Simulation Study Investigating Gas Cap Contamination Caused by Miscible CO2-WAG

Abstract

The gas cap contamination level and mechanisms caused by CO2-EOR operations in an oil field have been investigated by compositional reservoir simulations on a sector model of the Upper Tilje reservoir. The impact of different EOR and gas cap drawdown operational parameters on the contaminated gas volumes has been determined. The results show that the contaminated part of the produced gas from the gas cap, with more than 2.5% CO2, can be correlated to the final position of the CO2 front during the EOR-period. Massive CO2-breakthrough on the other hand, which is responsible for the contaminated part of the produced gas with more than 7.5% CO2, is a nearly linear function of the stored CO2 mass. This implies that EOR-operations which both increase oil recovery and CO2 storage, such as SWAG-injection, short-cycle WAG or oil producer gas shut-off will significantly increase gas cap contamination. Gas cap contamination however can be reduced by well-positioned horizontal gas producers and large gas blowdown flow rates. Introduction CO2-EOR has been applied on large scale in the US since the seventies and is still the most important EOR-technique in the US until today. So far, an estimated volume of one billion barrels oil has been recovered by CO2-flooding and 206 GSm3 CO2 have been sequestrated 1. In those American fields, mainly in the Permian basin of Texas and New Mexico, the CO2 is available from natural CO2-reservoirs and anthropogenic sources and is transported by extensive pipeline systems to the different fields. This is in contrast to the situation in the North-Sea where the CO2 is not naturally available and offshore pipelines have to be built to make CO2-injection possible. Nevertheless, CO2-injection for EOR in the North Sea has gained new interest due to the expected climate effect of CO2 emissions on the atmosphere. The Norwegian government has established a CO2-emission tax of around $50 per ton and together with the current high oil price, CO2-EOR could become an economically profitable recovery method for the North-Sea region. As a result, Statoil ASA and Shell have started a joint evaluation of the EOR-potential by CO2-injection in the Heidrun and Draugen fields respectively. Between 2 and 2.5 Million tons of CO2 will be available per year from 2012 on and will be delivered by a planned onshore electricity plant at Tjeldbergodden which is about 200 kilometres south-east of both fields. One of the drawbacks of CO2-injection in the Heidrun-field is the contamination of the gas in the gas cap by CO2, which will reduce the commercial value of the gas and cause operational problems. In this paper we have investigated the impact of CO2-injection on the gas cap contamination by compositional reservoir simulation. The simulation study has been performed on a sector-model, containing the H and I segment of Upper Tilje which is the main reservoir of Heidrun that contains a commercial gas cap. Different EOR and gas cap drawdown scenarios have been simulated, and the CO2-contamination levels of the produced gas have been compared for the different cases. Preliminary full-field production profiles by CO2 for Heidrun are reported elsewhere 2. Field Description Heidrun is a major Norwegian oil and gas field, discovered in March 1985 in Block 6507/7 in the Norwegian sea, about 150 km north-west of Trondheim. The average water and reservoir depths are 350 and 2500 metres respectively, giving a hydrostatic reservoir pressure of 250 bar. Initially, the field contained a volume of 440 MSm3 of oil and a gas cap containing about half of the 90 GSm3 of hydrocarbon gas (HC-gas). The production started in October 1995 and today, the production is 20,000 Sm3/day of oil and 5.5 MSm3/day HC-gas. The production strategy has so far been waterflooding with re-injection of some of the produced gas in the gas cap for pressure maintenance. Current predictions give an economical oil production till 2030 with a tertiary gas cap production at the end of the field life. To increase the oil recovery, several IOR-methods are currently investigated for Heidrun, such as continuous well optimisation, polymer and surfactant flooding and CO2-WAG injection.

Periodic CO2 Injection for Improved Storage Capacity and Pressure Management under Intermittent CO2 Supply

“…The other approach, called CO 2 -WAG (Water Alternating Gas), uses periods of CO 2 injection followed by water, displacing remaining oil and supporting reservoir pressure. There are several publications available covering CO 2 -WAG, ranging from the following general to more specific aspects: gas cap contamination [11]; combination of CO 2 -WAG with Soft Micro Gel (SMG) [12]; and rock wettability under CO 2 flooding [13]. Nevertheless, neither of these processes are designed for maximizing permanent storage of CO 2 .…”