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Seal Workshop - Overview
Seals are fundamental elements of hydrocarbon accumulations, and are known to control migration and charge volumes; the lateral and vertical distribution of hydrocarbons; percent fill of a reservoir; and the flow of hydrocarbons during production. Consequently,
the economic success, or failure, of projects is strongly dependent on proper seal risking. Despite the clear importance of seals, they remain the least studied element of the petroleum system. An understanding of both top seals and lateral (including fault) seals is essential to the analyses of exploration and development projects. The primary goals of workshop are to review analytical approaches for seal analyses and discuss predictive models for estimating top and fault seal capacity and seal risk analysis.
Top seals can leak because the buoyant pressure of the trapped hydrocarbon column is sufficient to inject enough hydrocarbons into the seal to cause membrane failure. The potential for membrane seal failure is evaluated using high-pressure mercury injection capillary pressure (MICP) analysis. Our top seal data are been derived mainly from MICP analyses of cuttings. Comparisons of MICP data from core plugs, cuttings, and artificial cuttings confirm the usefulness of cuttings in estimating seal capacity. MICP data are supplemented with SEM, XRD and thin section analyses to document compositional and textural variations. A sequence stratigraphic framework for top seal occurrence, based analyses of subsurface samples, and supplemented by outcrop analogs, provides the basis for top seal interpretation and prediction. The sealing capacity of marine shales, as determined from MICP varies with textural and compositional factors that appear to be controlled by sequence stratigraphic position. This sequence stratigraphic approach utilizes relationships between shale lithotypes and wire-line log patterns. Calculated column heights are dependent on rock properties and fluid properties (especially interfacial tension) as well as subsurface temperatures and pressures. Therefore, calculated column height values for a particular sample can vary widely, and the comparison of estimated column heights can be misleading. The comparison of measured values for 10% non-wetting saturation provides a more consistent and logical approach for comparing seal capacity. Maximum seal capacity typically occurs in silt-poor shales present within the upper parts of transgressive systems tracts. By comparison, silt-rich shales from highstand systems tracts and lowstand systems tracts generally have significantly diminished seal capacities.
Top seals can also leak where the buoyant pressure of the trapped hydrocarbon column increases to levels that induce seal failure by mechanical fracturing. The potential for mechanical failure can be evaluated using a compilation of leak-off test data (proxy for regional hydraulic fracture gradient) and/or seismically derived pore pressure. The hydraulic failure threshold is controlled by the tensile strength of a particular rock. Curvature (strain) analysis is another useful technique estimating rock strength (i.e., fracture potential). Both mechanisms (membrane failure and mechanical failure) need to be considered in a complete top seal evaluation.
A general correspondence between sealing capacity and measured petrophysical properties (bulk density, shear velocity, Young's modulus, and shear modulus) strongly suggests that seismic data can be applied to seal evaluation and seal prediction. Where wire-line logs and petrophysical data are calibrated to seismic data, the seismic response of potential sealing horizons may be recognizable (requires additional research).
The sealing capacity of faults constitutes another critical aspect of seal analysis. Faults may seal where a relatively low permeability lithofacies is juxtaposed with a reservoir or where low permeability gouge material inhibits fluid migration between adjacent 'reservoir' units. Fault seal may also occur when fault plane permeability is caused grain deformation or crushing or by cementation of fault zones. The literature suggests that most fault seals in siliciclastic sequences are membrane seals where the dominant control on seal failure is the critical capillary pressure of the seal rock.
In contrast to exploration where we are primarily interested in the static seal stability, production requires estimation of the average hydraulic properties of reservoir faults on a relatively short time scale. Juxtaposition diagrams indicate potential communication between reservoir zones. Fault-seal attribute analysis can be used as a guide to the behavior of these areas of potential communication under different production strategies. Faults that have an initial tendency to leak transmit fluid in proportion to fault plane permeability and inversely to the fault plane thickness. Pressure changes during production may greatly exceed the static pressure differences originally present across 'sealing' faults and may cause fault seal breakdown (leak) at some time after the initiation of hydrocarbon production.