Interactions of DTPA chelating agent with sandstone rocks during EOR: Rock surface charge study

Enhanced oil recovery (EOR) using low salinity water has been proven experimentally and at the field scale. One of the mechanisms behind recovery enhancement due to low salinity water injection has been related to the change of rock surface charge which might lead to wettability alteration. Low salinity water EOR has its own disadvantages such as the high cost due to the need for enormous amounts of fresh water and it may cause scale precipitation and fines migration. Therefore, efficient and alternative low-cost EOR methods are needed. In this work, chelating agent diethylenetriaminepentaacetic acid (DTPA) for EOR application in sandstone rock was investigated. DTPA is an efficient metal ion control agent and can seize certain cations (especially Fe3+) from rock surfaces. DTPA chelating agent is very stable up to 400 °F at high pH values. The effect of DTPA injection on the surface charge of the sandstone rocks was investigated. Zeta potential measurements, ions analysis after interaction with the rock, and coreflooding experiments were performed. Three different types of sandstone rocks were used to investigate the changes in rock surface after conditioning with DTPA. Several scenarios were tested to compare low salinity water, deionized water, and seawater with DTPA chelating agent. The comparison was made by measuring the zeta potential of the rock powder. Coreflooding experiment was conducted along with effluent analysis for SO42−, Ca2+, Mg2+, Fe3+, and Al3+ ions to observe the effect of injecting DTPA with seawater on oil recovery and permeability. The results showed that the addition of DTPA resulted in Fe+3 ions absorption from the rock surface. Absorbing such trivalent cation resulted in a significant change in surface charge. Zeta potential dropped towards a more negative value for all sandstone rocks samples used in this study. Zeta potential results showed that mixing Berea sandstone with 5 wt% of DTPA in seawater led to similar surface charge value as if it was mixed with low salinity water for the same period. This means that DTPA combats the effect of the concentrated salts in seawater. DTPA chelated all multivalent cations in seawater and eliminated their effects on the surface charge. Also, DTPA increased the pH of the seawater from 7 to 11 and this will drag the surface charge to more negative. From coreflooding experiment, we observed that injecting 5 wt% DTPA solution in seawater after seawater injection enhanced the oil recovery from 56% to 75% in Berea sandstone cores at 100 °C. The additional oil recovery can be explained by more negative values of the zeta potential of the rock surface. We expect that chelating agent EOR using DTPA is a viable alternative to low salinity water flooding where huge fresh water is required to dilute the seawater.