Integrated Downhole Fluid-Identification Applications for Complex Reservoir Development

摘要

A UAE offshore operator drilled a pilot well in a complex layered Middle Jurassic formation to confirm where to land a high inclination production drain.(fig.1) The reservoir underwent gas injection EOR. Reservoir simulation (fig.2 & fig.3) predicted possible gas fingering that required further confirmation before committing to drill the production drain. The objective was to positively identify all the intervals where liquid hydrocarbons could be produced avoiding the dry gas zones. The petrophysical analysis was complicated by rock heterogeneities and fluid mixture including gas, heavy and light oils plus water. A traditional approach based on standard logs was not conclusive. The addition of NMR data versus depth allowed carbonate texture differentiation and a derivation of a simultaneous multi-fluid solution that highlighted the zones with possible oil production. This initial information was used to select suitable intervals where to perform nuclear magnetic resonance stations with diffusion editing sequences designed for maximum sensitivity in the gas to light-hydrocarbon region. Several stations were acquired with success and processed in pseudo real time making use of a VSAT link to share data with wellsite and predict the most likely downhole fluid composition. With integrated depth and stationary information available, a few sets of sampling stations with real time downhole fluid identification were chosen to verify dynamically which formation fluid would finally flow. Sampling was achieved using either a single probe or dual packer, depending on rock permeability. The fluid was analyzed in-situ using an innovative downhole fluid separation technique and some samples were taken to surface for a final PVT confirmation. The combined information proved to be very consistent and provided a detailed reservoir description inclusive of fluid types.(fig.31, fig. 32 & fig.36) The immediate impact was the change of the target drain location to a deeper zone in the reservoir structure. In fact, the zone a priori selected would have produced dry gas, due to the poor miscibility of injected gas and relatively viscous hydrocarbons in place. The methodology used was well communicated to the various operator departments and had a broad acceptance, opening the way for a systematic similar approach in the future.