Cyclic steam injection has been performed since the early 60's in Lake Maracaibo reservoirs. The design parameters for a cycle have been generated from performance data gathered from stimulations of over 500 cycles. However, these design parameters include some statistical dispersion that might prevent attaining an optimum cycle design. While obtaining operating experience, Lagoven has also been using and developing numerical models to improve and optimize the cyclic-steam operating strategy.
A simulation and sensitivity study of the design parameters for the cyclic steam injection in the Bachaquero reservoir was performed to determine more accurate values than those obtained from field operations only.
These parameters are injection rate, heat or steam input, steam quality and soak time. Additionally, analyses were made of reservoir behavior during steam drives, gas injection with steam, heat transfer between layers and an investigation of the use of horizontal wells for oil recovery by steamflooding and cyclic steam, using a fully implicit thermal simulator.
The results of 3-D simulations mainly showed that horizontal well patterns improved productivity. Besides improving the reservoir volumetric sweep efficiency, use of horizontal wells has the advantage of requiring fewer wells per area swept, projects being completed in a significantly shorter time.
The Bachaquero reservoir, located in Lake Maracaibo in western Venezuela, originally contained 6621 million stock tank barrels (MMSTB) of 11.7 deg API crude oil with a viscosity of 635 cp, at an initial reservoir temperature of 128 deg F. Structurally, the reservoir is a simple monocline, dipping 2 deg to 3 deg to the southwest (Figure 1). The reservoir is comprised of nine producing intervals in unconsolidated Miocene sands of the Lagunillas formation (Figure 2). The sands, of fluvio-deltaic origin, are found at subsurface depths averaging 3000 ft.
The reservoir is limited on the south, west and northwest by a weak aquifer. The average properties of the reservoir are shown in Table 1.
Table 1. Reservoir basic data for initial conditions
Oil gravity deg API 11.7 Depth ft. 3000.0 Original pressure psia 1365.0 Permeability mD 4000.0 Porosity % 33.5 Nos thickness feet 200 Initial oil viscosity cp 635.0 Temperature deg F 128.0 Pore compressibility psia. 260.0 × 10-6 Gas-oil ratio scf/bbl 87.0 Oil saturation % 80.0 OOIP MMbbls 6621.0
As is common in the Bolivar Coast reservoirs, formation compaction has been the principal reservoir producing mechanism during the primary/steam soak phase.