Naturally Fractured Liquid Rich Shales: The Importance of Fracture Connectivity at Borehole and Matrix Levels
Recently, the unconventional resources landscape shifted towards Liquid Rich Shale development for commercial oil production. Most LRS plays produce through hydraulic stimulation except a few that can produce through natural fractures, without stimulation. Despite their attractive economics, naturally fractured LRS highlight several challenges and opportunities to their commercial development. The biggest being fracture prediction and production behavior that would ensure a repeatable success rate. To investigate that, we ran several dynamic models of fractured LRS and analyzed production data from LRS wells. Initial results distinguished between two modes of production: the first takes advantage of large natural fractures (big crack model) that intersect the borehole and contributes to a pulse of high initial production. The second contributes to long tails of low rate production, and is explained as the “matrix” contribution (small crack model). In over 90% of the cases, the matrix contributed over 75% of the produced oil although it is over a long period of time. These results are not surprising giving that fracture porosity rarely exceed 1% of total pore volume, and that does not typically account for the volumes produced. Maintaining high production rates in some LRS wells, requires the matrix to recharge the fracture system fast enough with oil, which is difficult to balance considering that shales are tight permeability rocks. This requirement leads us to suggests that matrix definition in LRS is complex and is poorly represented by routine lab measurements. Two reasons contribute this view: the inherent horizontal anisotropy characteristic of shales, and the abundant natural macro-micro fractures ins hale rocks. As an alternative, we propose that the matrix-level natural fractures connect with the shale anisotropy and redefines the overall “matrix” flow behavior enabling continuous, long-term drainage of a LRS play. When large fractures are absent, matrix connectivity to the wellbore is impaired despite the large storage and STOIIP and thus, stimulation becomes critical to produce the well. We also show that LRS with large natural fractures can efficiently drain deeper, more mature resources even if the well bore is placed in a less mature level. In contrast, the lack of large natural fractures limits production to oils sourced within a LRS layer and consequently ties well productivity to the layer's maturity level.
AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014