitation of any hydrocarbon or geothermal water reservoir, production of reservoir fluids is driven by natural energy in the form of reservoir pressure. Therefore, the primary task for production engineers is to maintain this reservoir pressure for as long as possible, ensuring long-term economic production. Unfortunately, sooner or later it becomes necessary to introduce mechanical methods of fluid lifting. Today, there are numerous solutions on the market for mechanical lifting of reservoir fluids (progressing cavity pumps-PCP, electric submersible pumps-ESP), and one of the oldest mechanical methods for producing reservoir fluids is the sucker rod pump. In the Republic of Croatia, sucker rod pumps are among the most commonly used mechanical methods for lifting reservoir fluids. Although Croatian production engineers have significant experience working with sucker rod pumps, operational problems are common and are accompanied by additional expenses related to workover operations. This paper presents an analysis of workover operations over a tenyear period (from 2011 to 2022) in an oil and gas field in the Republic of Croatia. Based on the collected data, an analysis was conducted to determine the volume of fluid not produced during equipment maintenance and repair, the waiting time for repairs, the duration of equipment maintenance as well as the cost-effectiveness of the repairs. Special attention is given to the analysis of the causes of problems in using sucker rod pumps and the identification of problematic wells.

Casing string is critical component in wellbore construction, defining wellbore diameter and providing structural integrity throughout drilling and production operations. Traditionally, steel casings are generally used due to their mechanical strength; however, problems connected with corrosion, particularly in environments containing hydrogen sulfide or saline formation water, presents significant operational and financial challenges. This paper explores the use of fiber-reinforced plastic (FRP) casing as a corrosion-resistant alternative, focusing on glass fiber-reinforced epoxy composites, especially concerning cementing operation. FRP casing properties depend on the fiber–matrix composition, fiber type, and manufacturing quality, which affect tensile and compressive strength, thermal stability, and resistance to environmental degradation. Laboratory and field studies, including pilot projects in Argentina, Oman, Brunei, and Kuwait, evaluated casing handling, rig down speed, anchoring, and cement bonding. Special attention was given to cementation procedures, fluid formulations for mud cake removal, and ultrasonic methods for assessing casing-to-cement bonding, adapted for the acoustic characteristics of FRP materials. Results indicate that while steel casings exhibit superior cement bonding, FRP casings can achieve adequate structural performance when mechanical stresses during handling and installation are carefully controlled and proper chemical compatibility between fluids and cement is ensured. Field data show that FRP casings can be deployed with only minor reductions in installation speed for smaller diameters and that optimized handling and cementation practices are crucial for operational success. These data demonstrate that FRP casings offer a technically feasible and economically advantageous alternative for wells in highly corrosive environments. However, their successful implementation requires tailored operational procedures, including modified handling equipment, optimized fluid systems, and specialized evaluation methods. Further field studies are needed to develop comprehensive guidelines for widespread adoption.