Oil & Gas - Maximising Recovery Programme (formerly SHARP) IOR Views

Introduction

Carbon dioxide has been used to improve oil recovery for more than thirty years. If injected at sufficient pressure, CO2 is an excellent hydrocarbon solvent. CO2 is miscible with a variety of intermediate (gasoline range) hydrocarbons, producing a lower viscosity, lower density hydrocarbon rich fluid that then mixes with and mobilises significant amounts of fresh oil.

Carbon capture and geological storage (CCS) is arguably the most important emission reduction technology available for large stationary CO2 sources. Over the next few decades depleted oil and gas reservoirs will provide the most suitable sites for geological CO2 storage. With useable infrastructure, numerous depleted fields and major CO2 sources nearby, maturing petroleum provinces are ideal areas for large scale geological storage.

Therefore CO2 injection for enhanced recovery and geological storage can yield both economic and environmental benefits. With the desire to reduce CO2 emissions and move to a lower carbon economy, alongside the need to maintain production from a maturing petroleum province, the time has come to begin injecting CO2 for enhanced recovery and geological storage in the UKCS.

Site Selection, Appropriate Reservoirs for Enhanced Recovery and CO2 Storage

As a rule of thumb, any reservoir buried deeper than 2500 feet and with oil lighter than 25ºAPI (the ‘25 rule’), that has a good historical response to water injection, is likely to be suitable for enhanced recovery via miscible CO2 flooding. Screening criteria for CO2 storage sites are similar to those used to select reservoirs for CO2 enhanced recovery. For storage, miscible CO2 flooding in reservoirs deeper than 2500 feet is preferred. At higher pressures, the denser CO2 phase makes better use of pore volume, while miscibility maximises the amount of CO2 that can dissolve in the oil. Additional areas of attention for storage site selection include: physical and chemical response of reservoir, seal, and overburden to CO2 injection, evaluation of potential spill points, and regional hydrodynamics. If managed prudently a reservoir that has retained oil and gas over geological time should provide an excellent storage site for CO2.

The Weyburn Project

Operated by EnCana Resources, the Weyburn field in Saskatchewan covers around 180 square km and produces 29°API gravity crude from Carboniferous shallow water carbonates at a mean depth of 1450 m. By the 1990s, primary and secondary recovery via water flood followed by an aggressive infill drilling programme produced around 25% of the 1.4 billion barrels OOIP. Following a successful CO2 injection pilot, the Weyburn field was re-engineered for a large scale CO2 enhanced recovery and storage project, with CO2 injection beginning in late 2000. Approximately 5000 tonnes of CO2 per day are transported by pipeline from the Great Plains Synfuel Plant, North Dakota. An agreement between industry, government and academia coordinated by the Petroleum Technology Research Centre (Regina, Saskatchewan) and the International Energy Agency Greenhouse Gas Research and Development Programme (IEA-GHG) implemented a C$40 million international research project into CO2 storage at Weyburn.

The reservoir has responded very well to the seven million tonnes of CO2 injected between 2000 and 2006, and is on schedule to produce around 15% of the OOIP over the next 25 years, equivalent to another 155 MMstb. CO2 flood monitoring via produced fluid and gas sampling, 4D seismic surveys and engineering data have helped improve reservoir performance and demonstrated that Weyburn is a secure storage site for CO2. Projected ultimate storage at Weyburn is approximately 25 million tonnes of CO2. The IEA GHG Weyburn-Midale CO2 Monitoring and Storage Project has now entered phase two with the inclusion of Apache’s CO2 enhanced recovery and storage project in the adjacent Midale field.

Storage of Injected CO2 in the Subsurface

The most important CO2 storage processes from an operational perspective are the short term ones, those that occur over the lifetime of CO2 injection and monitoring at the storage site (Table 1).

Table 1: Summary of geological CO2 storage processes

Short term storage.Both CO2 dissolved in oil and water (solubility storage), and CO2 that forms HCO3- (ionic storage) are retained securely in the storage reservoir for as long as the reservoir fluids remain in place. CO2 is much more chemically reactive than methane and other low molecular weight hydrocarbons, therefore any CO2 that migrates into the overburden will be immobilised by reacting with in situ minerals and fluids, and is very unlikely to reach the surface.

Longer term - mineral storage. Reactions that lead to mineral storage of CO2 are similar to the weathering reactions that take place at the earth’s surface. These reactions are most effective in the presence of the more reactive silicate minerals in the storage reservoir (i.e. feldspars and clays). Such reactions are generally slow and may require decades to centuries to form the minerals that incorporate CO2, gradually providing the most secure form of storage.

