The presentation summarized research on CO2/cement reactions and the development of CO2-resistant cement. The interaction between Portland cement and CO2 is a 3-step process:
• Carbonic acid diffusion,
• Cement (portlandite) dissolution and carbonate precipitation, and
• Leaching (calcium carbonate dissolution).
Cement sheath defects would cause acceleration of the degradation process. Potential
defects include:
• Inadequate placement of cement resulting in channels or mud films,
• Channels caused by gas migration during cement hydration,
• Cracks caused by cement failure in compression/traction, and
• Microannuli caused by lack of bonding at the interfaces with casing and/or rock.
Research is being conducted on a CO2-resistant cement formulation. It was concluded that sound cement design is required, both for the placement and post-placement phases.The presenter, representing the Southwest Carbon Sequestration Partnership, described the efforts of the Partnership regarding selection of sites for geological sequestration. One of the key aspects for site selection is the identification of the best sink for each CO2 source.The presenter provided an example of the well integrity analysis at a test site, the Aneth Unit in southern Utah, where well construction deficiencies may potentially affect a pilot test of CO2 injection.Selecting Sites for Geological Sequestration: Wellbore Integrity and Other Criteria Jason Heath, New Mexico Institute of Mining and Technology
Analysis of well construction deficiencies included:
• Calculation of the top of cement,
• Temperature and cement bond logs, and
• Information on the depth of surface or intermediate casing.
Wells vulnerable to interformational migration of fluids were identified by the screening analysis described above. However, no monitoring has been performed in wells identified as vulnerable.
Well Construction
• Industry has developed recommended practices and protocols for well
construction. However, much of the research upon which the protocols are based
is confidential.
• Experience from EOR and acid gas operations provides a good working basis for
well construction.
• Pilot tests with real-world volumes of CO2 are needed.
• Performance-based construction standards may be appropriate.
Research Need: Development of lower cost materials that perform as well as high-cost materials.
Casing
• Abandonment procedures may need to be more stringent for geological
sequestration.
• Casing specifications depend on possible impurities, formation brine, pressure,
temperature, and operational conditions.
• Casing options include chrome tubing, expandable tubing, titanium casing,
fiberglass casing, and inhibited packer fluid for additional protection.
Research Need: Study the impacts of injection at varying depths.
Cementing
• Cement specifications depend on CO2 impurities; formation brine; and pressure,
temperature, and operational conditions.
• Cement should run the entire length of the wellbore.
Research Need: Alternative (non-Portland) cements.
The number of seismic tests for oil and gas conducted on public lands in the United States is increasing, and this should concern anybody interested in conserving wildlife, plants and vegetation, soil, and the very character of these lands. As domestic oil and natural gas exploration increases, geophysicists seek to map and understand underground deposits that may hold valuable hydrocarbon resources.
Seismic testing is their preferred method, and one result is a proliferation of linear barriers across the landscape Seismic testing evolved from the discovery that when earthquakes occur, it is possible to capture the sound waves created and use the data to map geophysical features that lie underground. Much of what we know about the Earthøs core, mantle, and crust is the result of this discovery, and it follows that man-made seismic, or sound, waves can also be used to map subsurface geologic formations and locate stores of oil and natural gas. However, as seismic tests become more prevalent, there is growing concern about their impacts -- seismic testing requires intensive cross-country travel, often with vehicles that weigh 60,000 to 80,000 pounds.
How Itøs Done
To conduct a test using the preferred Three-dimensional (3-D) seismic method, long cables are first laid along a îreceiver line." Next, dynamite blasts or Thumper trucks (also called vibrasise trucks) are used along a îsource line" to create what is essentially a man-made earthquake, sending energy into the earth. The energy waves bounce off of the subsurface formations and back to the surface where they are captured by îgeophones," which are connected by the receiver lines to a îdoghouse," or data receiving truck. Knowing the frequency at which the energy is created, it is possible to analyze the frequency of the returning waves and create a map of the subsurface area.
