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In northeastern Sicily (Italy), sandstone rock masses widely crop out as cover deposits over crystalline terrains belonging to the orogenic belt. Despite being part of the same geological formation, these sandstones are characterized by highly different features in terms of texture and physico-mechanical properties. This poses a scientific question on the possibility of tracing these rocks to a single statistical model, which could be representative of their main engineering geological properties. Therefore, it is worth investigating on the possible reasons of such differences, that should be searched either in the current geographical sandstone distribution or in the rock texture. For this study, sandstone samples were collected from different sites and were analyzed at both the hand and thin section scales. Three sandstone types were recognized, characterized by a different texture. Then, the laboratory characterization allowed estimating their main physico-mechanical and ultrasonic properties, such as porosity, density, mechanical strength, deformability, and ultrasonic velocities. The rock mechanical strength proved linked to the rock compactness and to the presence of lithic fragments, while pores and a pseudo-matrix between grains represent weakening features. Rock data were also statistically analyzed by grouping the specimens according to a geographical criterion, with respect to their sampling area, but no link was found between location and rock properties. Finally, with the aim of achieving mathematical laws that could be used to predict some rock properties from others, useful for practical purposes when dealing with such a high property variability, single and multiple regression analyses were carried out. Results show that the Uniaxial Compressive Strength, porosity, and P-wave velocity are the best predictors for a quick, indirect estimation of the main physico-mechanical parameters. The methodological approach developed for this research can be taken as reference to study other worldwide cases, involving rocks characterized by a wide range of physico-mechanical properties and covering large regional territories.
In northeastern Sicily (Italy), sandstone rock masses widely crop out as cover deposits over crystalline terrains belonging to the orogenic belt. Despite being part of the same geological formation, these sandstones are characterized by highly different features in terms of texture and physico-mechanical properties. This poses a scientific question on the possibility of tracing these rocks to a single statistical model, which could be representative of their main engineering geological properties. Therefore, it is worth investigating on the possible reasons of such differences, that should be searched either in the current geographical sandstone distribution or in the rock texture. For this study, sandstone samples were collected from different sites and were analyzed at both the hand and thin section scales. Three sandstone types were recognized, characterized by a different texture. Then, the laboratory characterization allowed estimating their main physico-mechanical and ultrasonic properties, such as porosity, density, mechanical strength, deformability, and ultrasonic velocities. The rock mechanical strength proved linked to the rock compactness and to the presence of lithic fragments, while pores and a pseudo-matrix between grains represent weakening features. Rock data were also statistically analyzed by grouping the specimens according to a geographical criterion, with respect to their sampling area, but no link was found between location and rock properties. Finally, with the aim of achieving mathematical laws that could be used to predict some rock properties from others, useful for practical purposes when dealing with such a high property variability, single and multiple regression analyses were carried out. Results show that the Uniaxial Compressive Strength, porosity, and P-wave velocity are the best predictors for a quick, indirect estimation of the main physico-mechanical parameters. The methodological approach developed for this research can be taken as reference to study other worldwide cases, involving rocks characterized by a wide range of physico-mechanical properties and covering large regional territories.
Three-dimensional reservoir simulation models were developed to perform different simulation runs which aim to get an estimate for the optimal well productivity in such unconventional very low permeability reservoirs. It will take into consideration the limitations, uncertainties and capabilities of each parameter like matrix permeability, hydraulic fracturing spacing, half-length, height and conductivity to get an enhanced estimate of the optimal production ultimate recovery by using the uncertainty analysis. Hydraulic fracturing parameters with reservoir data were used to estimate the optimum well spacing distance between the horizontal wells in tight oil reservoirs. The estimated recovery factor and production forecasting using the reservoir simulation with its capabilities to get computational cases with different combinations will get the optimal well count with optimum economic evaluation. Different reservoir modeling scenarios were built with various arrangements of affecting factors. Characterizing hydraulic fracture parameters, such as fracture spacing, fracture height, fracture half-length and fracture conductivity, are studied by setting different ranges per parameter and results were investigated by comparing the oil production rate trends and total cumulative oil production after a ten-year lifetime. Sensitivity analysis was performed on the different hydraulic fracturing parameters by setting different ranges and setting the objective function to be the cumulative oil production. A complete statistical analysis for the results from the simulation runs using response surface analysis including Morris’s method and Sobol method which is a variance-based analysis was carried out. This study sensitivity analyses shed light on the long-term production behavior of tight oil reservoirs. The findings indicate that the most important parameters that influence the performance of such tight reservoirs are those related to fracture half-length and fracture height, fracture conductivity and matrix permeability. A general production trend analysis and comparisons are run for various well spacings with different numbers of wells per 100 acres (40.4687 ha). Because of their quick rates of decline, unconventional reservoirs typically favor high initial rates. Furthermore, drilling and fracturing procedures are expensive. As a result, it is crucial to quantify these fracture parameters using different data to optimize the fracture design for both single and many wells using economic analysis. The impact of the oil price and the other operational costs required for various instances will be considered in an economic evaluation based on a new well spacing optimization process. The net present value was calculated for several cases by changing the number of wells (2, 3, 4, 5, 6, 7 and 8 wells), and showed that the optimum well spacing is 300 feet (91.44 m) per 100 acres (40.4687 ha) for a tight oil reservoir.
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