Prolific Gas Production from Low-Permeability Sandstone Reservoirs - Part II: Reconciling Basin History, Fluid Saturations, Gas Shows, and Capillary Pressure

Shanley, Keith W.¹, Robert M. Cluff², John W. Robinson³ (1) Stone Energy LLC, Denver, CO (2) The Discovery Group, Inc, Denver, CO (3) Consultant, Littleton, CO

In many tight gas basins of the western USA, gas productive and non-productive sandstones are difficult to distinguish. Gas shows, calculated water saturations, and saturation-height profiles all appear similar, capillary pressure derived height functions are difficult to apply, and classic rock-typing procedures lack predictive capability.

Basin analysis of several Rocky Mountain basins in which low-permeability sandstone reservoirs produce gas suggests that hydrocarbon charge and migration occurred well in advance of maximum burial and uplift. Furthermore, these same models indicate that reservoir porosity and permeability were significantly greater than values commonly found today. We suggest that at the time of gas charge reservoir capillary pressure curves were characterized by lower displacement pressures. During burial, continued decreases in porosity and permeability caused both water saturations and gas column heights to increase. Upon uplift, gas accumulations either spilled or structurally readjusted resulting in capillary imbibition and, in some cases, secondary drainage then imbibition. Within trapped accumulations, gas expansion upon uplift also resulted in increased gas column heights.

We suggest the basin history of these low-permeability sandstones resulted in many non-productive sandstones being at, or near, residual saturations whereas many productive sandstones follow a capillary imbibition or secondary drainage-imbibition profile. Because of the complex pore geometry that characterizes many low-permeability sandstones, residual, or trapped gas, saturations fall within a similar range as the water saturations of the gas productive sandstones making the two difficult to distinguish. The lack of significant observable variation in saturation-height is a reflection of the imbibition or secondary drainage trend.

Conventional petrophysical evaluation is fundamentally based on drainage capillary principles. Low-permeability reservoirs that have experienced late uplift following an earlier phase of charge are unlikely to be characterized by capillary drainage and are more likely to be characterized by imbibition or secondary drainage and possibly imbibition.