Carbon dioxide storage potential of shales

SUMMARY Few data are available on shales in terms of seismic to ultrasonic properties and anisotropy, although all are important with regards to imaging problems often encountered in such lithologies. Additionally, mechanisms causing changes in these properties are not well documented due to the fine grain size of such materials and time required for testing under controlled pore pressure conditions. The results presented here are derived from a set of experiments run on Muderong Shale with pore pressure control in order to evaluate the effect of stress magnitude and stress anisotropy on ultrasonic response. This shale was noted to have a linear velocitymean effective stress response and extremely high anisotropy, both likely the result of the presence of fluid-filled cracks in a low- permeability porous medium. Changes in velocity and Vp/Vs ratios are dependent on both stress and smectite content. S-wave velocity is significantly affected by the presence of smectite in this and other shales and at low stress (<20 MPa) is less sensitive to stress change than P-wave velocity. Vp/Vs ratios are noted to increase in this shale up to 20 MPa effective stress, then decrease slightly due to stress-induced loss of interlayer water in smectite. Intrinsic anisotropy comes from composition, a strong compaction fabric and the presence of microfractures; changes to ultrasonic anisotropy are the result of the magnitude of the stresses, their orientation with respect to the fractures and the degree of stress anisotropy.

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Impact of fabric, microcracks and stress field on shale anisotropy

CO(2) capture and geologic sequestration is one of the most promising options for reducing atmospheric emissions of CO(2). Its viability and long-term safety, which depends on the caprock's sealing capacity and integrity, is crucial for implementing CO(2) geologic storage on a commercial scale. In terms of risk, CO(2) leakage mechanisms are classified as follows: diffusive loss of dissolved gas through the caprock, leakage through the pore spaces after breakthrough pressure has been exceeded, leakage through faults or fractures, and well leakage. An overview is presented in which the problems relating to CO(2) leakage are defined, dominant factors are considered, and the main results are given for these mechanisms, with the exception of well leakage. The overview includes the properties of the CO(2)-water/brine system, and the hydromechanics, geophysics, and geochemistry of the caprock-fluid system. In regard to leakage processes, leakage through faults or fracture networks can be rapid and catastrophic, whereas diffusive loss is usually low. The review identifies major research gaps and areas in need of additional study in regard to the mechanisms for geologic carbon sequestration and the effects of complicated processes on sealing capacity of caprock under reservoir conditions.

Comprehensive review of caprock-sealing mechanisms for geologic carbon sequestration.

Analysis of the Muderong Shale from the Carnarvon Basin suggests the shale is dominated by interstratified illite–smectite with a high percentage of illite interlayers. Capillary pressure measurements indicate that gas columns of c. 250 m could be sealed by such shale, although the choice of drying method used does influence the accuracy of this calculation. Freeze drying yielded the most consistent threshold pressure results, whereas air drying and vacuum drying showed a greater range of values. Similar calculations in regard to carbon dioxide sequestration indicate column heights of between 550 m and 750 m could be retained. Column height variation is primarily dependent on the contact angle of supercritical carbon dioxide with shale. Microstructurally, the shale is clay supported, exhibiting differential compaction of clays around more rigid grains and containing numerous high aspect ratio discontinuous fractures. These fractures do not affect the capillary properties of the shale, even when injection is fracture-parallel, suggesting they are unlikely to influence reservoir-scale fluid-flow properties. Comparison of the Muderong Shale laboratory data with hydrocarbon column heights from Carnarvon Basin discoveries indicate that top seal failure by capillary breakthrough is unlikely given the maximum lengths of hydrocarbon columns encountered to date. Potential for top seal failure is more likely to be influenced by formation integrity, pore pressure and in situ stress conditions.

Microstructural and petrophysical characterization of Muderong Shale: application to top seal risking

This paper presents reviews of studies on properties of coal pertinent to carbon dioxide (CO2) sequestration in coal with specific reference to Victorian brown coals. The coal basins in Victoria, Australia have been identified as one of the largest brown coal resources in the world and so far few studies have been conducted on CO2 sequestration in this particular type of coals. The feasibility of CO2 sequestration depends on three main factors: (1) coal mass properties (chemical, physical and microscopic properties), (2) seam permeability, and (3) gas sorption properties of the coal. Firstly, the coal mass properties of Victorian brown coal are presented, and then the general variations of the coal mass properties with rank, for all types of coal, are discussed. Subsequently, coal gas permeability and gas sorption are considered, and the physical factors which affect them are examined. In addition, existing models for coal gas permeability and gas sorption in coal are reviewed and the possibilities of further development of these models are discussed. According to the previous studies, coal mass properties and permeability and gas sorption characteristics of coals are different for different ranks: lignite to medium volatile bituminous coals and medium volatile bituminous to anthracite coals. This is important for the development of mathematical models for gas permeability and sorption behavior. Furthermore, the models have to take into account volume effect which can be significant under high pressure and temperature conditions. Also, the viscosity and density of supercritical CO2 close to the critical point can undergo large and rapid changes. To date, few studies have been conducted on CO2 sequestration in Victorian brown coal, and for all types of coal, very few studies have been conducted on CO2 sequestration under high pressure and temperature conditions.

