ABSTRACT

Bypassed, incompletely drained, and untapped gas reservoirs of distal-shoreface origin remain economically important targets for incremental reserve growth within North McFaddin field, located in Victoria County, Texas. The informally termed 4,200-ft No. 5 reservoir serves as an example to illustrate the methods adopted to identify and evaluate reserve additions. Recompletions in existing wellbores and an infill well were recommended to access potential reserves. North McFaddin serves as a model for evaluating the feasibility of developing additional thin-bed resources in other mature gas fields.

Recently developed wireline tools and log-evaluation techniques were fundamental in identifying resources. Reservoirs are typically 10 ft or less in thickness and are vertically compartmentalized by nonreservoir facies of subequal thicknesses. Superposed contour maps of net thickness, relative spontaneous potential, and resistivity, integrated with structure maps and well-test-production, wireline-formation-test, and sidewall-core data, enabled the potentially productive limits of each reservoir to be delineated. Potentially recoverable reserves were subsequently determined by comparing volumetric and material-balance calculations of original gas in place with historical production data. Engineering analyses indicated that most reservoirs are characterized by pressure-depletion drive mechanisms. Production and flowing-pressure data of recent thin-bed completions enabled estimation of productivity, drainage volume, and permeability thickness using a radial-flow simulator.

Potentially recoverable risked reserves were estimated at approximately 6.5 billion cubic feet of gas. Economic evaluation determined the reservoirs as a cost-effective resource. Thin-bed reservoirs in North McFaddin field illustrate the types of incremental resources that may be common in barrier/strandplain reservoirs from other mature gas fields of the Gulf Coast.