Abstract

Exploitation of natural gas in the eastern Greater Green River Basin (GGRB) has evolved significantly during the past decade. The initial challenge was to exploit the vast potential of basin-center gas, originally described by Law and others in pioneering resource-characterization papers. Significantly, this had to be accomplished within a persistent low price and a tight capital constrained environment. During this process, industry began to appreciate that large-scale exploitation of the basin's semi-continuous, low quality gas resource involved challenges much different than conventional development. During the decade of effort some of these challenges were met, others were not. In addition, a number of new challenges emerged. The shifts in exploration strategies and exploitation methods adopted by operators in response to these challenges continue to ripple through the industry today.

In 1990, Green River basin prospectors still used conventional exploitation concepts and techniques to develop the basin's tight Mesaverde gas reservoirs. Their primary emphasis was using stratigraphy to find thick, high porosity pay. Wellbores were individually designed, drilled and evaluated for completion, zone-by-zone. Completion efforts took weeks, involving numerous delays for hookup and production. Uneconomic results stimulated efforts to improve gas recovery and lower costs through the application of new technology. Core studies, horizontal wells, 3D seismic and slant hole production tests were attempts to improve reservoir understanding and prospect economics. Wellbore redesign, supply-chain management and "alliance" concepts were applied to control and reduce costs. By 1998, the exploitation model that emerged was a reduced cost, industrial process backed by large-scale 3D surveys and extensive operator-vendor alliances.

Industry followed this model to execute an aggressive development program in the eastern GGRB through the mid and late 1990s, even in the face of low to moderate wellhead prices. One operator alone drilled over 200 wells between late 1994 and late 1998. Well costs had been reduced significantly and per-well recoveries averaged over 2 BCF, generating economic returns for operators positioned to exploit economies of scale.

But, these successes came at a price. In a relentless effort to cut costs, industry slashed its technical and prospect generation staffs. Seismic vendors and drilling contractor prices were squeezed below replacement costs. And, the previously identified higher quality portions of the resource were consumed. Developing the remaining tight and deep Cretaceous age sands now required a new set of technologies and challenges. Among the most significant are effective prediction of fracture-enhanced reservoir permeability, reliable pay definition tools for shaley, water bearing sands and understanding the sources of water in overpressured basin settings. These technology challenges are being pursued by various industry and government R&D projects, helping achieve the full potential of the GGRB.