Digital Rock Extension of Laboratory Core Test Results for Acid Treatment Optimization

Abstract

Nowadays acidizing became one of the most common approaches used to increase the hydrocarbons production from carbonate reservoirs. An acid solution injected under pressures below the formation fracture pressures dissolves the rock matrix and, thus, facilitates the fluid flow. However, the overall treatment efficiency is crucially dependent on the acid composition and injection scenario, since the different dissolution patterns are created depending on the effective reaction rate (i.e. acid composition and matrix mineralogy) of the reactive fluid and the fluid injection rate. At slow injection rates, when the acid is spent before penetrating deep into the rock, the face dissolution scenario is observed. On the other hand, fast injection results in uniform distribution of the acid along the treatment zone and similar uniform dissolution of the matrix. The best result from production improvement point of view is achieved when the acid creates a set of thin channels - the so-called wormholes. This optimum regime corresponds to the minimum in the pore volume to breakthrough (PVBT) dependence on injection rate (Fredd, 1998; Zhang, 2021). Where PVBT is defined as the amount of treatment fluid (measured in core pore volumes) required to be injected before the appearance of macroscopic flow channel linking the opposite faces of the core. Thus, since the optimal acid composition and the injection rate are determined by geology and lithology of the reservoir, to achieve the best effect, each treatment should be preceded by experiments on representative rock samples. In addition to that, the parameters to be optimized for a typical acidizing job also include the sequence of injected fluids and the amount of the fluid to be injected (Yudin A., 2021), which requires an extensive laboratory study. Unfortunately, the amount of the core material available is usually not sufficient for such a comprehensive laboratory analysis. Moreover, the destructive nature of acidizing experiments imposes the fundamental limitation: experiments are performed on different core samples, which makes the results less conclusive.