Experimental and Empirical Observations Supporting a Capillary Model Involving Gas Generation, Migration, and Seal Leakage for the Origin and Occurrence of Regional Gasifers

Abstract

Low-permeability reservoirs in which gas is the regionally continuous phase (gasifers) occur over large areas in the Alberta Basin in Canada and the Rocky Mountain basins in the United States. These tight-gas reservoirs have also been called deep basin and basin-centered gas systems and contain very large resources of natural gas. Observation and theory show that a gasifer, or a regional low-permeability gas system, can be developed in a four-stage process: genesis, transition, steady state, and imbibition. This process involves the generation, migration, and leakage of gas, accompanied by the regional dewatering of the system, even in the siltstones and shales. The genesis stage contains both conventional gas pools, early in the development of the gasifer, and unconventional gas pools later. Late genesis gas pools are characterized by tall gas columns, with normal downdip apparent gas–water contacts. These tall gas columns generate enough capillary pressure to drain very low-permeability reservoirs and establish gas as the continuous fluid over a very large part of the basin. At this point, the gasifer is developed. The transition stage has normal and underpressured gas, with tall columns that crosscut waterlines on pressure-versus-elevation plots. In the steady-state stage, gas is underpressured, and the tall gas columns have updip gas–water contacts. The imbibition stage marks the decline of the gasifer and is characterized by shorter, underpressured gas columns and underpressured waterlines. Laboratory experiments based on a simple capillary tube model support the four-stage development of the regional low-permeability gas system and defined both a normally pressured and overpressured gas–water system in the genesis stage. These experiments demonstrated that the mechanism for the underpressuring of the gasifer in the transition and steady-state stages was gas leakage, which confirmed the conclusions based on capillary theory. The combination of the empirical approach using pressure-versus-elevation plots and the capillary theory with the laboratory experiments leads to several interesting concepts. For example, a regional low-permeability gas system can be viewed as a source rock undergoing primary migration. Gas generation may be thermal or biogenic. Therefore, this four-stage process would also apply to the shallow biogenic gasifers in the Milk River and Horseshoe Canyon formations in southern and central Alberta. The genesis stage will contain some moveable water, and there will be an overprint of structural and stratigraphic traps with higher water production downdip. Reservoirs in the genesis stage will have variable water saturations, and therefore, relative permeability should be a concern. A basin may have any one of these stages well developed, or all four may be present at various levels of development, as is the case for the Alberta deep basin. By knowing the stages, the gasifer can be defined, and an effective exploration strategy can be developed.