Laboratory Studies for Design of a Foam Pilot in the Snorre Field

Abstract

Abstract This paper presents a summary of laboratory work carried out to support the design of a foam pilot in the North Sea Snorre Field. Displacement tests in Snorre cores (60 cm long) at reservoir conditions (300 bar and 90 C) were conducted to evaluate the gas blocking and mobility reduction properties of foam as a function of surfactant concentration, foam quality, fluid injection rate, oil saturation, type of gas, and ageing time. Several different core flood modes were used: a) and b) gas injection or oil injection at constant pressure drop, c) foam injection at constant rate, and d) and e) gas injection or gas/oil co-injection at constant rate. The surfactant selected for the field test (a C14/16 alpha olefin sulphonate) provided high mobility reduction with the Snorre miscible hydrocarbon gas at oil saturations in the range of 8-19 %PV. Apparent viscosities up to 2500 cP were observed during foam flooding, with some dependence of apparent viscosity on oil saturation. Generally, shear thinning behaviour was observed for wet foam. Gas injection after foam resulted in a decrease in apparent viscosity by a factor of 10; however, significant residual gas mobility reduction was retained. Under an imposed pressure gradient, strong gas blocking by foam was observed. Foam properties do not appear to be a strong function of the surfactant concentration in the range of 0.5-2.0 wt% (which is significantly above cmc). Both surfactant ageing in the core and foam throughput have a positive influence on foam properties. In some experiments, foam propagation was slow compared to the injection rate and required several PV of foam flooding. C14/16AOS adsorption on the rock was in the order of 0.5 mg/g. Based on the results obtained, a 1 wt% surfactant solution will be recommended for a production well treatment. The foam injection strategy will be to follow alternating slugs of surfactant solution and gas with an equal volume of foam by co-injection. Introduction The Snorre Field, located in the Northern North Sea, 200 km north-west of Bergen, was put on stream in August 1992 with water flooding as the primary recovery mechanism. In February 1994 a WAG pilot project involving two injection wells and three production wells was initiated in the Statfjord Formation of the field. Extensive evaluation of the project plans and predictions showed that early gas breakthrough may be a potential problem. As a result, the use of foam for gas mobility control, fluid diversion, and blocking to reduce GOR in production wells has been considered as a means of improving the WAG process. Based on promising results from foam treatment evaluation research, including laboratory experiments and numerical simulation studies, it has been decided to perform a foam pilot to control the GOR in a producer, scheduled for the spring 1996. An injector treatment (foam assisted WAG) may realise much larger potential for improved oil recovery than a producer treatment, but with major technical and economic risks. (The interwell distance in the Snorre field is 700-1500 m). Therefore, to reduce the risk, a producer field test has been proposed as a first pilot. This test will give valuable operational experience for future foam projects, as well as important information regarding foam generation and stability in the Statfjord Formation. A previous paper provides a summary of screening laboratory work carried out to select a surfactant suitable for the field test. The surfactant, a C14/16 alpha olefin sulphonate (AOS), was selected based on criteria set to solubility, foam properties, health and environmental safety considerations, availability, and cost. The present paper summarizes optimisation studies for this surfactant and design recommendations for the producer treatment. These optimisation studies comprise gas blocking and mobility reduction properties of foam as a function of surfactant concentration, foam quality, fluid injection rate, oil saturation, type of gas, and ageing time. The data were used as input to a foam reservoir simulator for evaluation of various foam treatment strategies. Based on integrated results from laboratory and simulation studies, the pilot was designed. Experimental Core and Fluid Preparation. The foam experiments were performed using two composite cores, constructed from core plugs of similar properties from the Upper Statfjord Formation. The characteristics of the cores are summarised in Table 1. P. 563