New NMR technology detects OWC: A dual-method nuclear magnetic resonance (NMR) interpretation analysis helped characterize Niger Delta reservoirs

摘要

Even when several development wells have been drilled in certain Niger Delta reservoirs, key questions about the true irreducible water saturation and the point of the actual oil-water contact (OWC) remain. A lack of resistivity contrast may exist from high irreducible water saturation and from wireline sensor response masked by the invasion of drilling fluids such as oil-based mud filtrate (OBMF). One of a dozen prior wells, Well-Aa was far along in the development of this Niger Delta field and its log response was typical of some Niger Delta reservoir packages. Although the resistivity profile clearly showed an upper reservoir oil column, the fluid type in the sand reservoirs below remained questionable. Conventional log analysis indicated the lower reservoirs to be water bearing, but a lengthy testing program eventually proved the lower reservoirs to contain oil. The resistivity values may result from decreasing formation water salinity with depth, which is not uncommon in Niger Delta reservoirs.