Petro-Find Geochem Ltd very high-density soil gas surveys for light hydrocarbons and CO2 can play a significant role in the development of Carbon Capture and Storage (CCS), Enhanced Coalbed Methane (ECBM) and CO2 Miscible Flooding (EOR) projects. The ability to detect surface leakages from these projects can translate into more efficient operations with substantially reduced costs as well as provide a tool for measurement of environmental impacts. To receive carbon credits for CO2 sequestration projects, a suitable monitoring system needs to be put in place to verify storage of CO2 without leakage. Inappropriate methods for sampling surface leakages can lead to erroneous conclusions about the provenance of CO2 detected in soil gas.
Geologic carbon dioxide storage projects can be divided into two categories. The first category consists of value-added capture and storage projects, in which captured carbon dioxide is used to enhance resource recovery, such as enhanced coalbed methane production (ECBM) and CO2 miscible flooding (EOR). For these types of projects, the cost of carbon dioxide capture and injection is outweighed by the revenue from increased production of oil or natural gas.
The second category includes storage projects undertaken specifically to reduce carbon dioxide emissions, without the incentive of other value-added benefits. These include CO2 sequestration in depleted oil and gas reservoirs, unmineable coal seams, deep saline aquifers and salt caverns. The objective of sequestration is to permanently store injected CO2 by one or more of the following trapping mechanisms: permeability, solubility and mineralogical.
A major benefit from EOR projects is CO2 sequestration, which could contribute in a major way to reducing GHG linked to Global Warming. EOR has been practiced by the petroleum industry, mainly in the USA, for more than 30 years. Therefore, it is likely that a large proportion of early-generation sequestration projects will be implemented in oil and gas fields, where additional oil and gas produced as a result of CO2 injection can offset the costs of injection.
Soil gas surveys are ideal for monitoring of CO2EOR projects. To measure the effectiveness of storage it is important to establish baseline studies prior to injection. Follow-up surveys are conducted on a repetitive basis to identify at an early stage any major leakages along open conduits such as improperly plugged wells and fractures/faults. For examples see the August 2010 environmental study and February 2011 environmental study of a portion of the Saskatchewan Weyburn Oil Field currently undergoing EOR. For projects at the planning stage, potential CO2EOR sites should be screened by soil gas surveys for their storage suitability. This approach can save a great deal of time and resources, and can optimize planning.
It is entirely reasonable to expect that CO2 injected under high pressure in an oil reservoir will exhibit vertical migration through micro-fractures. The molecular weight of CO2 is about the same as Propane, which normally escapes from reservoirs along with heavier Butane to the surface. With flooding, open fractures can act as major conduits, both vertically and laterally, for CO2 and light hydrocarbons to migrate from the oil reservoir to the surface. Alternatively, closed fractures can act as barriers to the sweeping action of CO2 and a damming effect is likely to be observed at the surface.
Carbon capture and sequestration (CCS) is regarded as a feasible strategy for reducing large volumes of anthropogenic CO2 emissions. Although the purpose of geologic carbon sequestration is to trap CO2 in the subsurface, there is a risk that the injected CO2 will migrate away from the primary target formation and migrate to the surface. To monitor the possibility of CO2 leakages at the near-surface, a background database predating injection is first established. Follow-up surveys of a repetitive nature are conducted to identify at an early stage any surface seepages relating to open conduits such as improperly plugged wells and fractures/faults. CO2 plumes of less concentration emanating from target sites are slower and can take years to reach the surface in any significant amounts. A reliable and effective monitoring system is an important part of making geological sequestration a safe and acceptable method for the permanent disposal of CO2.
It should be noted that potential sites for sequestration should be screened by soil gas surveys for their storage suitability. Preliminary surveys can save a great deal of time and resources, and can optimize planning. Petro-Find can provide consultation services in this regard.
Petro-Find soil gas surveys can be adapted for Enhanced Coalbed Methane (ECBM) Projects. Repetitive very high-density surveys for Methane could prove to be a major tool for showing the lateral extent and direction of gas flooding fronts extending from the central injection well to the production wells. The released Methane from gas flooding moves laterally along the coal bed in the direction of the highest permeability and lowest pressure. It is reasonable to expect that the Methane migrates upwards as well though the hanging wall of the coal seam to the surface where it can be detected by soil gas surveys. It should be noted that Methane is reported in surface water wells in Wyoming CBM projects.
The gas front composed of Methane and CO2 may exhibit an elliptical pattern with the long axis parallel with face cleats. Lineaments or faults may serve as either conduits or barriers for the gases. Facies changes in the coal seam are also mitigating factors. The directional (anisotrophic) permeability of the cleat system and its control of gas fronts as it moves laterally along the coal seam have major implications for the spotting and timing of production wells.
The sampling method as practiced by Petro-Find is the same for all types of sequestration. Soil samples are taken by a non-plugging probe at least 3 feet below the surface or well into the C-zone to avoid contamination by CO2 from microbial and root respiration. CO2 from respiration can mask sequestration leakages, especially at the early stage, because of the wide fluctuations in CO2 from seasonal and weather factors.The soil gas method can monitor leakage patterns throughout most of the year even in winter when the ground is frozen and ice covered. A winter survey is very valuable for interpretation of provenance because little methane and CO2 is produced in biologically inactive soils. However, it should be noted that the higher alkanes - propane, butane and pentane - are severely attenuated. Surveys are not conducted during late March and April (in Canada) due to snowmelt and the infiltration of water into the frozen soils.
It should be noted that inappropriate sampling methods and equipment can lead to erroneous conclusions about the provenance of any high concentrations of CO2 found in soil gas above reservoirs undergoing CO2 injection. For example, vacuum pumps should not be used for probe sampling of tight soils because they quickly exhaust the soil gas at the tip of the probe resulting in the vacuum draw-down of ambient air around the annulus of the probe. Large soil gas samples should also be avoided because of the limited supply of soil gas at the tip of the probe and the lag time required for regeneration during the sampling process. For a complete picture, all active and abandoned well locations should be assessed for leakage.
It should be noted that depleted highly fractured oil reservoirs are normally not good candidates for CO2EOR projects because of possible leakage along faults. The suitability of oil reservoirs for CO2 injection can be assessed with soil gas surveys before beginning any CO2 injection. Excess CO2 from leakage in ambient air poses no problems unless it is drawn into homes by the negative pressures created by heating and air conditioning systems. Farm and ranch homes over reservoirs undergoing CO2EOR should be equipped with CO2 (not fire) detectors in case CO2 has migrated into those homes. It is well known that occupants can suffer neurological effects when the CO2 concentration exceeds a time-weighted average of 5,000 ppm over a 10-hour day during a 40-hour work week (U.S.National Institute for Occupational Safety and Health, 1997). Establishment of CO2EOR operations near populated areas should take into account that CO2 can migrate significant distances outside the surface projection of the reservoir being injected.
PETRO-FIND GEOCHEM LTD
215 Mallin Crescent
Saskatoon, Saskatchewan, Canada S7K 7X3
Contact: Paul Lafleur, President
Phone: (306)931-3156 Fax: (306)931-9773