Affiliation:
1. Heriot Watt University
Abstract
Abstract
CO2 can be injected into gas reservoirs for enhanced gas recovery. The main benefit of CO2 injection is pressure support to prevent subsidence and water intrusion. Enhanced gas recovery can be via both displacement and repressurisation of the remaining natural gas. The objective of this paper is to investigate the process of injecting CO2 into gas reservoirs using a compositional reservoir simulator. The effects of gas mixing, CO2 diffusion and CO2 solubility in formation water are investigated. Simulation studies are performed to assess the sensitivity of various design and operating parameters to the process. In general, an incremental gas recovery of 8% can be achieved by CO2 injection for a reservoir with a primary recovery factor of 85% by natural depletion. Economic feasibility of CO2 project is also investigated. The profitability of the project is sensitive to gas price, cost of CO2, original gas composition in the reservoir, and further processing of the produced gas.
Introduction
Most scientists believe that various industrial activities and in particular the burning of carbon-rich fossil fuels have caused the concentration of CO2 in the atmosphere to increase from 280ppm to 370ppm since the industrial revolution. The CO2 content in the atmosphere is expected to reach a critical point within 2–3 decades if nothing is done, which could trigger serious global warming and climate changes. In order to reduce these risks, a set of solutions is being investigated to reduce CO2 emissions into the atmosphere. Long-term oceanic and geological (underground) storage is considered as an important solution for CO2 storage. Although oceanic storage can provide greater storage capacity, there are some uncertainties about the retention time and their environmental impact. The subsurface storage is, therefore, deemed more realistic and reliable at least in the near future. The main promising targets are depleted oil and gas reservoirs, underground aquifers, coalbeds and abandoned and sealed mines.[1–3]
Injection of CO2 into oil reservoirs for enhanced oil recovery (EOR) has been broadly investigated. CO2 injection for enhanced gas recovery (EGR) is a new subject that has not been studied as extensively as EOR, and there are no reported field projects. This might be due to the high recovery of gas through natural depletion of the reservoirs and the concerns of potential excessive mixing of native and injected gases. Injection of CO2 for enhanced gas recovery can be an attractive option for many countries that have huge gas reserves but a limited number of oil fields. Gas reservoirs can offer vast storage capacity for CO2.
Many natural gas reservoirs contain a significant quantity of CO2, which has to be separated from the gas stream to meet the required export sale gas specification. It is expected that, in the wake of the Kyoto protocol, more companies will commit to greenhouse gas management in the development of CO2 associated natural gas fields. There are several on-going and planned projects worldwide that involve separation and geological storage of CO2. One of these projects is the Sleipner gas field in the North Sea in which the separated CO2 is injected into an underground saline aquifer.[4,5] Other projects, such as the Snohvit LNG project in the Barents Sea, the In Salah gas project in central Algeria, [4] the Natuna gas field in Indonesia,[6,7] and the Gorgon gas field in Australia,8 are under design and consideration to implement effective CO2 separation and geo-storage measures.
The subject of enhanced gas recovery has received significant attention in recent years as part of CO2 geo-storage study. A few experimental and simulation studies have been conducted to evaluate the feasibility of displacing natural gas with CO2.[6,9–12] Enhanced gas recovery can be via both displacement, analogous to water flooding in oil reservoirs, and repressurisation of the remaining natural gas. The effect of CO2 dispersion (diffusion) and mixing with the primary gas in the reservoir is the main concern in enhanced gas recovery process. The solubility of CO2 in formation water is also important for the potential of enhanced gas recovery.