An earlier testing method, 2-Dimensional (2-D) seismic, is conducted by placing a receiver line across an area of land, and creating energy along that same line. In other words, the receiver line and the 3-D seismic yields a picture that shows a volume of earth, which is much more valuable. In conducting a 3-D test, a number of receiver lines are placed parallel to each other across a landscape. The lines run at an angle (often perpendicular) to the source line1, in a îbrick pattern." To create the necessary energy, two methods are generally employed. In the vibrasise method, four trucks move in tandem along the source line, stop at a predetermined point, lower a self-contained platform, and vibrate in unison, sending energy into the earth. This is repeated hundreds or thousands of times in the course of one test. If dynamite is used, a drill rig creates a îshot hole" (50-100 feet deep) along the source line, into which a charge is placed. The charge is set off to create energy in what is called the îshot hole" method, and this process is repeated over the entire testing area. In addition to source vehicles, ATVs are also used during each test. These are driven along the receiver lines to troubleshoot problems.
Clearly, seismic testing is a vehicle-intensive process. In order to collect the most valuable data, it is not possible for the source lines to run along existing roads. With both methods (shothole and vibrasise) it is necessary for a number of vehicles to drive cross-country, causing potentially severe ecological impacts.
Under the National Environmental Policy Act (NEPA), federal agencies are required to analyze the potential impacts of proposed activities. In all seismic projects studied for this review, Environmental Assessments (EAs) were conducted, which are less thorough than Environmental Impact Statements (EISs). These documents outline the potential impacts to wildlife, soil, and vegetation, among others, and offer mitigation methods to minimize the effects.
A review of EAs conducted by the Bureau of Land Management (BLM) for projects in the Moab, UT and Green River Basin, WY areas reveals that BLM findings of no significant impact draw largely on anecdotal evidence and do not rely on verified science or cited references. In these EAs, interviews, internal agency documents, and observations from past projects are used to conclude that the impacts from seismic testing will be temporary and non-severe. In contrast, the documented science on linear disturbances like ORVs and roads is quite extensive, and suggests that long-term damage is occurring.
Ecological Impacts
Use of ORVs, for example, destroys habitat and forage for wildlife and disturbs threatened and endangered species. The impacts to soil and vegetation include compaction, which causes erosion and reduced plant growth, as less water is able to penetrate the surface. Ruts may also be caused if vehicles operate when the ground is wet, which can cause even greater problems with compaction and water runoff. The heavy vehicles used in 3-D seismic testing create twotracks that run across the landscape. If these tracks are not eliminated, unauthorized use of recreational ORVs may occur once testing has ended, and the impacts to wildlife, soil, and vegetation will be even more pronounced.
ORV use is known to destroy vegetation that serves as natural soil-protective elements, even after one pass of a vehicle (Wilshire 1983). This impact becomes more significant as the number of trips over an area increases (Payne et al. 1983). In arid regions, ORVs have been found to increase water runoff and erosion (Hinckley et al. 1983), a result of soil compaction and decreases in soil porosity and infiltration capacity (Webb 1983). Impacts were found even when use of such vehicles was slight, and the first passes of a vehicle over a landscape were found to be the most damaging (Iverson et al. 1981). It is estimated that recovery from soil compaction and a natural return to bulk density, strength and infiltration capacity make take a century to occur. In addition, invasive vegetative species were found in compacted areas within a few years, but native species were much slower to return (Webb & Wilshire 1980).
The impacts of roads, seismic lines and other linear disturbances have a number of impacts on wildlife populations, including individual disruption, habitat avoidance, social disruption, habitat disruption or enhancement, direct and indirect mortality, and effects on population. These impacts have received substantial treatment and attention (Jalkotzy, et al. 1997). Studies also show the dramatic effects that roads have on the movement and mortality of wildlife (Forman & Alexander 1998; Trombulak & Frissell 2000), and the balance of this evidence is so strong that policies have been enacted to reduce road densities in national forests to protect wildlife (Hourdequin 2000).
Impacts of seismic testing have been largely ignored. This is starting to change, however, as more attention is paid in areas where the testing is most prevalent, especially Utah and Wyoming. It is important that the lack of understanding and information about the processes and impacts of these projects be solved, and greater public pressure placed on the federal agencies conducting these reviews. Given the documented impacts of ORV use on wildlife, soils, and vegetation, it is imperative that the BLM and other federal agencies pay greater attention to these projects and the effects they are having on our federal lands. It also needs to be determined whether the two-tracks created during seismic testing are used for recreational purposes once testing is completed.
