Oil Rim Development by Waterflooding: Lessons Learned from the South Turgai Basin Case Studies

Abstract

Abstract Characteristics of two brown oil fields in the South Turgai Basin Field pose challenges for field development. Oil rims bounded by a gas cap on one side and an aquifer on another side are present in both fields. These features coupled with poor rock properties make the development of these fields challenging. As a result, enhanced recovery techniques are necessary for the economic production of oil and gas. Accounting for high saturation pressure and weak aquifer reservoir pressure maintenance is considered critical for the oil rims, as these have been observed twofold reduction in reservoir pressure from the initial one. To account for such effects, previous producing wells were converted to gas and water injection wells. Gas flooding of the gas cap is performed using a crestal injection pattern and water injection into the gas-oil zone is used to create a water bank between the gas cap and oil rim. Moreover, waterflooding is launched by peripheral water injection with additional advantages attained from gravity segregation. Changes in oil and gas production have been observed as a result of pressure maintenance. First, the gas injection implemented in the wells located 1.5-3 km from the oil rim have prevented gas breakthrough in producers, despite the significant injection rates. High injection rates are a result of the governmental ban on flaring, since this prompts engineers to perform faster pressure recovery and target a higher volume of utilization. Second, crestal and edge water injection have different results in different parts of the fields. However, in some cases rim flooding have devastating effects as producers have been watered out. On one hand, the goal of decreasing the gas production was met, which happened earlier and was one of the biggest challenges facing the company. Consequently, gas-oil ratios decreased almost ten times than what they were previously. On the other hand, water-cut on many producers increased substantially, up to 90-95%, and in some areas top injectors even killed the down-dip producers. Thus, despite having solved one problem, another one emerged. Overall, crestal water injection (i.e., barrier waterflooding) is risky, and requires stricter regulation and management. Case studies from the South-Turgai Basin show that water injection in top structure of oil rims for enhanced oil recovery can lead to positive as well as negative results. Overall, the paper demonstrates the cause-and-effect relationship of the previously described impacts of such water injection and provides recommendations for proper waterflooding management in oil rims based on the success in other parts of the field.