Abstract
The migration of expelled hydrocarbon from source rock into unconventional tight reservoirs is subject to different pore-scale fluid transport mechanisms as opposed to the conventional counterparts and therefore plays a crucial role in controlling the hydrocarbon distribution and accumulation in the former. One of the different mechanisms is related to the formation of a more viscous boundary layer (BL) of brine, i.e., wetting phase fluid on pore surfaces, giving rise to the so-called BL effect. In this work, a two-phase pore network model (PNM) that considers this BL effect is developed to study the influences of pore-scale characteristics on the oil migration process, manifested through the BL effect in tight-sandstone media. Good agreements are reached between experimentally derived relative permeability curves and predicted ones, by applying this model to the pore-network networks extracted from the same samples. Then, this validated model was used to evaluate the impacts of the following factors on the oil migration process: pore radius, coordination number, aspect ratio, brine viscosity, and wettability. The results show that all factors can influence the oil migration process but at different magnitudes. The applicability and significance of the developed tight oil migration PNM are discussed in this work.
Highlights
Following the shale gas breakthrough in the United States, tight oil has become the focus of unconventional petroleum exploration worldwide [1, 2]
The principal objectives of this work are as follows: (1) to extend the application of pore network model (PNM) to analyze the oil migration in tight formations; (2) to study the influences of pore-scale characteristics on oil migration using PNM method, which includes the radius of pores, average coordination number, aspect ratio, brine viscosity, and wettability conditions; and (3) to discuss the significances and limitations of the conducted PNM simulations
Note that the low Swi here does not mean most of water could be replaced by the injected oil during the oil migration process in the natural system, which is only an idealized calculation value under the ideal hypothesis conditions
Summary
Following the shale gas breakthrough in the United States, tight oil has become the focus of unconventional petroleum exploration worldwide [1, 2]. Tight oil reservoirs are usually characterized by low porosity, low permeability, complicated pore-throat structures, and strong heterogeneity ([4,5,6]). Of many factors that control this process and the resultant reservoirs in terms of their sizes, fluid saturations, and fluid properties within them, which are of crucial importance for hydrocarbon exploration and exploitation [8, 9], the tightness of pore space within carrier and reservoir rocks is of pivotal importance. Oil migration behaves distinctively in tight reservoirs in the following aspects [5, 10]: (1) there is no clear demarcation between the primary migration and secondary migration process, (2) the migration distance is typically short which induces hydrocarbon inner source accumulation or near-source accumulation, and (3) the impact of capillary pressure is significantly important during the migration process while the effect of buoyancy is limited
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