Quantification of Performance Heterogeneity in a Devonian Shale Gas Play: Improving Development Drilling Results Using Statistics
Authors: Stell James R. , Fischer Bradley W. 
Published: December 2007

The fractured Upper Devonian Black Shale in western West Virginia and eastern Kentucky is a mature target for natural gas exploration and development. Risk of a dryhole is limited — a feature that attracts interest. However, ultimate reserves are not related to pay in predictable relationships, particularly on a location to location basis. The average well may recover sufficient gas over its life to provide only a little over hurdle-rate economics. If a development well recovers reserves (EUR) similar to those estimated for the average neighboring well, it may be a viable candidate for investment. In these cases, field development often proceeds based on the arithmetic average of earlier wells. However, production from well to well varies considerably throughout the area. In spite of the best efforts of the technical staff, the "next" development well may be marginal because of the unpredictable nature of the fractured reservoir rock system providing the ability of gas to flow to the wellbore.

Economic success in this play is extremely sensitive to an overall ability to achieve expected production from the fractured shale. Areas with similar average reserves may have different economic successes based on their production heterogeneity. Variability or heterogeneity of gas reserves from groups of wells may be measured. The heterogeneity of EUR's is quantified using the Dykstra Parsons variation coefficient, V, which is a function of the standard deviation divided by the median and is calculated from cumulative frequency plots. The coefficient of variation (CV), equal to the standard deviation divided by the mean EUR, is also used to quantify variability.

When drilling infill wells, particularly later in the development program, consistency in reaching the area's average EUR was better in areas with lower heterogeneity. Area drilling results are more reliable and may often be improved over time using prediction techniques when the area's Dykstra Parsons variation coefficient V is less than 0.5 and CV is less than 0.7.

Plotting techniques have been developed to correlate gas recovery with three important variables: thermal maturity, organic-shale thickness, and heterogeneity. Average ultimate gas recovery was related to organic-rich, radioactive shale thickness and to the heterogeneity of wells in the group. The overall average EUR was 1.1 million standard cubic feet (MMCF) per foot of organic-shale (223 MMCF/well from 208 feet of organic-shale). Thermally mature areas that were homogeneous, CV &lt; 0.4, will produce 1.4 MMCF per foot. Heterogeneous areas with CV &gt;0.7 were underachievers, producing 0.8 MMCF per foot.

The Devonian Shale is fracture stimulated predominantly by using foam with proppant or straight Nitrogen. Study of EUR's of 320 wells in Pike Co., Kentucky, indicated identical average reserves for wells stimulated with either foam or Nitrogen—280 MMCF/well.

Introduction

The Devonian Shale is a widespread, under-pressured, dry gas-producing formation in the study area which spans 90 miles across 3 counties in eastern Kentucky and 5 counties in western West Virginia in the Appalachian Basin. The study included Ashland Exploration, Inc. (Ashland) development wells that were completed in the Devonian Shale and commingled Devonian Shale-Berea Sand completions during the period 1980–94. Each well's projected ultimate gas recovery (EUR) was calculated by Netherland, Sewell & Associates, Inc. as of October 1994. Reserves vary from county to county, as might be expected, since shale character, depth, depositional factors and the mechanism and intensity of natural fracturing change greatly across the area. Well productivity also changes dramatically from well to well in localized areas in a seemingly random manner. P. 541^