New Methods for the Nonequilibrium Initialisation of Reservoir Models With Lateral And Vertical Variations in the Initial Fluid Composition

Abstract

Abstract New methods have been developed to initialise a compositional model for a giant Middle East reservoir where the initial H2S mol% varies laterally from close to 0 to around 20 and vertically from close to 0 to around 20, being highest at the lower parts of the reservoir. Conventional initialisation methods assume equilibrium, assigning the same composition to any grid block at a given depth. Two different new method, triangulation and surface-fit, are used to capture observed lateral non-equilibrium variations. Vertical variations in fluid composition are simulated using a gravity segregation model as in the conventional method. The non-equilibrium methods use data on the variation in fluid composition and pressure derived from 18 representative fluid samples collected from wells early in the field history. The stability of both non-equilibrium methods was initially verified through no-production reservoir simulations. Simulations were carried out to compare results using the conventional equilibrium method and the new non-equilibrium method. An improved level of detail was verified by monitoring the development of H2S mol% in the produced fluids in three selected groups of wells from regions of the reservoir with different initial levels of H2S mol%. Field development planning is greatly improved as production predictions now incorporate the regional variations in H2S mol% into the future. Same level of detail can be obtained for other components from the same simulations. As the more sour areas of the field are developed, the new model enables the prediction of H2S mol% and CO2 mol% for the engineering design of new facilities at remote stations and at the main processing plant. Introduction Most hydrocarbon reservoirs exhibit variations in composition with depth and thereby also variations in GOR, saturation pressure and fluid type. In an isothermal homogeneous non-producing reservoir the variations are due to gravity segregation (Schulte, 1980) and the reservoir is at thermodynamic equilibrium. Such fields can be handled by compositional reservoir simulators using a built-in gravity segregation model. In addition to vertical variations some reservoirs exhibit lateral variations in composition, which can be due to an ongoing inflow. To properly capture the influence of these lateral variations on the production a non-equilibrium initialisation may be needed that initialises each grid block explicitly with reservoir fluid composition, pressure, and gas and oil saturations. This paper deals with a reservoir extending several kilometres in all areal directions. The shape is a shallow dome and the vertical distance from top of the gas cap to the oil-water contact (OWC) is approximately 800 ft. There is a secondary inflow of an H2S-rich fluid from one corner of the reservoir. The inflowing fluid has not yet flushed or diffused through the reservoir and pronounced variations in the H2S mol% of the produced fluids are seen across the reservoir. At the point furthest away from the point of inflow H2S is almost absent while close to the point of inflow the H2S mol% can reach as high as 20. Proper predictions of the H2S level in the produced fluids require that the simulation model be initialised to reflect the observed variations in H2S mol% in the reservoir. Since this reservoir exhibits lateral variations, the initialisation was done by explicitly initialising each grid block with composition, pressure and gas and oil saturations. This is referred to as a non-equilibrium initialisation. An isothermal reservoir at thermodynamic equilibrium would not show any lateral variations in fluid composition.