Abstract
The Vaca Muerta formation is a shale oil and gas reservoir under active tectonics created by the Andes which are compressing the basin in an east to west direction and creates a varying stress regime both in horizontal and vertical directions. Additionally, this formation is composed of alternating layers of three litho-types with contrasted mechanical properties. These are organic shale, carbonate and ash beds. Slippage or debonding at weak-strong interfaces separating laminations is suspected to further contribute to the development of horizontal fracture components or T-Shape fractures during hydraulic fracturing when the fracturing pressure reaches the overburden (Zhang et al. 2007).
Selecting the proper landing position of horizontal wells in these compressive environments depends not only on the reservoir quality but also in selecting a layer which combines lower stress with sufficient vertical connectivity, thus minimizing detrimental effect from T-Shape propagation behavior.
Closure pressure can be obtained by performing a Dynamic in Situ Stress Test by force-closing the fracture (Nolte 1979; Raaen and Brudy, 2001). To this effect, YPF initiated in 2015 a campaign of in situ stress measurements across the Quintuco-Vaca Muerta system in vertical exploration and pilot wells. The objective was not only to predict a more accurate mud weight window to drill through the Vaca Muerta but mostly to measure as best as possible the difference between the overburden and the minimum horizontal stress to build a mechanical earth model accurate enough to help select better landing points for horizontal wells.
There are 3 scales at which dynamic closure tests are performed. At the smallest scale, a micro-fracture is created by pumping a little amount of drilling fluid at few liters/minute in 1m open hole isolated between straddle packers. At the largest scale, in a cased-hole perforated well, the dynamic closure test is performed as part of fracturing data acquisition before fracture operation: at this scale the test involves tens of bbls pumped at 5-10 bpm. Finally stress can be measured during Extended Leakoff Test (ELOT or XLOT) acquired in open hole at the casing shoe. After a literature review of the development of the flow back method this paper presents extensive details and operational procedure recommendations that were obtained from successful XLOT combining multiple cycles with injection, forced flow back and reopening sequence in the Neuquén basin.
