Experimental optimization of acid system for acidizing in mud-damaged deep fractured carbonate formation

Abstract

On account of mud loss occurred during drilling in Dengying formation in Gaomo district of Sichuan Basin, factors affecting mud loss and optimization of acid system for acidizing in Mud-damaged deep fractured carbonated formation were studied. Carbonate core plugs with a diameter of 25.4 mm and a length of 50 mm were used as experimental samples to simulate natural fractures by artificially splitting, and experiments of mud loss in fractures varying from 10 μm to 250 μm were carried out. Results showed that fractures wider than 150 μm are main channels for mud loss. As fractures widen, the volume of mud loss increases while mud damages intensify first and then relieve; when the width is 150 μm, damages reach the top rate of 95.8%. When the fracture surface become rougher, the volume of mud loss decreases; in this case, the surface roughness has less impact on mud loss with widening of fractures. Then acidizing experiments of gelled acid, divert acid, and compound acid were carried out respectively in order to optimize acidizing system. Results are as follows: when fractures are narrower than 50 μm, acidizing has poor effect since all acid can hardly break through the mud damage zone to enter fractures; fractures at the width of 50 μm–200 μm enjoy better acidizing effect, and the fracture permeability recovery rate reaches 100%~400%. Since acidizing by gelled acid caused the most obvious wormholes that were thick and deep, gelled acid shows better acidizing effect than divert acid and even compound acid. As the width of fractures wider than 200 μm increased, wormholes parallel to the direction of oil and gas seepage changed into uniform etching that was wide and shallow, thus acidizing has poorer effect. Hence, selection of acid system for acidizing of mud-damaged deep fractured carbonate formation can take reference from this study.