Experiments and modeling of wettability alteration in low permeability porous media
Naturally fractured reservoirs contain a significant amount of global hydrocarbon reserves. In fractured reservoirs, the efficiency of water flood is governed by spontaneous imbibition of water into oil-containing matrix blocks. When the matrix is oil-wet or mixed-wet, little oil can be recovered by imbibition. Wettability alteration provides a possible solution to enhance oil recovery in oil/mixed-wet fractured formations. Different chemicals such as surfactants, enzymes, selective ions can be used to alter wettability from oil-wet towards more water-wet which can substantially increase the oil recovery. Understanding recovery mechanisms for these processes at different inverse bond numbers (ratio of capillary to buoyancy forces) and developing scaling rules are critical for estimating feasibility at field scale.
Surfactants were identified which altered the wettability of a low permeability (0.03 – 0.23 mD) mixed-wet/oil-wet sandstone reservoir. Static imbibition experiments in the surfactant solution resulted in high oil recovery (42-68% OOIP) compared to 15% OOIP in formation brine. High (>240) inverse bond numbers for these experiments indicate recovery mechanism as counter-current imbibition driven by capillary forces. Numerically simulated saturation and velocity profiles on validated datasets were analyzed to study the recovery mechanisms. Velocity profiles indicate counter current flows with velocity vectors pointing outwards. Similar visual observations were made during experiments, which were captured through images. The saturation front moves radially inward with symmetric profiles at the top and bottom. An analysis of scaling laws for the capillary driven flow suggests that imbibition recovery curves do not correlate with traditional scaling groups (Mattax and Kyte, 1962; Ma et al. 1997). The scaling equations analyzed are for strongly water-wet porous media and are insufficient to explain the dynamics of changing wettability from oil-wet to water-wet. The recovery data shows that oil recovery varies linearly with square root of time. It was observed that the rate of recovery was higher for the higher IFT cases in experiments performed on cores with almost same initial oil saturation using the same surfactant, but at different salinities. As a result of varying the salinity, interfacial tension between oil/water is varied. To evaluate the application of wettability altering processes at larger scales experiments were performed on outcrop cores of different dimensions and at dynamic conditions. Surfactant formulation was developed which altered the wettability from oil-wet to water-wet on outcrop rocks Estaillades Limestone and Texas Cream Limestone. Using the surfactant formulation static and dynamic imbibition experiments were performed on cores with different dimensions and boundary conditions. It is observed that dynamic imbibition process recovers oil faster than static imbibition. Imbibition experiments performed on cores with varying height and diameter show that oil recovery decreases with increasing diameter and height. Study of numerically simulated velocity and saturation profile on validated input datasets established the recovery mechanism as gravity dominated flow. Analytical scaling groups for gravity dominated flow were tested considering pressure drop only in water phase, pressure drop only in oil phase, and pressure drop across both water and oil phases. The model with pressure drop in both phases captures the decrease in recovery with increase in diameter and height of the core. Sensitivity to change in oil recovery with change in height is fairly accurate whereas the model over-predicts oil recovery with change in diameter. A new space-time scaling function (t/DH) is proposed for surfactant aided gravity dominated processes. Data with same boundary conditions, rock, fluids and varying dimensions can be correlated with the scaling function at early times with no fitting parameters involved. A good correlation is obtained with the data from different studies indicating the effectiveness of the scaling function. The scaling is applicable to both static as well as dynamic imbibition cases. Corefloods were performed on cores from different reservoirs to study the effect of wettability altering surfactant flood in a viscous pressure gradient driven process (as opposed to capillary or buoyancy driven imbibition process). Incremental oil recoveries over waterflood were analyzed for different injection schemes. Incremental recoveries over waterflood of 16% and 11% were obtained for secondary surfactant flood and slug process (surfactant slug injection after short initial waterflood) respectively for carbonate reservoir 1. Similarly, incremental recoveries over waterflood of 11% and 7% were obtained for secondary surfactant flood and slug process respectively for carbonate reservoir 2. The incremental oil recovery due to surfactant injection is attributed to the favorable increase in the relative permeability values of oil as the wettability is changed from oil-wet to water-wet. Experiments indicate that surfactant performance at the reservoir conditions (temperature, salinity, heterogeneity) is a key variable in these processes. Despite the differences in these conditions, for both the reservoirs oil recovery is more in the secondary surfactant injection mode compared to the slug process.