A review of coal properties pertinent to carbon dioxide sequestration in coal seams: with special reference to Victorian brown coals

Geological storage of CO2 in the offshore Gippsland Basin, Australia, is being investigated by the Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) as a possible method for storing the very large volumes of CO2 emissions from the nearby Latrobe Valley area. A storage capacity of about 50 million tonnes of CO2 per annum for a 40-year injection period is required, which will necessitate several individual storage sites to be used both sequentially and simultaneously, but timed such that existing hydrocarbon assets will not be compromised. Detailed characterisation focussed on the Kingfish Field area as the first site to be potentially used, in the anticipation that this oil field will be depleted within the period 2015–2025. The potential injection targets are the interbedded sandstones of the Paleocene-Eocene upper Latrobe Group, regionally sealed by the Lakes Entrance Formation. The research identified several features to the offshore Gippsland Basin that make it particularly favourable for CO2 storage. These include: a complex stratigraphic architecture that provides baffles which slow vertical migration and increase residual gas trapping and dissolution; non-reactive reservoir units that have high injectivity; a thin, suitably reactive, lower permeability marginal reservoir just below the regional seal providing mineral trapping; several depleted oil fields that provide storage capacity coupled with a transient production-induced flow regime that enhances containment; and long migration pathways beneath a competent regional seal. This study has shown that the Gippsland Basin has sufficient capacity to store very large volumes of CO2. It may provide a solution to the problem of substantially reducing greenhouse gas emissions from future coal developments in the Latrobe Valley.

Site characterisation of a basin-scale CO2 geological storage system: Gippsland Basin, southeast Australia

Overpressure in ‘old’ sedimentary basins that have not undergone rapid, recent sedimentation cannot be easily explained using traditional burial-driven mechanisms. The last significant burial event in the Cooper Basin, Australia, was the Late Cretaceous deposition of the Winton Formation (98.5–90 Ma). Maximum temperature in the basin was attained during the Late Cretaceous, with cooling beginning prior to 75 Ma. Hence, overpressure related to rapid burial or palaeomaximum temperatures (e.g. hydrocarbon generation) must have developed prior to 75 Ma. Retaining overpressure for 75 Ma in ‘old’ basins such as the Cooper Basin requires extremely low seal permeabilities. An alternative explanation is that overpressure in the Cooper Basin has been generated because of an increase in mean stress associated with an increase in horizontal compressive stress since Late Cretaceous times. Structural observations and contemporary stress data indicate that there has been an increase in mean stress of approximately 50 MPa between Late Cretaceous times to that presently measured at 3780 m. The largest measured overpressure in the Cooper Basin is 14.5 MPa at 3780 m in the Kirby 1 well. Hence, disequilibrium compaction driven by increasing mean stress can explain the magnitude of the observed overpressure in the Cooper Basin. Increases in mean stress (tectonic loading) may be a feasible mechanism for overpressure generation in other ‘old’ basins that have undergone a recent increase in horizontal stress (e.g. Anadarko Basin).

The origin of overpressure in ‘old’ sedimentary basins: an example from the Cooper Basin, Australia

Geosequestration CO 2 in the offshore Gippsland Basin is being investigated by the CO 2 CRC as a possible method for storing the very large volumes of CO 2 emissions from the Latrobe Valley area. A storage capacity of about 50 million tonnes of CO 2 per year for a 40-year injection period is required, which will necessitate several individual storage sites to be used both sequentially and simultaneously, but timed such that existing hydrocarbon assets are not compromised. Detailed characterisation focussed on the Kingfish Field area as the first site to be potentially used, in the anticipation that this oil field will be depleted within the period 2015-25. The potential injection targets are the interbedded sandstones, shales and coals of the Paleocene-Eocene upper Latrobe Group,regionally sealed by the Lakes Entrance Formation. The research identified several features to the offshore Gippsland Basin that make it particularly favourable for CO 2 storage. These include: a complex stratigraphic architecture that provides baffles which slow vertical migration and increase residual gas trapping; non-reactive reservoir units that have high injectivity; a thin, suitably reactive, low permeability marginal reservoir just below the regional seal providing additional mineral trapping; several depleted oil fields that provide storage capacity coupled with a transient flow regime arising from production that enhances containment; and, long migration pathways beneath a competent regional seal. This study has shown that the Gippsland Basin has sufficient capacity to store very large volumes of CO 2 . It may provide a solution to the problem of substantially reducing greenhouse gas emissions from the use of new coal developments in the Latrobe Valley.

