Hydraulic fracture and stimulation efficiency are critical to well productivity in unconventional reservoirs. Ideally horizontal wellbores are drilled along the direction of minimum principal stress so that hydraulic fractures can be placed transversely, connecting reservoirs while opening against the least resistance. However, it is not uncommon wells are drilled at angles to the in-situ stress for various reasons, such as lease holdings, operation constraints, and/or geomechanics uncertainties. Based on comprehensive well data from eight major US unconventional plays, this study investigated the extent and timing of misaligned well orientations on production.
Over 71,000 wells from eight US unconventional plays, such as Bakken, Barnett, Eagle Ford, Haynesville, Marcellus, Granite Wash, and Utica, were analyzed for the misalignments of well horizontals with minimum horizontal stresses. The in-situ stress directions were identified from the World Stress Map (WSM), through borehole breakouts, focal mechanism analysis, induced fractures, core, and other geological indicators. Well directional surveys and three years of accumulated production were obtained from public databases, normalized by lateral length and proppant intensity (pounds per foot). To investigate depletion effects, wells were grouped based on starting time, especially for the years with significant populations.
Misalignments between well orientation and the direction of minimum horizontal stress is referred as "well-stress misalignments". It was found that unconventional fields exhibit different relations of well productivity with well-stress misalignments. Majority of unconventional fields exhibited negative relations: the increase of misalignment angles led to a polynomial decrease in well productivity. The extent of production reductions varied across different plays. For examples, the misalignment angles as small as 10 degrees had reduced 12% hydrocarbon production in Marcellus, 25% in Bakken, and 50% in Utica. In comparison, wells from Barnett and Haynesville fields had been little affected by the well-stress misalignments. Due to depletion effect, the negative impacts of misalignments diminished with time in Eagle Ford, Granite Wash, and Haynesville. However, the effect was not clear in Bakken, Barnett, and Wattenberg while an opposite trend was observed in Utica.
The large-scale data-driven study provides new insights of key drivers for unconventional productivity, especially when well horizontals are not aligned with in-situ stress. The findings can help optimize well design to maximize production potential in US unconventional fields.