Probabilistic Original Oil in Place (OOIP) in Producing Clastic Reservoirs of the Niger Delta

Randle, J. M., D. B. Faparusi, R. E. Fitzmorris, K. D. Kelsch, Chevron Nigeria Ltd, Lagos, Nigeria

A process is described to probabilistically evaluate Oil-In-Place (OOIP) in mature, pro­ducing reservoirs of the Niger Delta. While mature, producing reservoirs can be well-con-strained by many wells, 3-D seismic, and other data, they still have uncertainty in OOIP and reserves that can effect reservoir management decisions. The process described uses two independent, probabilistic methods: Volumetric OOIP and Material Balance OOIP. The Volumetric OOIP method uses earth models to sample uncertain factors such as area, thick­ness, water saturation, and net pay cutoffs. For reservoirs with sufficient production histo­ry, OOIP is also calculated using a probabilistic Material Balance method. Uncertain variables for Material Balance calculations typically include initial pressure, pressure history, volumes of fluids produced (oil, gas, water), PVT properties, and aquifer size. For both methods, a Design of Experiments technique is used to sample the factor space for case selection and to determine response functions. Monte Carlo simulation is then used to sample distribu­tions of input factors to determine probabilistic OOIP for each of the two methods. A cross­functional, integrated team examines the two independently calculated distributions of OOIP to reconcile results, recycle as needed, and select earth model cases for reservoir simula­tion. Results from earth modeling, material balance analysis, and flow simulation are incor­porated into uncertainty management plans and overall reservoir management strategies.