A Large Heavy Oil Reservoir in Lake Maracaibo Basin: Cyclic Steam Injection Experiences

Abstract

Abstract The Bachaquero-01 reservoir is located in the Lagunillas field within the Lake Maracaibo Basin in western Venezuela. This reservoir is divided into two zones: the lake area operated by Lagoven, S.A. and the land area by Maraven, S.A. both companies are PDVSA (Venezuela Oil Holding) subsidiaries. The Cyclic steam injection experiences of the Lake Maracaibo area are discussed in this paper. This part of the reservoir covers 19,540 acres of unconsolidated sand, with an Original Oil In Place (OOIP) of 6,621 million stock tank barrels (MMSTB) of 11.7 API with a viscosity of 635 cps, at initial conditions of 1,360 psia and 128 F. Cyclic steam injection as a development strategy in Bachaquero-01 began in February 1971. From that date until December 1995 a total of 325 wells have been steam stimulated with 860 cycles. Currently, the reservoir produces 40,000 BOPD with 285 active wells, which is the world's largest offshore operation of th is type for a single reservoir. This report describes the experiences with surface and completion equipment for steam injection wells, fluids for thermal insulation (casing-tubing), offshore equipment for steam injection, steam injection with additives (gas, solvents and surfactants); also good results of cyclic steam injection into two horizontal wells completed in this reservoir in 1995 are presented. Introduction The Bachaquero-01 reservoir is located on the Eastern coast of Lake Maracaibo approximately 75 miles southeast of Maracaibo City, Venezuela. It represents one of the most important single accumulations of heavy oil in the Bolivar Coastal Field (BCF) of western Venezuela (Fig. 1). The lake area of this accumulation, comprising 53% of the total reservoir, covers 19,540 acres of unconsolidated sand with an Original Oil In Place (OOIP) of about 6,621 million of Stock Tank Barrels (MMSTB) of 11.7 API oil with a viscosity of 635 cps at the initial conditions of 1360 psia and 128 F. Structurally, the reservoir is a simple monocline, dipping 2 – 3 to the southwest. It is bounded on the south, west and northwest by an aquifer with moderate activity (Fig. 2). It is comprised of nine producing intervals of unconsolidated Miocene Sands of the Lagunillas formation. The sands, of fluvio-deltaic origin, are found at subsurface depths averaging 3,000 ft (See Figure 3). Average properties of the reservoir are shown in Table 1. The Bachaquero-01 was discovered in 1934, however, during the first 20 years it was poorly developed due to low well productivity and high sand production. In the 1960's new sand control techniques were developed and the reservoir exploitation increased. The daily production rate increased from 5,000 BOPD in 1963 to 10,000 BOPD in 1965. In 1971 an extense drilling program was initiated to complete the development with normal spacing (19.3 acres/well). In 1982 well spacing was optimized to 6.4 acres/well in order to increase the primary recovery factor up 14%. Figure 4 presents the original and current well spacing in Bachaquero-01. As is common in BCF Miocene reservoirs, primary production is characterized by an initial period predominated by solution gas drive. This mechanism contributes 0–5% of OOIP, depending on oil viscosity (Bachaquero 01: 3.4%). After that, due to the unconsolidated nature of reservoirs sediments and its shallow depth, formation compaction became the principal reservoir producing mechanism as a consequence of pressure decline caused by fluid withdrawals. In Bachaquero-01 reservoir the compaction mechanism may contribute up to 9.2% of OOIP. Cyclic steam injection as a development strategy in Bachaquero-01 began in 1971, and until December 1995 a total of 325 wells have been stimulated with 860 cycles (see Figure 5). Currently the reservoir produces 40,000 BOPD with 285 active wells. Figure 6 shows total production and active wells for Bachaquero-01. P. 337