As part of our investigations involving surfactant imbibition to stimulate oil recovery from shale, we previously studied oil flow through, and especially, surfactant formulation intake into shale to displace oil from Bakken shale. Our earlier studies in this area focused on the microdarcy permeability of Bakken cores to Bakken oil (with no water present). Permeabilities were basically independent of flow rate, in agreement with expectations from the Darcy equation. At optimal salinity, the incremental oil recovery (during imbibition into Bakken cores at 120°C) was up to 18% OOIP more than that observed during comparable experiments using previous formulations at 15–30% TDS.

In this paper, we test flow behavior using porosity and effective permeability measurements, and the degree of imbibition for available waters in the Niobrara chalk/shale of Colorado at different depths. Unlike Bakken shale and brine, Niobrara chalk/shale formation water is less saline (2.5–15% TDS) and the formation more lithologically complex. The effective permeability to oil, water and surfactant formulations in the Niobrara are greater. We also observed the following:

  1. As lithology changed with formation depth from chalk to marl to ultra-low permeable sandstone, the absolute water permeability, effective oil permeability, and effective surfactant permeability increased.

  2. Increased fracture/rock surface area assists liquid flow into the matrix and increases oil displacement.

  3. Surfactant solutions did not block oil flow within the matrix (formation damage)—instead it promoted oil displacement.

  4. For the selected surfactant, the oil recovery factor for Niobrara cores ranged from 13% to 80% OOIP, depending on salinity and conditions of the cores. For cores from the Middle Member of the Bakken, the average oil recovery for spontaneous imbibition of selected surfactants reached 57% OOIP at optimal salinity.

In summary, for a tight chalk/marl/shale reservoir like the Niobrara, the surfactants tested exhibited a favorable potential to promote flow rate behavior, as was shown previously for the Bakken.

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