Monitoring Enhanced Recovery and CO2 Storage

Monitoring production during all stages of recovery is a vital part of reservoir management. Monitoring helps refine and validate production and storage simulations and indicates when things aren’t going as expected. If emission reduction credits are to be issued and storage liability transferred from the site operator, monitoring must demonstrate that the injected CO2 is securely stored. At a recent international CCS monitoring and verification meeting, a US government environmental regulatory delegate suggested that; ‘without a monitoring program injected CO2 would be considered 100% vented’.

To maximise the value of monitoring, each storage and enhanced recovery site should have a tailored monitoring programme based on defined risks, and both operational and regulatory requirements. If monitoring confirms that the site is behaving as expected, then the monitoring programme can be scaled down.

A range of techniques are available for monitoring CO2 storage and enhanced recovery. 4-D seismic has imaged the injected CO2 flood and identified infill drilling targets at Weyburn. Single well borehole and crosswell seismic have been successfully applied at a number of pilot and full scale projects. Of the many monitoring techniques available, produced fluid and gas sampling has been one of the most economical and effective.

Produced fluid and gas monitoring; hydrocarbon derived CO2 is a ready made tracer. Carbon dioxide from hydrocarbon combustion inherits a fingerprint of the carbon from the original organic molecules in the source rock. This fingerprint results from a distinctive abundance ratio of the two stable isotopes of carbon (carbon-13/carbon-12), incorporated by plants and algae during photosynthesis. Therefore CO2 derived from industrial use of hydrocarbons is distinct from the vast majority of CO2 found in the subsurface, which has a significant inorganic carbon component. Using produced fluid and gas monitoring it is straightforward to track the injected CO2, quantify CO2 storage and evaluate CO2 partitioning between the reservoir phases.

Figure 1: Detection of small volumes of injected CO2

Using produced fluid and gas monitoring to improve reservoir management. Produced fluid and gas monitoring provides accurate and timely information on injector – producer communication, and early warning of problems such as channelling. The example in Figure 1 shows that, following injection of around 750 tonnes of CO2 over one month, the injected CO2 was detected at two producing wells 100 m and 500 m from the injection well. The injected CO2 and the original formation CO2 have very different carbon fingerprints (carbon-13/carbon-12) so despite the fact that there was no significant increase in gas production at the wells compared with pre-injection conditions, the injected CO2 was clearly identified at the production wells, highlighting highly communicative zones within the reservoir.

Figure 2. Storage of injected CO2 as HCO3-

Quantifying ionic storage of injected CO2 as HCO3-. The storage of injected CO2 as HCO3- (ionic storage) is shown by the data in Figure 2. Over four years and four million tonnes of CO2 injection at Weyburn, the concentration of HCO3- measured in the produced fluids increased to around five times the pre-injection level. The carbon fingerprint (carbon-13/carbon-12) of the HCO3- became almost identical to the injected CO2 (the solid curve on Figure 2) demonstrating that large amounts of injected CO2 are stored securely as HCO3-.

A UKCS Perspective

Monitoring geological CO2 storage and enhanced recovery in the North Sea.Monitoring needs to be reliable and cost effective for widespread offshore deployment. Produced fluid and gas monitoring is routinely used to evaluate water flooding, associated gas injection, and well performance, while 4D seismic reservoir monitoring is now becoming established as a normal practice during water flooding. These monitoring techniques are ready to be applied to full scale geological CO2 storage and enhanced recovery projects and will be effective in the vast majority of North Sea fields.

Leading the world? The UK is well placed to become the leader in CO2 injection for storage and enhanced recovery. The increased costs of drilling and production in the hostile offshore environment have led to world-leading expertise in reservoir monitoring and well maintenance, two vital operational aspects of CO2 storage and enhanced recovery. The North Sea as a province is maturing with recent screening studies highlighting a number of suitable fields for CO2 enhanced recovery and storage, while the ageing offshore infrastructure represents a one off opportunity for conversion to geological storage operations. On the legislative side, there has been some good news over the last few months, including an indication from the EU that emissions credits for carbon storage may be granted under the EU ETS (Emission Trading Scheme). In addition, the Parties to the London Convention on the Prevention of Marine Pollution have adopted a proposal to legalise storage of CO2 in sub-seabed geological formations (effective February 2007). To allow industry to take advantage of the huge opportunity that CO2 enhanced recovery and geological storage represents, clear encouragement from the UK government is now essential.

Further Information

Guoping Li, 4D seismic monitoring of CO2 flood in a thin fractured carbonate reservoir, The Leading Edge, July 2003; 22: 690 - 695.

Mark Raistrick, published Weyburn monitoring work etc.