The combined effect of permeability and pore structure on carbonate matrix acidizing
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Determination of optimal acidizing conditions through laboratory experimental study is crucial for designing matrix acid stimulation jobs in the field. Optimal interstitial velocity, vi-opt(the velocity of injected fluid that yields the minimum volume of acid needed to propagate wormholes) can be determined though curve fitting of experimental data. This optimal interstitial velocity coincides with the minimum volume of acid required for wormhole breakthrough, and therefore, the most efficient stimulation design. Optimal interstitial velocity determines the injection rate for a treatment, and the optimal pore volume to breakthroughbreakthrough of pore volume, PVbt-opt,suggests the total volume of acid needed for a treatment. Studies of carbonate matrix acidizing have focused on the role of many parameters, such as acid concentration, acid type, temperature, and core dimensions. However, under the same experimental conditions, different limestone rocks exhibit different optimal conditions. It is important to explore how changes in rock properties, such as permeability and pore structure, can also impact the efficacy of matrix acidizing techniques. In this work, a series of linear core-flood experiments on relative homogenous Indiana limestone and Desert Rose limestone cores were performed at ambient temperature. Cores of 1.5-in. diameter by 8-in. long were acidized using 15% hydrochloric acid. The average permeabilities of the cores tested were 6, 11, 33 and 239 mD with varied porosity. To help understand the effect of permeability on the wormholing process, thin sections were analyzed to characterize the pore structure. Based on experimental results, a detailed explanation of the pore structure and permeability effects on optimal interstitial velocity is presented. The core-flood acidizing results show that at low permeabilities, high permeability rocks exhibit higher optimal interstitial velocities. When permeability of the rock reaches a certain value, the effect of permeability on the optimal injection condition diminishes. The implications and applications of this work are far-reaching; better understanding of optimal acidizing conditions based on the studied rock properties has significant potential economic and operational impact.
