Influence of Wellhead Pressure and Injected Gas Specific Gravity on Oil Production Rate from a Gas-Lifted Well in the Niger Delta

Abstract

The Niger Delta, a prolific hydrocarbon province, relies heavily on gas lift systems to enhance oil recovery from wells with declining reservoir energy. While the benefits of gas lift are well-established, its optimization is complex and influenced by key operational and fluid properties. This study uses PROSPER software for nodal analysis to optimize gas lift in a marginal well in the Opolo field, producing 2652.3 STB/day. The analysis examines the combined effects of wellhead pressure (WHP) and injected gas specific gravity (SG) on oil production, using calibrated reservoir, well, and Pressure-Volume-Temperature (PVT) with industry-standard correlations. A sensitivity analysis was performed by simulating three wellhead pressures (100, 250, and 400 psig) and three injection gases, methane (SG: 0.554), nitrogen (SG: 0.967), and carbon dioxide (SG: 1.519), at injection rates of 1, 2, and 3 MMscf/day. The results confirm a strong inverse relationship between WHP and production rate for all gases, with lower WHPs yielding significantly higher oil rates. Gas SG emerged as a critical factor: methane delivered the highest absolute production (up to 7,332.1 STB/day at 100 psig), while nitrogen provided the greatest relative gain (36.09% at 400 psig), reflecting an optimal balance of density and flow characteristics. Carbon dioxide, with the highest SG, consistently showed the smallest production improvements. The study concludes that optimal gas selection is not universal but is contingent on specific well conditions: low-SG gases are generally preferred, with nitrogen being particularly effective for oil rate gains under high wellhead backpressure (400 psig) and high injection rate (3 MMscf/day); and methane excels at low WHP (100 psig) and high injection rate (3 MMscf/day). These findings offer valuable guidance for engineers to optimize gas-lift designs, focusing on key operational parameters and gas properties to boost production efficiency in the Niger Delta and similar regions.