Abstract
The energy cost of drilling a natural gas well has never been publicly addressed in terms of the actual fuels and energy required to generate the physical materials consumed in construction. Part of the reason for this is that drilling practices are typically regarded as proprietary; hence the required information is difficult to obtain. We propose that conventional tight gas wells that have marginal production characteristics provide a baseline for energy return on energy invested (EROI) analyses. To develop an understanding of baseline energy requirements for natural gas extraction, we examined production from a mature shallow gas field composed of vertical wells in Pennsylvania and materials used in the drilling and completion of individual wells. The data were derived from state maintained databases and reports, personal experience as a production geologist, personal interviews with industry representatives, and literature sources. We examined only the “upstream” energy cost of providing gas and provide a minimal estimate of energy cost because of uncertainty about some inputs. Of the materials examined, steel and diesel fuel accounted for more than two-thirds of the energy cost for well construction. Average energy cost per foot for a tight gas well in Indiana County is 0.59 GJ per foot. Available production data for this natural gas play was used to calculate energy return on energy invested ratios (EROI) between 67:1 and 120:1, which depends mostly on the amount of materials consumed, drilling time, and highly variable production. Accounting for such inputs as chemicals used in well treatment, materials used to construct drill bits and drill pipe, post-gathering pipeline construction, and well completion maintenance would decrease EROI by an unknown amount. This study provides energy constraints at the single-well scale for the energy requirements for drilling in geologically simple systems. The energy and monetary costs of wells from Indiana County, Pennsylvania are useful for constructing an EROI model of United States natural gas production, which suggests a peak in the EROI of gas production, has already occurred twice in the past century.
Highlights
Natural gas dominates the well-derived fossil fuel production of the United States; the number of wells drilled for natural gas overtook the number of wells drilled for crude oil in 1993 and accounts for nearly 70% of the wells drilled annually [1]
Individual well production data from Indiana County are for the years 1984 through 2003 while material data are for the years between 1964 and 2005
The materials consumed in natural gas production have never before been incorporated into an energy return on investment (EROI) analysis
Summary
Natural gas dominates the well-derived fossil fuel production of the United States; the number of wells drilled for natural gas overtook the number of wells drilled for crude oil in 1993 and accounts for nearly 70% of the wells drilled annually [1]. Natural gas is currently the most widely used fuel by the manufacturing industry in the United States [2]. It becomes prudent to analyze energy resources in terms of physical constraints and requirements. This situation is more serious if we consider arguments about whether the most important fields have reached maturity and are in decline, i.e., peak gas [4]
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