The absence of scientific research on these issues is disturbing, and until more study is done, it is hard to justify that these impacts are short-term and unimportant. One way this might be corrected is to force the BLM to conduct EISs when considering seismic projects, which would result in much greater scrutiny and require a higher threshold of scientific evidence before determining that a project will have no impact.
Footnotes
1. Actual cables are placed along a receiver line, but a source line is a theoretical line along which vibrasise trucks will drive or dynamite blasts will be placed to create the energy necessary.
"5.2.3 Pressure-Induced Physical Effects
Injecting CO₂ into geologic formations will in most cases cause subsurface changes in pressure. As
discussed above, induced fracturing and fault reactivation can occur if injection pressure exceeds fracture pressures, which may in turn result in the opening of fl uid migration pathways. If pressures are great enough, they could in extreme cases cause earthquakes (Healey et al., 1968).
There may be greater uncertainty about evaluating pressure effects in GS systems when examining the potential for pressure-induced regional scale impacts that do not involve fracturing or faulting.
As discussed in Chapter 4, pressure changes in the injection zone could cause regional impacts on overlying aquifer systems, including changes in groundwater fl ow directions and water table levels. These may result in alterations in the distribution and fl uxes of groundwater. This could in turn have other impacts, for example, changing the quantity of groundwater that is available for municipal drinking water supplies. There also could be pressure-induced migration of brines and other fluids through the pore structure of overlying formations into groundwater receptors, which may impact water quality. Furthermore, pressure-induced fluid displacement could result in the release of brine at locations where injection zone formations outcrop at the land surface. Regional pressure effects have been the focus of relatively few studies, and uncertainties and vulnerabilities associated with this subject should be addressed through additional research."
"At room temperature and ambient pressure CO2 is a colourless, odourless gas that will not support combustion or human life. CO2 has been recognised as a workplace hazard for over a century. It is significantly heavier than air and many fatalities from asphyxiation have resulted from entry into pits, tanks, sumps or cellars where CO2 has accumulated and displaced oxygen.
It is also possible for dangerous levels of CO2 to form out-of-doors in trenches, depressions or valleys. This is particularly likely when the gas is colder than the surrounding air, which may occur following pressurised release.
In 2000, a US Environmental Projection Agency study on CO2 related incidents in fire scenarios reported that since 1975 there were 51 recorded incidents involving the discharge of CO2 fire extinguishing systems resulting in 72 deaths and 145 injuries.
There is no significant inherent human response to CO2 that could be useful as a detection mechanism. Human response to hydrogen sulphide by smell occurs at very low (ppm) concentrations, similarly with ammonia and sulphur dioxide.
In contrast, CO2 is present in the air we breath (0.037%). This may cause problems with instrumented detection because the 'background' CO2 levels are so high. In addition, the cooling effects of a pressurised CO2 leak may have an adverse effect on the accuracy and operability of CO2 gas detection systems.
The recognition of the dangers of CO2 has prompted much research into its toxicity in both human volunteers and animals. It is now known that, in addition to the problem of asphyxiation due to the displacement of oxygen, the inhalation of elevated concentrations of CO2 can increase the acidity of the blood triggering adverse effects on the respiratory, cardiovascular and central nervous systems. Data from published research reports has been used by HSE to quantify the toxicity of CO2 in the form of Dangerous Toxic Load (DTL)1 values.
The DTLs have been used in calculations by the Health and Safety Laboratories (HSL) to demonstrate that CO2 exhibits major accident potential, when transported by pipeline in large quantities at ambient temperature and at a pressure of 7 bar or more, well below the dense phase or supercritical region. It is not yet clear whether controls should be applied to the transport of CO2 in this context but pending further research, it is possible that HSE will propose amending the Pipelines Safety Regulations to include CO2 as a dangerous fluid.
For economic and technical reasons it is likely CO2 will be handled close to or above its critical pressure (73.82 bar) where many of its properties are similar to that of a liquid. In this state it is often referred to as a dense phase fluid, whereas above critical temperature (31.04oC) and pressure it is referred to as supercritical. Most of the additional hazards associated with dense phase or supercritical CO2 arise when this pressure suddenly falls or is lost completely.
In the event pf a major pressure loss, e.g a pipe rupture or containment failure, the depressurisation will result in an increase in the volume occupied by the CO2 of several hundred fold as the escaping fluid undergoes a rapid expansion (and phase change) as a proportion essentially 'boils' and becomes a gas while the remainder forms solid particles. This rapid, violent expansion causes the temperature of escaping CO2 to fall very rapidly, frequently below -80°C. while the particles of solid CO2 formed (dry ice) will result in projectiles expelled at very high velocities.
