A WATER FOOTPRINT CONCEPT FOR SUSTAINABLE OIL & GAS DEVELOPMENTS

A “Water Footprint represents the changes in one or more water reservoirs as caused by a human enterprise. Although a water-footprint methodology has been proposed for agriculture, it does not address energy-based enterprises and it continues the tradition of carbon-footprint methodologies by ignoring metrics for beneficial use of the water.

A Water Footprint Methodology for Oil & Gas Developments should account for how drilling and hydraulic fracturing (hydrofrac) water is sourced and how produced/flow-back wastewater is managed. And to allow intercomparisons among oil and gas developments, as well as with other categories of water use, the water footprint also should include a measure of the hydrocarbon resource value produced by the footprint. A quantitative description of key water footprint (F*) terms is:

F* = [V_Fni + (1 – Q_SR)V_SR + V_UIC] / P_EUR [1]

where V_Fni = new intake water (in addition to any recycled volumes); V_SR = surface release of wastewater which has been treated to acceptable water-quality standards; Q_SR = a water-quality metric; V_UIC = wastewater which is isolated by injection wells as an alternative to treatment and surface release; P_EUR = the estimated ultimate recovery (EUR) of oil (Bbl) or gas (Mcf), or their energy-content equivalents (Dth).

Water Footprint Optimization for Shale Gas involves significant volumes of water for drilling and completion, including multiple hydrofrac stages, and must consider not only the volumes of water and wastewater involved but also their associated costs. Examples of water footprint calculations are presented for three different shale-gas wells which were completed in the same resource play by three different operators (X, Y and Z). Using the water-footprint calculation protocol (Equation 1), Company Y arrived at the lowest physical water footprint (F*_p = 3.0 million gallons per well) whereas Company Z achieved the lowest economic water footprint (F*_e = 0.97 gal / Dth). Company X kept water costs low but was the worst performer overall with regard to physical water footprints (F*_p = 4.8 million gallons per well). Company Z achieved low water footprints but carried the largest burden for water costs. Company Y approached a more optimized outcome where clear gains were demonstrated, relative to peers X and Z, both for water footprints and water costs.