Investigating Sequestration Potential of Carbonate Rocks during Tertiary Recovery from a Billion Barrel Oil Field , Weyburn , Saskatchewan : the Geoscience Framework ( IEA Weyburn CO 2 Monitoring and Storage Project )
Introduction In Western Canada the application of CO2 injection for enhanced, ‘tertiary’ oil recovery is a relatively recent addition to the arsenal available to reservoir engineers. The first successful application of CO2 as a miscible fluid in Western Canada began in 1984 at Joffre Field, a Cretaceous marine siliciclastic reservoir in southern Alberta. A significant portion of the remaining proven conventional oil reserves in Western Canada, however, reside in large mature Paleozoic carbonate fields, such as the Mississippian Weyburn Field in SE Saskatchewan (Figure 1). In combination with economically dictated tertiary recovery schemes, these large fields could provide attractive geologic sites for future sequestration of greenhouse gases. An immediate challenge to geoscientists is to assess the ability of these geologic containers to store CO2 for geologically significant lengths of time.
Can a fractured caprock self-heal?
© 2015 Elsevier B.V. The ability of geologic seals to prevent leakage of fluids injected into the deep subsurface is critical for mitigating risks associated with greenhouse-gas sequestration and natural-gas production. Fractures caused by tectonic or injection-induced stresses create potential leakage pathways that may be further enhanced by mineral dissolution. We present results from reactive-flow experiments in fractured caprock (dolomitic anhydrite), where additional dissolution occurs in the rock matrix adjacent to the fracture surfaces. Preferential dissolution of anhydrite left a compacted layer of dolomite in the fractures. At lower flow rate, rock-fluid reactions proceeded to near equilibrium within the fracture with preferential flow paths persisting over the 6-month duration of the experiment and a negligible change in permeability. At higher flow rate, permeability decreased by a dramatic two orders of magnitude. This laboratory-scale observation of self-healing argues against the likelihood of runaway permeability growth in fractured porous caprock composed of minerals with different solubilities and reaction kinetics. However, scaling arguments suggest that at larger length scales this self-healing process may be offset by the formation of dissolution channels. Our results have relevance beyond the greenhouse-gas sequestration problem. Chemical disequilibrium at waste injection sites and in hydrothermal reservoirs will lead to reactive flows that may also significantly alter formation permeability.
Paleogeomorphology and its Application to Exploration for Oil and Gas (With Examples From Western Canada)
Under the term paleogeomorphology are grouped all geomorphological phenomena which are recognizable in the subsurface. Buried relief features with a marked three-dimensional geometry are of importance to the petroleum geologist whenever they lead to the trapping of hydrocarbons. Paleogeomorphic traps form a third and distinct group which ranks in importance with stratigraphic and structural traps as a major mechanism for localizing hydrocarbon occurrences. They are not simply another type of stratigraphic trap, but form by themselves a category of considerable economic significance. Paleogeomorphic traps can not be analyzed, nor can their occurrence be predicted, by stratigraphic or structural methods of study, but must be treated as a geomorphological problem. Of the man types of buried relief features, some are of greater interest to the petroleum geologist than others. In this paper, the morphology of buried erosional landscapes is discussed in greater detail. Other types of buried relief features are fossil reefs, barrier beaches, and submarine canyons. Hydrocarbons may be trapped either directly or indirectly as a result of paleogeomorphological processes. In either case, the traps may occur below or above (against) the morphological surface. Numerous examples of the effects of erosion are known from the Paleozoic and Mesozoic buried landscapes of western Canada and the Mid-Continent area of the United States. Hydrocarbon traps occur below the highs on such erosional surfaces as well as in sandstone bodies deposited in the lows on these surfaces. The rules which govern the formation of ancient landscape forms are worked out in detail, with particular emphasis on the application of quantitative geomorphology to the pattern of the ancient drainage system and on such features as summit levels and the influence of geological factors (er sion-resistant levels, influence of faults and fractures, etc.). Weathering and underground solution also play a role in providing both reservoirs and buried traps for oil and gas. The method of analysis must take into account (1) the structural attitude of the strata below and above the erosional surface, (2) the lithology of the formation overlying the unconformity (if used for isopachous mapping), (3, 4) synsedimentary structural movements and the compaction factor in the formation overlying the unconformity, (5) problems of correlation, (6) reservoir development, (7) presence of a "seat seal," and (8) other factors. Paleogeomorphology provides an interesting evaluation of "modern" geomorphological thinking. The fossil relief forms "frozen" by the transgression of younger rocks neither disprove the validity of the classical peneplain concept, nor do they support fully the newer ideas regarding slope retreat. The actual conditions found are described by a re-definition of the old term paleoplain. The summit level is an intrinsic part of this feature, and not a dissected earlier peneplain. A new geomorphological concept introduced here concerns the alternation of obsequent and resequent interfluve spurs. Buried landscapes should provide a high percentage of the future oil and gas fields yet to be discovered in North America. The most important stratigraphic levels at which buried landscapes occur are those that formed after major periods of orogenesis. Their geographical locus corresponds to the broad belts of subsequent transgressions.