Pore-scale modelling of wettability alteration in microporous carbonates
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While carbonate reservoirs are recognized to be weakly- to moderately oil-wet at the core-scale, wettability distributions at the pore-scale remain poorly understood. In particular, the wetting state of micropores (pores <5 μm in radius) is crucial for assessing multi-phase flow processes, as microporosity can determine overall pore-space connectivity. Nonetheless, micropores are usually assumed to be water-wet and their role in multi-phase flow has often been neglected. However, oil-wet conditions in micropores are plausible, since oil has been detected within micropores in carbonate rocks. Modelling the wettability of carbonates using pore network models is challenging, because of our inability to attribute appropriate chemical characteristics to the pore surfaces in the presence of the oil phase and over-simplification of the pore shapes. First, we carry out an investigation of the prevalent wettability alteration scenario due to heavy polar compounds (e.g. asphaltenes) adsorption from the oil phase onto the surface, which occurs strictly after oil invasion. We develop a physically-plausible wettability distribution that we incorporate in a quasi-static two-phase flow network model which involves a diversity of pore shapes. The model qualitatively reproduces patterns of wettability alteration recently observed in microporous carbonates via high-resolution imaging. To assess the combined importance of pore-space structure and wettability on petrophysical properties, we consider a homogeneous Berea sandstone network and a heterogeneous microporous carbonate network, whose disconnected coarse-scale pores are connected through a sub-network of fine-scale pores. Results demonstrate that wettability effects are significantly more profound in the carbonate network, as the wettability state of the micropores controls the oil recovery. Second, we develop a novel mechanistic wettability alteration scenario that evolves during primary drainage, involving small polar non-hydrocarbon compounds present in the oil (e.g. alkylphenols, carbazoles, etc.). We implement a diffusion and adsorption model for these compounds that triggers a mild wettability alteration from initially water-wet to more intermediate-wet conditions. This mechanism is incorporated in the quasi-static pore-network model to which we add a notional time-dependency of the invasion percolation mechanism. The model qualitatively reproduces experimental observations where an early rapid wettability alteration occurred during primary drainage. Additionally, we are able to predict clear differences in the primary drainage patterns by varying both the strength of wettability alteration and the balance between the processes of oil invasion and wetting change, which control the initial water saturation for waterflooding. In fact, under certain conditions, the model results in higher oil saturations at predefined capillary pressures compared to the conventional primary drainage. In particular, it leads to the invasion of micropores even at moderate capillary pressures in the microporous carbonate network. Additionally, the model results in significant changes in the residual oil saturations after waterflooding, especially when the wetting state is altered from intermediate-wet to more oil-wet conditions during ageing.