Southwest Section AAPG Annual Convention

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The recent oil boom in the US has been attributed to the result of hydraulic fracturing and horizontal drilling of shale plays, however despite this boom, production and maximum recovery is still limited as only few percentage of the original oil in place is being produced. The prolific Bakken petroleum system of North Dakota is one of the biggest discovery in the US petroleum system and despite producing about a million barrels of oil daily, it is producing only 8.9±5.3% of the oil in place (North Dakota Industrial Commission) and this is because of the ultra-tight pores and poor connectivity.

Enhancing the oil recovery in Bakken has led to different studies, however one area that has not been given consideration is the structure of the nanopores storing and transporting the hydrocarbons. The implications of these nanopores structures can be felt in fluid flow and mass transportation as a function of pore geometry (e.g., pore size and shape, pore-size distribution) and topology (e.g., pore connectivity). This work is focused to study the connectivity of the nanopore structures and associated fluid migration of Bakken units using cores of the three Bakken units obtained from a producing well in North Dakota. For these tight rock samples, we have studied pore structure and edge-accessible porosity, from the following complementary tests: mercury intrusion porosimetry, fluid and tracer imbibition into partially-saturated shale, vacuum saturation followed with tracer introduction, and tracer diffusion into fluid-saturated shale. These tests use tracer-bearing fluids (API brine or n-decane) to examine the association of tracers with mineral or kerogen phases, using a combination of elemental laser ablation-ICP-MS mapping.

These innovative approaches bridges the gap between pore structure and other fluid behavior studies and indicates the limited accessibility and connectivity of nanopores in shales, as it affects migration of hydrocarbon molecules from the shale matrix into the stimulated fracture and overall oil recovery.