Tight-Sand Development Potential in the Southern Rulison Area, Garfield County, Colorado
Authors: Duda J.R. , Hancock J.S. 
Published: December 2007

Summary
Reservoir characterization and simulation of natural gas production were used to assess the production potential for an extension area within the Piceance Creek basin (Garfield County, CO). Many of the basin's reservoirs are lenticular sands characterized as having low porosities, low permeabilities, and relatively high water saturations. One such horizon is the Mesaverde group, which consists of discontinuous sandstone lenses distributed within a shale matrix and is present throughout the Piceance Creek basin. Prior studies of the commercial Rulison field in the east-central portion of the basin indicated that compressional forces created an anticlinal nose aligned northwest/southeast through part of the field. These forces created a conductive natural fracture system that enhances the permeability of the Mesaverde. This paper shows a relationship between Rulison field and an area south of Rulison that encompasses about 36 sq miles [93.2 km]. The area has been sparsely drilled (averaging one well per two sections and nonuniformly spaced); thus, the possibility exists for further commercial development through infill drilling. The southern Rulison area is characterized in structure, percent sand, and critical reservoir parameters including permeability. Production rates, cumulative gas production, and drilling and completion data are used to infer permeabilities by history matching with a dual-porosity reservoir simulator. Results include a reservoir description of the area and economic evaluations of several investment scenarios that will help to assess the potential resource.

Introduction
The Piceance Creek basin of northwest Colorado contains an estimated 33 Tcf [934 × 10–9 m3] of maximum recoverable gas and is the setting for the U.S. DOE's Multiwell Experiment (MWX). The Natl. Petroleum Council estimated the volume of recoverable gas at 12.9 Tcf [365 × 10–9 m3), assuming a gas price of $2.50/Mcf [$8.83/100 M], a 15% discounted cash flow rate of return, and the current level of technology. The basin comprises Garfield, Mesa, and Rio Blanco counties with minor extension into Delta, Gunnison, and Moffat counties. The Tertiary Age Wasatch formation and Cretaceous are Mesaverde group are typically the gas-productive horizons throughout the basin.
Situated near the center of the basin in Garfield County is the Rulison field. Discovery of the field and its early development occurred in the late 1950's and early 1960's, with sparse drilling in the southern sections of Township 6 South, Range 94 West (T6S, R94W) and scattered drilling throughout T7S, R94W and T7S, R95W. Wasatch completions have generally been restricted to T6S, R94W with development on 160 acres/well [65 ha/well]. The Mesaverde has been drilled on 320 acres/well [130 ha/well] and is the focus of this study. Most of these wells penetrated the entire lenticular section of the Mesaverde (Williams Fork), with a few deeper wells reaching the marginal marine deposits in the lowermost Mesaverde. Northwest Exploration (now part of the Williams Co.) further developed the area during the early 1980's. Thirteen Mesaverde wells were completed in T6S, R94W. For this investigation, T6S, R94W will be referred to as the Rulison field and T7S, R94W and T7S, R95W as the southern area.
The geology and natural gas production of the study area were evaluated to determine why the southern area had not been developed to the extent of the Rulison field. The Rulison field has been well characterized; therefore, the present effort concentrates on the southern area. In appraising the southern area as a developmental prospect, similarities to the Rulison field are identified and quan-tified. Fig. 1 is a generalized map of the study area, the two townships of interest are southwest of Rifle, just south of the Colorado River.