NMR Logging Through Fiber-Reinforced Plastic Casing For an ASP EOR Monitoring
This poster addresses sweep efficiency evaluation of an Alkali-Surfactant-Polymer (ASP) multi-wells field pilot using NMR logging (Al-Yaarubi, et al. 2015 & Guntupalli, et al. 2018). The subject field is a large brownfield in the south of Oman. The targeted reservoir is in Al-Khalata sandstone formation. Initial screening using coreflood investigations and single-well tests indicated significant additional recovery due to the ASP flooding. Due to the large capex involved for full-field application, a multi-well field pilot was first implemented to assess operational challenges and the economic attractiveness. The pilot was aimed to estimate the ultimate recovery, optimize chemical formulation and highlight operational and technical challenges that may arise if expanded to a larger area of the field. The pilot comprised a 1.4-acre (75 m x 75 m) inverted 5-spot pattern that consisted of seven wells. These included four corner producers, one center injector, one data gathering observation well, and one sampling observation well. The pilot was executed with several phases, starting with pre-ASP waterflooding. This was followed by ASP slug injection, which, in turn, was followed by polymer chase and, ultimately, by chase water injection (Al-Shuaili, et al. 2018). The surveillance strategy is designed to monitor lateral and vertical flooding conformance across the area and throughout all phases of the pilot. Surveillance objectives included oil desaturation monitoring at the data-gathering observation well and the EOR fluid front monitoring as it advances from injector towards producers. Chemical EOR processes often cause alteration of original rock and fluid properties, such as wettability and salinity. Thus, the traditional carbon-oxygen and salinity-dependent resistivity and sigma logging, have limited applicability to quantitively monitor time-lapse saturation at the desired accuracy. NMR logging, which is sensitive to fluid volumes, was envisioned to provide a robust alternative to the traditional methods. Prior to field implementation, NMR was successfully used to measure desaturation during ASP coreflood experiments (Mitchell et al. 2012). The same technique was applied during the single-well experiment (Arora et. Al. 2010). NMR logging essentially requires an open hole or wells completed using magnetic-transparent materials. The data-gathering observation well was purposely drilled at 10-deg deviation and located midway between the injector and a producer of the pilot. This well was completed with fiber-reinforced plastic casing (FRP) to allow time-lapse saturation monitoring using the field-gradient NMR device. This device operates at multiple frequencies and provide measurements at multiple depths of investigation from the casing wall in a form of thin “shells” of sample volume. The well design and completion was made such that deeper shells measure entirely within the formation, which is beyond the casing and cement. NMR showed that waterflood reduced the average oil saturation from 81% to 59%, amounting to 22% desaturation. NMR showed the passage of oil bank and the subsequent desaturation of additional 31% due to ASP. Independently, total pattern BS&W was reduced by about 20% as measured by multiphase flowmeters. The estimated incremental recovery due to ASP is about 30% over that of waterflooding as estimated from decline curve analysis (Al-Shuaili, et al. 2018).
AAPG Datapages/Search and Discovery Article #90328 © 2018 AAPG Middle East Region GTW, Maximizing Asset Value: Integrating Geoscience with Reservoir Management & Facilities Optimization, Muscat, Oman, April 30 – May 1, 2018