Recovering medium to heavy oil from fractured carbonate reservoirs, which typically have a low matrix permeability, is challenging. Steam-injection is a promising EOR application for such reservoirs, as it initiates beneficial processes like thermal expansion and viscosity reduction of the oil, leading to an increased rate of gas-oil gravity drainage, resulting in accelerated recovery. In addition thermal solution-gas drive and distillation of lighter components can be activated upon heating.

We present experimental results and thermal simulation results of oil production from carbonate core-plugs during heating, which were designed to determine the contribution of the various thermal recovery mechanisms. Two carbonate core-plugs were cleaned, aged with formation brine and live-crude to restore original wettability, and were subsequently heated from 50 C to 235 C at a constant pressure of 31 bara, in a dedicated set-up.

The liquids and gas expelled from the core-plugs were continuously collected in a separate collection vessel, also at 31 bara.

The recovered oil volumes were substantial. We simulated the experimental results by thermal k-value simulation and obtained good agreement with experimental results. Progressive heating activates several recovery mechanisms: thermal expansion, thermal solution-gas drive and distillation. The temperature ranges where each of these recovery mechanisms are dominant are clearly distinguished in the experimental and simulation results.

The results provide quantitative estimates for the contribution of the various thermal recovery mechanisms in carbonate reservoirs subject to steam-injection.

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