## 3 Results and Discussion
### In-Situ Acidizing Micro-Model and Acid-Rock Interaction  
Several workers have studied the reservoir characterization to understand the surface alteration of heterogeneity of carbonate rock through different kinds of parameter, e.g., porosity and permeability. To study these alterations, workers have been utilizing different kinds of application, including well log data, such as gamma ray, sonic, neutron and bulk density data; litho-log with a set of subsurface logs; numerical measurement of porosity and permeability from large-scale core samples as widespread conventional ways of understanding the complex subsurface settings of carbonate rocks. However, to draw a better model for subsurface rocks, a microscale study of these rocks is required through high-resolution tools and the observation of fluid-rock interactions. In general, fluid-rock interactions have been studied intensively in a wide range of experiments and considered essential in the following: subsurface applications, oil migration associated with hot fluids, enhanced oil recovery and enhanced porosity-permeability properties in shallow and subsurface conditions ([34; 35; 36]). Therefore, this study focuses on acid-rock interactions utilizing the new in-situ micro-model setup. The in-situ micro-model design gives us a high-resolution tool and shows a microscale alteration on the surface of reservoir core samples obtained from subsurface carbonate rocks. A moderate-to-high acid-rock interaction within a considerable size of dolomite and calcite crystals was observed under optical microscope. The petrographic fabric of dolomite is mostly euhedral, in places with euhedral crystals, fine-to-coarse crystalline-sized in a tight carbonate rock, while limestone is smaller in size, with an anhedral crystal shape, less compacted than dolomite fabric, having more pore spaces than in the dolomite lithology. To understand the phenomena of enhancing the porous media by matrix acidizing, combined acids of different formulations (12% HCl: 3.2-12% Oxalic acid) at different temperatures (starting from ambient temperature until 78.8 \({}^{\circ}\)C) were injected into micrometer-sized reservoir carbonate core samples under high-resolution micro-design, where thin sections were observed and recorded continuously under optical microscope to measure the rate of alteration of lithology of carbonate rocks. When the acid was injected into the thin section, the interaction rate seemed to change differently according to the acidizing operating conditions (acid concentration and acidizing temperature). During the micro-injection, the most significant alteration and interactions observed were dissolution, precipitation and replacement processes. These processes during the acid injection are strongly effective leading to intensive increase in solubility and dissolution of carbonate crystals, thus enhancing the porous media of the samples from the Qamchuqa Formation reservoir. Additionally, the precipitation/re-precipitation and accumulation of new crystals in places were observed and probably influence the porous media enhancement and occlusion of pore spaces. The dissolution and replacement processes of carbonate reservoir rocks during acid interaction could be varied according to the rate of elemental/mineral exchange of carbonate composition and other secondary compositional associations. Therefore, the fluid-rock interaction rate was more intensive with calcite compared to dolomite crystals and shows a remarkable dissolution at various ranges of microscale under optical observation. ImageJ software allowed us to observe the high resolution of the lithological alteration and improved porous media (Figure 6a,b). Nonetheless, the diversity in dissolution is also related to impurities of calcite and dolomite crystals within their lattice [37]. In addition, the elemental and mineralogical exchange is also observed under the micro-model (Figures 6 and 7).