Cryogenic burns and impact injuries from extremely cold jet of gas and entrained missiles are serious hazards to personnel. Cryogenic embrittlement of structural steelwork and adverse effects from the impingement of extremely cold gas jets on safety-critical equipment are major threats to the structural and functional integrity of nearby plant unless appropriately designed or protected.
Supercritical CO2 is a highly efficient solvent. When supercritical CO2 undergoes a significant pressure reduction it moves from its supercritical state with super solvent properties to a gaseous state with virtually no solvent capability. In any environment where other substances are present with supercritical CO2 their solvation will occur resulting in fluid medium or "solution" containing various compounds or elements many of which may be extremely toxic. Any toxic substance held in such a pressurised 'solution' will 'precipitate' out on loss of pressure or containment and is likely to result in harmful human exposure or environmental damage due to the contamination of the area of deposition unless appropriate measures are taken.
Where captured CO2 may be present with solid particles such as reservoir-derived sand and other solid debris, loss of containment may result in these combining with the dry ice formed to produce particles of a much greater abrasive capability than dry ice alone. This would enhance the erosion effects on process pipework and vessels adjacent to the leak which could lead to further damage to equipment and hence risk to people.
Whilst the processes that make up Carbon Capture and Storage (CCS) are not novel in themselves there is relatively little experience worldwide in managing the risks associated with CO2, compared with oil and gas. The major accident hazards presented by handling high pressure CO2 offshore or onshore need to be considered in the context of about 10,000 years' operating experience in managing hazards associated with hydrocarbon processing offshore alone, and probably much more if onshore processes are included2. In comparison there are probably less than 100 operating years for handling CO2 and significantly less in dealing with supercritical CO2.
The ability to anticipate foreseeable major accident scenarios and accurately predict the consequences of these hazardous events is a fundamental element in the assessment of the risk. A lack of substantial operation experience in a novel process or technology generally leads to significant difficulties in identifying accurately the hazards associated with that process or technology.
We do not yet fully understand the behaviour of CO2 when released from dense phase. Industry is researching appropriate models which will need to be validated. There is a need for appropriate scale experimental work to provide HSE and duty holders with a thorough understanding of how CO2 behaves during foreseeable large releases.
Whilst there are applicable general engineering standards, there is a lack of internationally recognised standards and codes of practice specifically for dense phase or supercritical CO2 plant and equipment. When designing, fabricating and maintaining plant for handling and transporting CO2 it is important that the full significance its physical properties, at the temperatures, pressures and inventories required are fully recognised and managed accordingly. Where applying standards developed for other substances including hydrocarbons, such as natural gas, extreme caution is advised as even the highest standards for many other substances may not be sufficient to ensure adequate containment for CO2 under the expected, and unexpected operating envelope(s).
Until recently, however, few cared so much about how long the carbon dioxide stayed put.
That is starting to change. Since 2000, the North American energy company EnCana Corp. has boosted oil production 50% at
So far, monitoring indicates that most of it will stay underground but, by one report, about 2,500 tons a day bubble to the surface where it must be recaptured and re-injected.
Critics of the storage operations worry about the long-term safety of the reservoirs. No one knows whether excess carbon dioxide will remain stable underground for hundreds or thousands of years.
"If it can find any well, crack or conduit in the rock, it will escape," said Harvard carbon storage researcher Kurt Zenz House.
With more than 3.5 million oil wells drilled in the
Experts also worry how so much carbon dioxide will alter the chemistry of the storage formations themselves. Bubbles composed of millions of tons of sequestered CO2 could form an acid that could etch away the confining rocks or erode the concrete caps on well heads.
To test the effects of carbon dioxide storage, researchers funded by the
After monitoring the site for two years, researchers at the U.S. Geological Survey found no leaks.
But in a study made public in July, they did discover that the buried CO2 increased the acidity of the saltwater in the rock enough to dissolve the surrounding minerals. Should enough minerals be eaten away, the gas could seep slowly into the atmosphere again, they reported. The acidic solution also could combine with trace metals and organic compounds to contaminate groundwater."
