Novel laboratory investigation of huff-n-puff gas injection for shale oils under realistic reservoir conditions

Abstract

Enhanced oil recovery (EOR) from shale oil formations is recognized as the next frontier in the exploitation of unconventional resources. Current approaches for performing EOR experiments using fractured rocks can generate misleading results as the fractured cores cannot be saturated with live oils. In this work, a novel laboratory methodology is developed to quantify the efficiency of gas huff-n-puff in shale oil formations under realistic reservoir conditions, i.e. live oil and hydraulic fracturing reservoir conditions. This new approach involves first saturating the shale core with oil and then hydraulic fracturing the oil-saturated core by increasing pore pressure above the core confining pressure. To demonstrate the viability of this new methodology, shale cores from the Haynesville formation with a permeability of 0.0001 mD were tested.Two huff-n-puff experiments were performed by the injection of associated gas into cores saturated with either dead crude oil or live oil. 48% additional oil recovery was achieved from the shale core saturated with live oil whereas, the shale core saturated with dead oil yielded a 33% enhanced oil recovery, which is significantly lower than that of the live oil. Furthermore, the pressure decay profiles during the soaking periods indicate a comparatively higher gas penetration into the live oil system despite the higher gas solubility in the dead oil. The in-situ gas formation can have a strong influence on the efficiency of huffn-puff. The notable differences in the results obtained on live compared to dead oil experiments demonstrate the need to perform more realistic laboratory experiments to better optimize shale oil EORbased extractions in the field.

Experimental and computational evaluation of cyclic solvent injection in fractured tight hydrocarbon reservoirs

“…The previous laboratory studies represent a step change in the thinking around experimental evaluation of corebased HNP process through suggesting alternative flow geometries 7,20 , use of large-scale samples/fractures 7 and combining 'outside-of-coreholder' drilling and advanced core-based hydraulic fracturing under stress inside the coreholder to induce fractures 8 . However, the current work is unique due to (1) the proposed methodology used for creating fractures under stress and (2) its integrated experimental and simulation approach in which a compositional model is calibrated to laboratory data (porosity, matrix/relative permeability, etc.).…”

“…There are now numerous studies of improved oil recovery in a variety of North American tight oil reservoirs that have examined the feasibility of incremental oil recovery using gas injection [1][2][3][4][5][6] . These studies have primarily focused on numerical 2,3,6 or laboratory-based approaches 1,[7][8][9] . Laboratory-based studies are important for assessing (1) the key mechanisms controlling injected gas transport into the reservoir, and miscibility with the oil 7,8,10 and (2) the influence of operational parameters including injection pressure/time, soaking time, production pressure/time-among other factors (e.g.…”

“…These studies have primarily focused on numerical 2,3,6 or laboratory-based approaches 1,[7][8][9] . Laboratory-based studies are important for assessing (1) the key mechanisms controlling injected gas transport into the reservoir, and miscibility with the oil 7,8,10 and (2) the influence of operational parameters including injection pressure/time, soaking time, production pressure/time-among other factors (e.g. PVT)-on recovery performance 7,11 .…”

“…Further, the laboratory methodologies are primarily focused on intact ('unfractured') core plug samples using either the 'flow-through-matrix' 12,18,19 or 'flow-around-matrix' 1,20,21 schemes, which do not realistically represent the fracture-matrix contact or sequences associated with a typical cyclic solvent injection (HNP) process. While seminal works have been conducted to mimic HNP process on fractured cores 7,8 , the fractures were created outside of the coreholder either fully using saw 7 or partially using drilling bits 8 (combined with hydraulic fracturing under stress inside coreholder). These approaches, though more representative, are prone to the creation of stress-induced micro-fractures, particularly in organic-rich shales (the primary target plays for field-scale HNP application).…”

“…In addition, previous laboratory-based HNP experiments were typically not simulated using a compositional (lab-calibrated) model. More importantly, the experimental durations of previous laboratory techniques are extensively long, even for fractured cores (typically 15-20 h per cycle 7,8 ). This limitation hinders the experimental throughput, and the capacity for characterizing the HNP mechanisms/controls over diverse experimental conditions in a timely fashion.…”