Evaluating the Effects of Acid Stimulation Treatment Before and After Fines Migration on Petrophysical Properties in Sandstone Reservoirs

Abstract Restoring formation damage by matrix acidizing is one of the common ways to increase productivity in sandstone formations. While sandstone formations are typically composed of the same types of minerals, the concentration and composition of these minerals, as well as the permeability and porosity of the matrix, can vary widely. Such variable petrophysical and compositional properties of sandstone formations can affect the severity of fines migration and the performance of acid stimulation treatments. To evaluate the effect of acid stimulation on petrophysical properties before and after fines migration, coreflood experiments were conducted on Bandera sandstone cores. Half of the treatments were conducted on cores that were damaged by triggering fines migration by injecting deionized water, and the other half were conducted on undamaged cores. These cores contain a wide range of clay content, mineral compositions, permeability, pore size distribution, and porosity. A series of characterizing tests and measurements were performed before and after acid treatments. The objectives of this work include: (a) analyzing X-ray diffraction (XRD) and X-ray fluorescence (XRF) measurements to determine the mineralogy and elemental composition of the rocks, (b) interpreting computational tomography (CT) scans to evaluate pore-scale heterogeneity and acid effect on cementing material, and (c) evaluating permeability by flooding the core samples with brine. More than 20 coreflood experiments were conducted at constant temperatures of 72, 150, and 250°F and constant acid concentration on 6 in. x 1.5 in. core samples at different injection rates. The CT- scans proved, quantified, and localized the fines migration in the damaged cores. The effect of fines migration on the acid stimulation treatment success in terms of increasing permeability and ultimately productivity was determined. Additionally, acid reaction increases porosity and permeability. However, treatment of damaged cores yielded different pore-size distribution and permeability compared to undamaged cores. Furthermore, combining CT-scan and inductively coupled plasma (ICP) results proved an extremely high sensitivity of fines migration toward HCl at temperatures 150 and 250°F resulted in porosity deterioration toward the outlet compared to the gained porosity at the inlet of the treated cores. The literature is rich with discussions about sandstone acidizing and acid systems. However, this comparative study provides a more comprehensive understanding of the effect of fines migration on the success of the stimulation treatment and its effect on petrophysical properties. The outcome of this work will lead to a reliable design of matrix acid treatments and, hence, increase the chances of successful acid stimulation treatment that optimizes well productivity.