Comparison of pressure front with tracer front advance and principal flow regimes in hydraulically fractured wells in unconventional reservoirs

Pressure depletion patterns near hydraulically fractured wells in ultra-low permeability reservoirs occupy a much larger region than outlined by fluid withdrawal contours. When used for reservoirs with moderate or high permeability (say k > 1 mD), industry never needed to caution using pressure depletion plots as a proxy for fluid removal or drained rock volume (DRV). For such conventional reservoirs, pressure depletion plots are good proxies for fluid removal or drained rock volume (DRV). The fluid that moves from the margins of a pressure front, indicative of the so-called diffusive depth of investigation (DOI), will reach the well on the time-scale of economic life of the well. However, a whole new way of thinking is required when studying ultra-low permeability shales (nanoDarcy range), where the distinction between pressure depletion zones and the DRV regions (where fluid is recovered from) becomes crucial to understand which factors control the hydrocarbon recovery process. Our detailed analysis compares the diffusive DOI, of the leading edge of the pressure depletion front, and that of the convective DRV given by a tracer front. Results show that the pressure depletion front between the fractures quickly approaches the adjoining fracture pressure, and most of the production originates from the fracture tips after 12 months. The pressure depletion pattern vastly overestimates the DRV, whereas the tracer front shows the actual produced rock volume. A final section evaluates the specific flow regimes that correspond to certain diffusive DOI and convective DRV growth stages in the life of the well. The discussion section explains how production diagnostic plots can be used to identify the moment in the well life when enhanced oil recovery methods should be applied. This study can facilitate a better understanding of the fluid flow in fractured porous media and can be used to optimize fracture and well spacing to maximize the DRV.