Differential Depletion in Heavy-Oil Sands, Lagunillas, Venezuela

Abstract

Abstract Cyclic steam stimulation has proven to be an effective and flexible process for recovering heavy oil. However, similar to the other projects in the Bolivar Coast Field, vertical steam distribution in the Laguna reservoir is poor due to thicker pay zone divided into two different main sand bodies, an upper sand unit a lower sand unit. Difference fluid and rock properties for the lower sand have contributed to production, meanwhile, the upper sand stayed at original conditions, This work illustrates how an interdisciplinary team can set up new approaches to a mature reservoir, in order to optimize the exploitation of heavy oil sands. This approach was within the context of areal definitions, steam soak scheduling and production forecast. It was concluded that the Laguna upper sands should be treated as an individual reservoir. Introduction The Laguna reservoir, within the Bolivar Coast Field (Fig. 1), initially contained 9791 MMSTB of heavy oil in place (11,5API). Reserves are for 1928 MMSTB (14 % STOIIP) due to solution gas drive and compaction stimulated by steam soak. The reservoir is made up of two sand intervals separated by a continuous clay unit throughout the entire field. Steam soak in open hole gravel pack completion has been applied extensively to the entire section without an effective selective completion. Due to differences between oil-sand properties (viscosity, porosity and permeability), a strong differential depletion situation has occurred where most of the recovery has come from the lower sands leaving the upper sands almost at original conditions. This in turn has led to major production difficulties and compromise reserve on the order of 40%.In order to resolve this problem, an interdisciplinary team was set up to study the geological correlations and areally delineate the upper sands in the Laguna reservoir. New petrophysical data coming from cores and recent logs were studied to establish relationships between resistivity and hydrocarbon pore volume. Based on this, net oil sand maps and volumetric could be derived for the upper sands. A numerical simulation was conducted in order to estimate the production response and the maximum oil recoverable of a well completed in this sand. Simultaneously, a pilot test was proposed in the field to evaluate reservoir parameters and production performance in two wells completed in the upper sand. GEOLOGICAL AND PETROPHYSICAL ASPECTS The Laguna reservoir (located in Lagunillas Field) is a monoclinal structure with dips of 3 to 4 SW which extends to the Maracaibo Lake concessions. Laguna reservoir is part of Lagunillas formation (Middle Miocene) and is located between updip fresh water in the North, the Alto Pueblo Viejo in the East, a limit concesion to the West and an aquifer to the South [1]. P. 217^