Gippsland Basin geosequestration: A potential solution for the Latrobe Valley brown coal CO2 emissions

A small, but significant fraction of wells drilled in the Northern Carnarvon Basin have encountered problems with overpressure: better pore pressure prediction would improve safety and economy for drilling operations. In the Northern Carnarvon Basin the occurrence of overpressure and likely mechanisms are under investigation as part of the Australian Petroleum Cooperative Research Centre (APCRC) Research Program on Pore Pressure Prediction. Previous workers have proposed a number of mechanisms as the main cause of overpressure including undercompaction, hydrocarbon generation, horizontal stress and clay reactions. A preliminary regional study was undertaken incorporating over 400 well completion reports which identified approximately 60 wells with mud weights greater than 1.25 S.G. A subset of these wells was investigated and more reliable but much scarcer pressure indicators such as kicks or direct pressure measurements were examined. Depth-pressure profiles of wells across the region are variable and commonly show pressure compartmentalisation. Using a range of indicators, it was observed that overpressured strata in the Barrow Subbasin: occur over a wide depth range (2,500 to 4,000+ mbsl); occur over a wide stratigraphic range (Late Triassic to Late Cretaceous); are not regionally limited by major structural boundaries; are associated with sequences dominated by finegrained sediments with variable clay mineralogy; and in depositionally, or structurally, isolated sandstones; and mainly to the west of the Barrow and Dampier Subbasins around the Alpha Arch and Rankin Trend, coinciding with thickest Tertiary deposition. Previous published work in the study area has tended to support hydrocarbon generation as the primary cause of overpressure, though more recent publications have emphasised compaction disequilibrium. The log response (DT, RHOB and NPHI) of overpressured clay-rich strata has been investigated to constrain the type of overpressure mechanism. A normal compaction trend has been derived for four stratigraphic groupings; Muderong Shale, Barrow Group, Jurassic and Triassic. All overpressure occurrences were accompanied by an increase in sonic transit time. Not all wells have suitable log data for evaluation, but all stratigraphic groups show some evidence of elevated porosity associated with overpressure consistent with disequillibrium compaction as a dominant mechanism. Overpressures in the Barrow Group in Minden-1 and the Jurassic section within Zeepaard–1 do not have accompanying porosity anomalies suggesting a different overpressure mechanism model is needed.

Geological controls on overpressure in the Northern Carnarvon Basin

The presence of smectitic shales often causes serious problems for drillers in the form of both overpressures due to low permeability and wellbore stability problems resulting from the presence of swelling clays. Prediction of overpressures in such sediments is hampered by interlayer water in the mineral structure that is not accounted for in porosity determinations whether by wireline methods or laboratory drying techniques. This study looks at the detailed physical properties and microstructure in relation to acoustic response of the Muderong Shale, a smectitic shale from the Carnarvon Basin on the Northwest Shelf of Australia. Porosity determined from acoustic or standard drying techniques is far in excess of that determined by both helium and mercury porosimetry, but correcting for interlayer water results in the values determined by the latter techniques. Physical properties are intimately tied to the acoustic response of the Muderong Shale at both sonic and ultrasonic frequencies. The presence of smectite results in anomalously low velocities in the Muderong Shale and the development of both compaction fabrics and induced microfracture generation result in significant acoustic anisotropy. Velocity variations from downhole shales in wells from the Carnarvon Basin can mainly be tied to smectite content but also to overpressure. The high degree of anisotropy of the Muderong Shale at ambient pressure can be related to the presence of both a sedimentary compaction fabric and high aspect ratio compliant microfractures. Determination of the relative contributions of these two causes of anisotropy in shales may provide a method by which to distinguish overpressure-generating mechanisms from acoustic data.

P. Van Ruth

Papers

Site characterisation of a basin-scale CO2 geological storage system: Gippsland Basin, southeast Australia

The origin of overpressure in ‘old’ sedimentary basins: an example from the Cooper Basin, Australia

Gippsland Basin geosequestration: A potential solution for the Latrobe Valley brown coal CO2 emissions

Geological controls on overpressure in the Northern Carnarvon Basin

Acoustic properties of muderong shale