Use of NMR Logging for Formation Damage Prevention: Water-Flooding Case Study in Cañadón Seco, San Jorge Basin

Abstract

Abstract Water flooding is a well-established secondary oil recovery technique used in many reservoirs around the world. A key issue involved in water flooding is the prevention of the invasion of solids, which may cause formation damage and, consequently, inhibit injection and cease production. To remove fine particles cost-effectively from the flooding fluid, it is necessary to determine the maximum size of the particles that can flow with the fluid without plugging the formation. Determination of the threshold particle size, however, is often very challenging. In this paper, we report a successful water flood case study for San Jorge Basin reservoirs. Nuclear magnetic resonance log (MRIL [R]*) data were used to obtain the pore size distribution. Core NMR measurements were conducted to validate the T2 distributions derived from the MRIL logs. In addition, the same core samples were analyzed with conventional methods, including capillary pressure, SEM, thin section, and mineralogical analyses. Based on the MRIL log and core data, we were able to establish the correlation between the T2 distributions in the formation and the threshold solid particle size allowed in the injection water, which is important to prevent formation damage. A critical step in the process is the accurate determination of T2 distribution. Different inversion techniques were used for data processing from which we found that the inversion with gamma-density functions is the most suitable for matching the NMR-based pore size distributions with core-based pore body and throat size distributions. The approach was applied to the water-flood projects for Mina el Carmen formation reservoirs, San Jorge Basin, Argentina, in a pilot project involving multiple wells. The initial estimate of lifetime cumulative oil production was estimated with conventional-analysis prediction. Based on MRIL and core data analysis of a pilot well, the water-flood filtration program was changed, and successful water injection has continued for over two years. The cumulative oil production exceeds the initial estimate by more than a factor of 2. The particle-size correlation derived from the key well was found applicable to other wells in the same formation and water injection was applied to four oil-bearing zones. NMR logs were acquired for five of the seven wells in the project. The pore-size distributions derived from these logs are used to adjust the threshold particle size in the filtration program of the individual wells. To date, all of these wells have been producing successfully.