Improvement in the Characterization of the Acid Wormholing by "In Situ" X-Ray CT Visualizations

Abstract

Introduction Acidizing of carbonate reservoirs is a widely used process to improve reservoir production. Acid dissolves portions of the rock, making channels; the result is an increase in permeability. Two different stimulation processes deal with acids: matrix acidizing and fracture acidizing While in matrix acidizing deep, highly ramified channels are desirable around the wellbore in fracture acidizing, compact patterns are preferable since they limit fluid loss through the fracture walls and thus increase acid efficiency. A large number of studies deals with acidizing patterns. Core observations after lab experiments consist in most cases in replica of the acid etched channels by filling the channels with resins or melting-point alloys and dissolving the surrounding rock material. Compact patterns, wormholes of limited size, highly branched wormholes, i.e. trees like patterns have been observed with this technique. Also, very pitted core faces have been mentioned. On another hand, modeling studies try to reproduce the pattern observed. Network models show tree like pattern. Capillary models put forward the dissolution mechanisms and the determinant effect of the acid filtration from the wormhole wall on the acid propagation. However, the way of formation of these facies and the different parameters influences on the pattern size are not clearly established. Mechanisms involved in carbonate dissolution by acids depend on the nature of the chemical reaction and on the flowrate. For limestone, the dissolution is mass-transfer limited. For this material, the acid flowrate has a determinant effect on the acid propagation rate defined as the ratio of the core length to the acid breakthrough time. At low flow rate the dissolution is compact and no wormhole forms; at high flowrate, the dissolution is mass-transfer limited: it is the wormholing regime; P. 9