Steam-Drive Pilot in a Fractured Carbonated Reservoir: Lacq Superieur Field
Authors: Sahuquet Bernard C. , Ferrier Jerome J. 
Published: December 2007

Summary
In 1977, a steam-drive pilot was initiated in the Lacq Superieur field in southwest France. The unusual aspects of this pilot test lie in the characteristics of the reservoir, which is carbonated, dolomitized, and highly fractured in the pilot zone. The pilot pattern [35 acres (141 × 103 m2)] has an irregular configuration delineated by six old producing wells; only the injection well was specially drilled. A stable steam-injection rate [176 ton/D (1.85 kg/s)] has been maintained since the pilot began in Oct. 1977. Oil production increased very early and now the oil/steam ratio is about 0.2. A GOR evaluation induced by an increasing CO2 content was noted on several wells. Only a small temperature evolution has been observed at one producing well. This test shows that the fracturing of a reservoir, at least when it is intensive, does not prevent an efficient steamdrive.

Introduction
Lacq Superieur field, on the north side of the Pyrenees Mountains, is in a region where frequent heavy oil shows were observed in calcareous or dolomitic formations that were often heterogeneous and fractured. Relatively good reservoir characteristics and the importance of the OOIP [about 125 MMbbl (20 × 106 t)] justified production of the field from its discovery in 1950. In 1975, after 25 years of production, the low recovery (17% OOIP) and the importance of remaining oil prompted research for a method to increase recovery of this field. A reservoir engineering study and experimental research on enhanced recovery processes showed that the most favorable process for the calcareous, fractured parts of this field would be steam drive. The original and new conditions of the steam-drive application in a calcareous, fractured reservoir caused us to conduct a pilot test to evaluate such operating points as pattern size and injection rate. This steam-drive pilot was started in Oct. 1977, and an extension phase is projected from the favorable results obtained.

The Lacq Superieur Field
The Lacq Superieur oil field (Fig. 1) is in southwest France near the Pyrenees Mountains. Production began in 1950, and 80 wells were drilled for its development. The saturated horizon is the Lower Senonian limestone of the Upper Cretaceous age. Overburden is provided by Campanian argillaceous, calcareous rocks or by Paleocene marls. The reservoir is limited by underlying carbonates of Turonian, Cenomanian, and Albo-Aptian ages. These structures form a huge aquifer, more than 6,500 ft (2000 m) thick, that maintains the pressure of the oil reservoir. The Lacq Superieur structure is an anticline with a northwest/southeast axis. The oil-bearing area is about 1,400 acres (5.7 × 106 m2). Depth is 1,970 to 2,300 ft (600 to 700 m), and thickness is about 400 ft (120 m). Reservoir limestones exhibit two facies with very different characteristics:The central portion contains very fractured, occasionally karstic, dolomites with a matrix porosity of 10 to 15% and a very low matrix permeability (about 1 md). However, the high fractionation of this zone results in a very high productivity,The northwest and southeast parts of the field are constituted of calcareous rocks with an average porosity of 20% and permeability of 1 to 10 md.
This facies has only some microfissures, and productivity is low. The OOIP was about 125 MMbbl (20 × 106 m3), of which 17% currently is produced.
JPT
P. 873^