A Comparative Study of Fishbone Wells and Multi-Fractured Horizontal Wells for a Tight Carbonate Reservoir Case Study of the Sadi Formation in Iraq

Development of a tight reservoir characterized by low permeability poses significant challenges to the oil and gas industry due to the low productivity and the excessive costs. This study explores the potential of utilizing "Fishbone Wells" compared to multi-fractured horizontal wells to maximize the net present value (NPV). The development approach is assessed for implementation in a tight carbonate reservoir in southern Iraq. Numerical reservoir simulation models were utilized to predict oil production rates and optimize key parameters such as the number of branches for the fishbone well. Analytical models were used to calculate behaviour for both types of wells then, the outcomes were compared. Our findings indicate that both multi-fractured horizontal wells and fishbone wells can achieve a positive Net Present Value (NPV) in the Sadi reservoir. However, the fishbone well consistently delivers higher NPV under specific conditions. Fishbone wells utilize smaller lateral branches, known as 'subs,' each containing 3 or 4 needles (or ribs) with a diameter of half an inch and 40 ft length. These needles usually have a 70% success rate in reservoir penetration. The well design limits each stage to12 subs because of the 40-foot spacing between subs and the 480-foot distance between hydraulic fracturing stages. Although 12 subs yield the highest production, our analysis found that a configuration with six subs optimizes NPV compared to one stage of hydraulic fracturing. It is important to note that the fishbone technology is most effective in reservoirs with permeability of 0.5 millidarcy or higher. In formations with lower permeability, hydraulic fracturing, which can be implemented below 0.5 millidarcy, may be the more reliable approach. While fishbone wells offer significant profitability potential, they also come with marginal risks and require favourable reservoir conditions to ensure success. As such, careful reservoir assessment is critical before selecting this technology for development. This is the first study that compares the economic feasibility of fishbone stimulation technology to the multi-fractured horizontal wells implemented in a real case for tight reservoir formation.