Successful Miscible-Gas Injection in Rhourde El Krouf Field, Algeria

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 94138, "Successful Peripheral Miscible-Gas Injection in RKF Field, Berkine Basin, Algeria - Field Case History," by H. Cubillos, SPE, D. Stright, SPE, and J.M. Navarro, SPE, CEPSA, prepared for the 2005 SPE Europec/EAGE Annual Conference, Madrid, Spain, 13–16 June. The full-length paper discusses the discovery, development, and operation of a miscible-gas-cycling project in a highly under-saturated, volatile-oil Trias Argilo-Gréseux Inférieur (TAGI) reservoir in the center of the Berkine basin, Algeria. Miscible gas injection was initially considered to be high risk and marginally economic; however, with a flexible development plan and careful reservoir management, a successful project was implemented. Introduction The Rhourde El Krouf field (RKF) in Block 406A of the Berkine basin, 480 miles southeast of Hassi Messaoud, has been producing under partial pressure maintenance by miscible-gas injection from the TAGI since 1996. Deeper volatile-oil and retrograde-gas-condensate reservoirs provide makeup gas for the project. After 9 years of peripheral, downdip, high-pressure gas injection, RKF has recovered more than 50 million bbl of oil with 90 Bcf of gas injection. Primary recovery was predicted to be less than 40 million bbl of oil. The field still is producing at the 20,000 BOPD designed plateau rate. Background The RKF was discovered in 1992 by the Rhourde El Krouf 1 exploration well drilled to a total depth of 16,670 ft. Sustained production from this well was 5,000 BOPD from two siliciclastic reservoirs, the TAGI and the Carboniferous, from 9,843 to 11,155 ft. The RKF was brought on stream in 1996 in a record time for a remote area with no infrastructure. In 1997, the field was placed on full production with seven producing wells and three gas-injection wells. As dynamic production data were acquired, facilities were expanded and wells added. Reservoir Description The TAGI reservoir consists of stacked fluvial deposits, including porous sandstone units interbedded with claystones, resting unconformably over Carboniferous rocks. The TAGI was deposited over a relatively stable continental surface closely related with the regional Hercynian unconformity. A combination of well logs and core descriptions was used to subdivide the 492-ft TAGI section into three well-constrained stratigraphic sequences (Upper, Middle, and Lower) that have excellent lateral continuity. Pressure and tracer data indicate good lateral and vertical communication. Significant anisotropy with a southwest/northeast azimuth has been observed that is aligned with the main fault patterns. Vertically, high-permeability layers exist together with lower-permeability sublayers within individual flow units that are, on average, 33 ft thick. Porosity varies from 12 to 18%, with a 16%average. Permeability varies from 10 to 1,000 md, with an average of 200 md. Producing-sand bodies are mainly composed of quartz (94%) and clays (6%), which are mainly chlorite, illite, and kaolinite.