Identification of CO2 with Magnetic Resonance Key to Understanding Production Behavior in Campo Palo Azul, Ecuador: A Case Study

Abstract

This paper presents a case study focusing on the identification of CO2 in the Hollin reservoir, based on nuclear magnetic resonance (NMR) responses, its effect on petrophysical parameters, and the negative effect on production wells. The paper also discusses the strong relationship between the high CO2 concentration in the reservoir fluids and its effect on the formation damage resulting from pore throat plugging from organic and inorganic compounds. After NMR processing, it was possible to identify the CO2 response on porosity related to low hydrogen index; the PVT analysis reveals approximately 75% of CO2 in the reservoir conditions and more than 95% of CO2 in the surface conditions. Asphaltene stability was evaluated through four methods, including the Leontaritis method, colloidal stability index, Stankiewicz method, and stability crossplot (SCP), which enabled the determination of an unstable probability by more than 75%. Palo Azul wells have been completed with production enhancement techniques, such as fracturing, to minimize the formation damage and its negative production effect; until now, however, the unique hypothesis considered formation damage to be related to fine-grain migration without considering that the primary problem could be connected to asphaltene stability. The deposition of the asphaltene can plug pore throats, reducing wellbore permeability and dramatically reducing fracture wings conductivity. This case study provides several important contributions. First, NMR results were successfully used in the Oriente basin of Ecuador to identify reservoirs with high concentrations of CO2. Second, four different diagnostic methods were used to determine asphaltene stability; this has altered common paradigms regarding production reservoir behavior. Finally, this case study enabled the development of a new technical hypothesis concerning additional factors that can contribute to formation damage in the Hollin reservoir.