Integrated Approaches for Wells Interference and Drainage Assessment in Vaca Muerta Shale Play, Argentina.

One of the greatest challenges in the development of unconventional shale resources through factory-mode horizontal drilling is determining the optimal well spacing and stimulation design that maximize hydrocarbon recovery while minimizing field development costs. The Vaca Muerta Formation in the Neuquén Basin, Argentina, is a world-class unconventional shale play where commercial hydrocarbon production has been successfully achieved through the drilling and hydraulic stimulation of horizontal wells. To quantify vertical and horizontal interference between wells completed in different landing zones and to estimate their Effective Drainage Height (EDH), multiple diagnostic techniques were integrated across several well pads located in both the oil and gas windows. These techniques included production allocation using geochemistry, chemical tracers, pressure interference analysis, and well performance evaluation.

In the shale oil study, production allocation by geochemistry was conducted using core material from a vertical well and oil samples collected from well pads producing from three target intervals. Organic extracts from lightly crushed rock samples were obtained using carbon disulfide (CS₂) extraction in sealed vials. Oil samples, solvent extracts from rock and mud samples were subsequently analyzed using high-resolution gas chromatography with flame ionization detection (HRGC-FID). For the shale gas study, gas samples were collected at the wellheads using pressurized cylinders. The chemical and isotopic compositions of the gas samples were determined using gas chromatography coupled with a combustion/pyrolysis interface and a continuous-flow isotope ratio mass spectrometer. Carbon isotope ratios were reported relative to the Vienna Pee Dee Belemnite (V-PDB) standard.

In the shale oil area, integration of geochemical data with completion and well design information indicated generally weak vertical interference. The estimated EDH varied among the tested intervals within the same pad and was observed to change over the production period. In contrast, significant vertical interference was identified in the shale gas development area. A pilot study was therefore conducted to determine the optimal landing position within a specific target zone and to evaluate modifications to the completion design aimed at reducing interference. By adjusting the landing point by approximately 10 m, modifying the number of clusters per stage, and optimizing the injected volume per cluster, interference was successfully minimized, as confirmed by gas geochemistry and the other diagnostic methods applied.

The integration of production fluid and core geochemistry, pressure interference analysis, chemical tracers, and well performance data enabled the operator to evaluate different development hypotheses and optimize field development strategies, including well spacing and completion design, in areas surrounding the pilot projects.