Exploring EnKF stability under different type reservoir model

Abstract

Reservoir properties estimation aims to maximize the recovery, and least squares history matching is one of strategies to achieve this target. A model is matched on historical data until it reproduced the production history. The model can be used to simulate future reservoir production. The Ensemble Kalman Filter (EnKF) is a method designed for non linear history matching. The success of EnKF affected by non linearity of reservoir model. If the reservoir model is highly non linear the EnKF might be unstable. This paper aims to explore the stability of EnKF under different type reservoir models. Three type reservoir models are considered: (1) line source (2) bounded constant rate no flow (3) three grid reservoir. The permeability is estimated based on the matching of observed pressure with predicted pressure (obtained from reservoir model). A simulation experiment is set up to show how EnKF iteration may be used to estimate the permeability. The simulation is set up for an experiment time T, time step Δt, and number of observations m. The prior for permeability is drawn from normal distribution. The stability of the results is presented as plots of pressure history match and permeability updating. The three grid reservoir model shows instability of EnKF method, and an iterative EnKF was proposed to improve the stability of EnKF. The proposed method is able to history match the reservoir models, and exploring the stability of a more general reservoir model is needed for the success of the method.