Experimental study on the micro alkali sensitivity damage mechanism in low-permeability reservoirs using QEMSCAN

Low permeability reservoirs are extremely sensitive to injected fluids because of small pore throat sizes and complex connections. Alkali sensitivity damage is the primary damage mechanism in low permeability reservoirs and strongly influences rock permeability. Therefore, it is significant to study alkali sensitivity damage mechanism to prevent capacity attenuation. In this paper, we propose a method combines macroscopic core flow experiments and microscopic mineral analysis to study alkali sensitivity damage mechanism. Core flow experiments are designed to evaluate the alkali sensitivity damage degree. QEMSCAN technique is introduced to quantitatively compare changes of porosity, mineral contents and distribution forms at inlet and outlet sides before and after alkali sensitivity damage. The results indicate that pore and throat plugging is the root cause of core alkali sensitivity damage. Big pores are plugged and segmented into significantly narrow flowing channels. The inlet side is plugged mainly by precipitates generated by the reaction of hydroxide and matrix minerals including dolomite, muscovite, and K-feldspar as well as dispersion of clay minerals in alkali environments including illite and chlorite. Meanwhile, the outlet side is mainly plugged by the dispersion and migration of clay minerals. Micro minerals are selected as the research object to study the alkali sensitivity damage at both inlet and outlet sides in low permeability reservoirs. This provides references for alkali sensitivity analysis in reservoirs with similar mineral compositions. Moreover, alkali sensitivity damage mechanisms are clarified at both inlet and outlet sides in the core flow experiment, which can be correlated to actual reservoirs near and far away from wells. Then specific measures corresponding to different types of reservoirs in terms of distances to wells can be taken to restore the damage and thus achieve the recovery of reservoir capacity. The method proposed in this paper is also applicable to other reservoir sensitivity damage, such as salt and acid sensitivity damage. (C) 2016 Elsevier B.V. All rights reserved.