When considering the wettability state during steam applications, we find that most issues remain unanswered. Removal of polar groups from the rock surface with increasing temperature improves water‐wettability; however, other factors, including phase change, play a reverse role. In other words, hot water or steam shows different wettability characteristics, eventually affecting the recovery. Alternatively, wettability can be altered using steam additives. The mechanism of this phenomenon is not yet clear. The objective of this work was to quantitatively evaluate the steam‐induced wettability alteration in different rock systems and analyze the mechanism of wettability change caused by the phase change of water and by chemical additives.
Heavy oil from a field in Alberta (27,780 cp at 25°C) was used in contact‐angle measurements conducted on quartz, mica, calcite plates, and rock pieces obtained from a bitumen‐containing carbonate reservoir (Grosmont). All measurements were conducted at a temperature ranging up to 200°C using a high‐temperature/high‐pressure interfacial tension (IFT) device. To obtain a comprehensive understanding of this process, different factors, including the phase of water, pressure, rock type, and contact sequence, were considered and studied separately.
To study the effect of pressure on wettability, we started by maintaining the water in liquid phase and measuring the contact angles between the oil and water at different pressures. Next, the contact angle was measured in pure steam by keeping the pressure lower than saturation pressure. The influence of contact sequence was investigated by reversing the sequence of generating steam and introducing oil during measurement; these measurements were repeated on different substrates. Different temperature‐resistant chemical additives (alkalis, surfactants, ionic liquid) were added to the steam during contact‐angle measurement to test the wettability alteration characteristics at different temperatures and pressure conditions (steam or hot‐water phase). In addition to these wettability‐state observations, surface‐tension experiments were conducted to evaluate the performance of additives in reducing surface tension for the oil/steam system. The results showed that the wettability of the tested substrates is not sensitive to pressure as long as the phase has not been changed. The system, however, was observed to be more oil‐wet in steam than in water at the same temperature in the calcite test. The wettability state could be altered by using chemical additives in certain ranges of concentration; moreover, the optimal chemical‐additive concentration was also observed from both contact‐angle and surface‐tension measurements.
Analysis of the degree of wettability alteration induced by steam (or hot water) and temperature was helpful to further understand the interfacial properties of the steam/bitumen/rock system, and proved useful in the recovery‐performance estimation of the steam‐injection process in carbonate and sand reservoirs, specifically in chemically enhanced heavy‐oil recovery.