The Use of Formic Acid and Organic Mud Acid for Stimulation of Volcanic Rocks in Minami-Nagaoka Gas Field, Japan

Unlike acid stimulation in sandstone and carbonate formations, acid stimulation of volcanic formations is not well documented in the literature, and its effectiveness and applicability are not well understood. This study aims to evaluate acidizing of volcanic rocks (especially rhyolite) as a well stimulation technique through a comprehensive experimental and modeling investigation using formation samples from a volcanic reservoir, the Minami-Nagaoka gas field in Japan. The experimental study consists of rock characterization, solubility tests, coreflooding tests, and batch reactor tests. The rock samples are investigated with computed tomography (CT) for textural characteristics and with X-ray diffraction (XRD) analysis for lithological characteristics. With these results, candidate acid systems are selected, and their effectiveness in terms of the capability of dissolving volcanic rocks is evaluated through acid solubility tests. Acid coreflooding tests are performed using undamaged plug cores to evaluate permeability responses caused by acid/rock reactions under high-temperature and high-pressure conditions (300°F and 3,000 psi, respectively). Batch reactor tests are conducted to quantify damage due to secondary/tertiary reactions. The mineralogical and textural characteristics of the rock samples let us select formic acid as the preflush acid and a mixture of formic acid and hydrofluoric acid (HF) called organic mud acid (OMA) as the main treatment acid. The composition of OMA was a mixture of 9% formic acid and 1% HF or 10% formic acid and 0.5% HF in this work. Results of the coreflooding tests with the preflush acid indicated permeability enhancement in all the samples and, especially in cores with cemented fractures filled by carbonate minerals, substantial permeability enhancement was observed. On the other hand, cores treated with OMA after the preflush indicated further permeability enhancement in some cases without cemented fractures, whereas other cases showed permeability impairment after the OMA injection. Furthermore, results of the batch reactor tests with formic acid indicated low precipitation risks, whereas those with OMA suggested higher precipitation risks. Detailed analysis on the thin sections and residuals of the batch reactor tests with OMA highlighted the precipitation of unique fluorides, and the precipitation risk was modeled and quantitatively evaluated with geochemical simulations. Although there is more room to investigate the risks of the usage of OMA for the volcanic rocks, the results in this work suggest the use of formic acid as a main treatment acid, as in carbonate acidizing, for wells with abundant cemented fractures in near-wellbore regions. This paper provides insights on acid stimulation in volcanic rocks (especially rhyolite). The results provide a fundamental understanding on the acid/rock reactions and the potential benefits/risks for productivity enhancement of wells in the subject volcanic reservoir.