Acid-Fracturing Effectiveness in High-Temperature and High-Stress Volcanic Reservoirs: Experimental Insights from Formation Samples

Abstract

Abstract

Abstract Acid-fracturing effectiveness in volcanic formations is rarely documented as a well stimulation technique. This study aims to evaluate acid-fracturing effectiveness in volcanic formations through experiments using formation samples from a high-temperature and high-stress volcanic reservoir. It investigates the influence of carbonate mineral types and their spatial distributions in acid-etched fracture conductivity under high temperature and stress of the field (175°C, 10,000 psi). Furthermore, the results are benchmarked against baseline and propped fracture conductivities. In total, five rock samples (slab pairs) were prepared from whole cores of three wells in a volcanic reservoir. The rock samples (slab pairs) were investigated using medical computed tomography, X-ray fluorescence, rock embedment strength, and surface profilometry. Acid-injection tests used either 10% formic acid or 15% HCl to generate acid-etched fracture surfaces under leakoff and no leakoff conditions. The rock characterization was repeated for post-acid-injection slabs to evaluate acid-rock interactions. Acid-etched conductivity tests were performed using a modified API cell at high temperature and a series of closure stresses (i.e., 175°C and 6,500/9,000/12,000 psi). Prepared samples are categorized into two sets: one sample with a carbonate vein (mainly ankerite) along the slabs and four samples with carbonate minerals (mainly siderite) in patches. The first subset showed almost complete dissolution of the carbonate vein with 10% formic acid creating a void channel along the sample. The subsequent acid-etched conductivity indicated relatively higher value (255 mD-ft) compared to propped fracture conductivities at 6,500 psi (8.2 and 1,308 mD-ft with 0.2 and 2.0 lbm/ft2 of ultra-strength 35-mesh proppant), though the acid-etched conductivity steeply declined to a value equivalent to that of the propped fracture with 0.2 lbm/ft2 at 9,000 psi. On the other hand, the second subset dissolved the carbonates in patches more uniformly without significant leaching regardless of acid type and leakoff conditions. The two samples (with main productive lithology) showed resultant acid-etched conductivity even at the high closure stress (e.g., 0.79-1.40 mD-ft at 9,000 psi) suggesting the etched fracture remained open with conductivities equivalent to the propped one with 0.02 lbm/ft2. The acid-etched conductivity was 3-5 times higher than the (pre-acid) baseline one at the low stress range (1,000-3,000 psi); the acid-etching enhanced the fracture conductivity though its ultimate effectiveness in productivity at higher stress is not conclusive. The work provides insights and precautions on acid-fracturing treatments in volcanic reservoirs that rock texture matters; carbonate minerals are not equally amenable to acid fracturing. The presence of ankerite dominated vein networks near the wellbore can potentially deliver high acid-etched conductivity, whereas siderite dominated carbonates in patches have less chance. Furthermore, the study provides evidence on the distinction of the design parameters in stimulation of volcanic formations, which have been rarely investigated.