Abstract

Tanos Exploration II, LLC (Tanos) along with partner Breitburn Energy Partners (Breitburn) conducted a microseismic survey in 2015 at Overton Field, Smith and Cherokee counties, Texas. The survey was acquired in two horizontal wells drilled in opposite directions (north and south) from the same surface pad. Both wellbores targeted the same stratigraphic interval within the Taylor sandstone of the Jurassic-aged Cotton Valley formation. Stimulation spacing, perforation clusters, pump rates and proppant concentrations were fairly consistent between wells. The McElroy-Swann #1H, the northern lateral, was drilled between twelve existing vertical wells that were completed with single stage fracture stimulations over the entire Taylor interval approximately 10 yr earlier. In contrast the southern lateral, the Wilkinson-McElroy A #1H was drilled within a relatively undrained area of the field. The survey was conducted to compare and contrast the fracture stimulation results for the two wells, one drilled within “partially-drained” versus one drilled within “undrained” areas of the field.

The McElroy-Swann #1H was drilled with a total displacement of 6640 ft and an effective lateral length (first perforation to last perforation) of 5803 ft along a 358° azimuth to a total depth of 17,999 ft. Completion design utilized 5 1/2 in. P110 casing cemented in place and a “plug and perf” methodology with three or four perforation clusters sixty feet apart per stage. The existing vertical wells ranged from 410 to 1265 ft away from the horizontal wellbore. Cumulative production from the existing vertical wells totaled over 4.6 billion cubic ft (BCF) of gas and 98,650 barrels of condensate (BC) (5.2 BCFE [BCF gas equivalent]). The microseismic event mapping for the northern lateral indicated the original stress field of the Taylor sandstone reservoir had been significantly altered by the fracture stimulations and associated production from the vertical wells. Instead of “well-behaved,” predictable fracture propagation along a consistent orientation, results indicated a “random” orientation with different widths and half-lengths resulting in highly complex fracture patterns.

The Wilkinson-McElroy A #1H was drilled with a total displacement of 7253 ft and an effective lateral length of 6433 ft along a 169° azimuth to a total depth of 18,600 ft. Plug and perf methodology within 5 1/2 in. casing was employed as above. In contrast to the McElroy-Swann #1H, the southern horizontal wellbore drilled through a relatively undrained area of the field. Three wells along the wellbore, all completed in 2004, had produced less than one BCFE of gas combined. The Wilkinson-McElroy A #1H results indicated a more predictable fracture propagation orientation consistent with an earlier 2005 study at Overton Field.