Feasibility of Using Alternative Water Sources ...

 

 

 

Millions of gallons are needed to perform the completion phase before gas wells are put online, and, in the past years, gas operators have mostly used (1) groundwater from dedicated supply wells tapping the Trinity Aquifer, (2) surface water from large reservoirs and rivers, purchasing it from water-rights owners (private or state agencies such as river authorities), and, to a lesser extent, (3) surface water from private ponds and other water bodies, (4) treated water from municipalities and industrial users, and (5) water recycled from previous fracking operations. As gas production moves away from the core area toward the north, south, and west to access the remainder of the play, gas operators are faced with two challenges: (1) increased water scarcity and (2) measured reluctance to impact domestic and public water supplies.

The study analyzes three sources with the potential to meet those goals: (1) treated wastewater outfalls from waste water treatment plants; (2) small water bodies outside the State regulation of surface water, and (3) smallish groundwater aquifers in Paleozoic-age disconnected sand bodies west of the more plentiful Trinity aquifer. Treated waste water amount and chemical quality was obtained by mining TCEQ and federal databases and their characteristics are fairly constant throughout the year. All non-State surface water bodies were inventoried through satellite orthoimagery coverage. They were sorted into 6 size categories and computed for two selected years assumed to be representative of Texas climate cyclicity (?normal? wet and ?normal? dry). The study assumes that the only water available from the surface water bodies is the balance between wet and dry conditions and that that volume is to be spent in the course of two years. The surface water quality is understood to be fresh. Groundwater availability was inferred from the construction and running of a numerical model of aquifers covering pre-Trinity Paleozoic formations making up the subsurface geology of the area of study. The development of the model was done following informal guidelines set by the State GAM program and entailed (1) development of a conceptual model that consists of a shallow flow system quickly discharging baseflow water to the streams with a general head gradient to the north; (2) a thorough and lengthy phase of data gathering (top and bottom of layers, hydraulic conductivity values) from the literature and from public-domain databases; and (3) a calibration phase of matching modeled heads to observed heads. Those aquifers provide fresh to brackish water from generally low-yield wells. The fact that the State of Texas has never catalogued North-Central Texas Paleozoic aquifers even as minor aquifers, and this is borne out by our study, strongly suggests that they can only provide small amounts of water. This, in turn, suggests that obtaining meaningful amounts of water will require long and deep screened intervals tapping into more brackish sections and producing a degraded water quality. The amount of water available was computed by taking into account projected water use from all other users (as prepared by the TWDB) and assuming that an average cumulative drawdown of 5 feet across the entire study area in the next 50 years was acceptable to all stakeholders. This value is consistent with values accepted in the State for other small aquifers. Although aquifers do contain immense amounts of water, the amount that can be withdrawn is quickly limited by negative environmental and other impacts.

We developed an Arc-GIS tool to determine the amount of water available at any point of the study area from the three characterized water sources within a given radius (note that water might be available too from more common sources such as large reservoirs). They were chosen at 5, 10, and 15 miles. To understand the adequacy of the resource, ~1000 points were selected on a regular grid covering the area of interest and statistics on water availability were then derived. To allow for comparison of water availability all sources use the same reference unit of million gallons per month. Results suggest that, assuming a dense development of the gas resource, in most cases, enough water is theoretically available on average (~40 million gallons available vs. ~10 million gallons used per month in a 5-mile radius). However, more than half of the total is surface water making it very susceptible to droughts. Droughts will do more than drying up the surface water resource, it will also limit access to treated waste water and groundwater as conservation takes place and as more users rely on groundwater, respectively. From an economic standpoint all the alternative sources described in the study are very fragmented leading to a diffuse ownership and a likely expensive water gathering system