Print this pageKey Unresolved Issues in Tight-Gas Exploration and Production
Keith W. Shanley1, Robert M. Cluff1, Stephen P. Cumella2, and John W. Robinson3
1The Discovery Group, Denver, CO
2Bill Barrett Corporation, Denver, CO
3North Ranch Resources, Littleton, CO
Over the last few years our perceptions regarding the controls on prolific gas production from tight-gas sandstone reservoirs have changed dramatically. Many tight-gas fields are now recognized as conventional traps which have been complicated by fluid-flow behavior in very low permeability rocks.
Basin-history modeling suggests the timing of oil and gas generation precedes the onset of structural uplift and reorganization. Critical questions that emerge are: (1) did hydrocarbon charge precede most loss of porosity and permeability; (2) in response to uplift did structural reorganization create reservoirs and traps whose water saturation is not characterized by primary drainage capillary pressure equilibrium i.e., conditions of imbibition or secondary drainage; (3) if reservoirs are not in primary drainage equilibrium, how do we recognize producible hydrocarbons; and (4) if hydrocarbon generation ceased during uplift what is the cause/source of overpressure?
Other questions independent of burial history include: (1) how much of the poor-quality reservoir volume contributes to ultimate recovery? Initial estimates suggest that lowering porosity cutoffs by 1-2% may increase storage capacity by 2X or more. The consequent impact on perceptions of drainage and spacing are dramatic. (2) Within thick alluvial successions such as the Lance, Williams Fork, or Almond, are sandbody geometries best modeled as through-going bodies with varying sinuosity or as small, disconnected bodies; (3) how does water move in microdarcy reservoirs - and to what extent do natural fractures influence gas production; and (4) does the hyperbolic decline exponent ‘b’ change with time, perhaps varying from > 2 to < 1.
AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas