Marcellus Shale Hydraulic Fracturing and Optimal Well Spacing To Maximize Recovery And Control Costs

Abstract

EQT wants to maximize recovery and control development costs in its Marcellus shale development program, which consists of multi-stage fractured horizontal wells. To achieve this objective the asset team considered tighter well spacing. But at what distance do fracture-stimulated wells directly interact? Microseismic data implies that the radius of influence is greater than 1,000 ft. However, after 6 months of production, a study using a nodal analysis software model suggests an effective frac half-length less than half of this, a finding that has been supported in other EQT wells across Pennsylvania and West Virginia. To test this concept for horizontal wells with multi-stage slickwater, sand-propped frac completions in the Marcellus shale—where well spacing is typically 1000-1200 ft—well spacing was reduced to just 500 ft, in three different areas of the Marcellus. The goal was to better understand: How do the hydraulic fractures interact? Are the closer spaced wells more likely to share reserves? Does the frac design need to be adjusted for tighter spaced wells? To answer these questions, 500-ft and 1000-ft offset wells were monitored during completion with microseismic, radioactive tracers, chemical tracers, and pressure gauges. Production logging was initially scheduled to begin 4-6 months after production, however, had to be postponed. With more than 6 months of production data in the first of the three development areas, the 500-ft-spaced wells and 1000- ft-spaced wells are performing similarly, despite frac communication during the completion of the 500-ft wells. Estimates of effective frac half-length from the nodal analysis model support this conclusion. A unique, comprehensive data set is collected across the Marcellus shale to document how hydraulic fracturing influences well performance, and the study attempts to reconcile conflicting frac half-length interpretations to identify optimal well spacing. The results to date support tighter spacing, which maximizes recovery and facilitates multiple wells to be drilledfrom the same pad, thereby minimizing the footprint for "greener" operations.

Potential Contaminant Pathways from Hydraulically Fractured Shale to Aquifers

“…Fully developed shale typically has wells spaced at about 300‐m intervals (Edwards and Weisset 2011; Soeder 2010). Up to eight wells may be drilled from a single well pad (NYDEC 2009; Arthur et al 2008), although not in a perfect spoke pattern.…”

“…Up to eight wells may be drilled from a single well pad (NYDEC 2009; Arthur et al 2008), although not in a perfect spoke pattern. Reducing by half the effective spacing did not enhance overall productivity (Edwards and Weisset 2011) which indicates that 300‐m spacing creates sufficient overlap among fractured zones to assure adequate gas drainage. The properties controlling groundwater flow would therefore be affected over a large area, not just at a single horizontal well or set of wells emanating from a single well pad.…”

Water Pollution Risk Associated with Natural Gas Extraction from the Marcellus Shale

“…( 43 ) Proponents of hydraulic fracturing posit that there is no known mechanism by which fractures and fluid can propagate through over 1,000 m of sedimentary strata; some recent data even suggest that Marcellus Shale wells should be more closely spaced due to shorter effective fracture lengths than originally estimated. ( 44 ) In reply, critics say that several potential mechanisms cannot be ruled out: ( 10,45 ) unexpected vertical fracturing through overlying strata, ( 46,47 ) and fracturing fluid preferentially traveling through naturally occurring fractures and faults. ( 44,48 ) Besides, critics argue, current conventional fracturing models are not appropriate for shale gas reservoirs, ( 49 ) and the interpretation of microseismic data used to monitor hydraulic fracturing is still controversial.…”

A model describing flowback chemistry changes with time after Marcellus Shale hydraulic fracturing

“…Microseismic data from hydrofractured wells yields an idea of the stimulated reservoir volume (Edwards et al, 2011;Fisher, 2010). To a first-order approximation, hydrofracturing around a horizontal borehole of length 1.1 × 10 3 m (3609 ft) located in a well field with one horizontal bore every 300 m (984 ft) in the Marcellus will result in a stimulated layer of shale 45 m (148 ft) in the vertical dimension and 300 m (984 ft) in the horizontal dimension.…”