ABSTRACT

Production from "unconventional" shale gas reservoirs has seen increased attention and economic viability in recent years through the use of horizontal development wells. Maximizing horizontal permeability in such wells has been found to be one of the largest keys to unlocking shale gas reservoirs and producing them at economic rates. Currently, a world-class shale gas accumulation is being developed in and around the Dallas-Fort Worth Texas, area using horizontal well applications. A multi-component approach to derive reliable and accurate predictions of horizontal shale gas well performance is presented here, using open hole well log data from several vertical pilot bore holes that have been drilled targeting the Barnett Shale. Fundamentally, this approach is founded upon four key elements: a thorough probabilistic volume estimation of lithology, calculation of total porosity, estimation of free gas present using a modified dual water saturation model and prediction of kerogen and total-organic-car bon (TOC) of the interval. From the primary evaluation from open hole logs, net kerogen-feet, TOC-ft and adsorbed gas-feet have all been identified as meaningful measures of shale "quality" that are useful as a vehicle to obtain accurate predictions of horizontal well ultimate recovery gas volumes (EUR). These factors have been weighted as a function of drilled lateral length for the horizontal production hole that has been subsequently drilled from each pilot. Varying the values calculated at the pilot well according to horizontal length is a key aspect of this evaluation technique and the weighting of these parameters found to correlate to decline-curve based EUR volumes for the study wells. The net product has been a reliable indicator for visualizing and quantifying well performance in five existing Barnett Shale horizontal wells, using standard open hole well log data acquired in the vertical pilot holes.

THE BARNETT SHALE, METROPOLITAN DALLAS-FORT WORTH, NORTH TEXAS

The Mississippian Barnett Shale is currently under going prolific development as a gas resource in and around the Dallas-Fort Worth area, north east Texas (Figure 1). One of the largest development challenges to the Barnett Shale has been its proximity to urban areas in and around the greater Dallas-Fort Worth area. Creative development plans and operations have been specifically tailored to the Barnett Shale play to minimize environ mental impact on the area, including the use of horizontal wells drilled from multi-well pad sites.

Geological setting of the Barnett Shale

Stratigraphically, the Barnett sits unconformably above the Viola and/or Ellenburger Formation. The Ellenburger has seen prolific production in the north Texas area over many decades. In this particular study area, the Ellenburger is known to be water bearing and is a significant risk for hydraulic fracturing. Historically, fracturing horizontal wells drilled in the Barnett in proximity to the water bearing Ellenburger has resulted in excessive wellbore loading or hydraulic lifting issues when these wells are put on production due to high water rates. Above the Barnett lies the Marble Falls Lime stone, a low porosity thick lime stone interval (Figure 2).

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