ABSTRACT An accurate estimation of the CO2 storage capacity in saline aquifers is critical for the successful implementation of geological CO2 sequestration projects. Although volumetric methods provide quick and accurate estimates for the storage capacity, the storage efficiency factor (SEF) relies on empirical correlations or computationally expensive numerical simulations. To overcome these limitations, this study proposed a novel regression-based prediction approach for directly predicting SEF in vertical and horizontal well scenarios using simulation-derived data. Four regression models were developed: multi-linear regression (MLR), exponential (Exp), and response surface methodology models (RSM-1 and RSM-2). Among them, the RSM-2 models, which incorporate linear, interaction, and quadratic terms, demonstrated the highest accuracy, achieving mean absolute percentage errors of 1.1% and 3.4% for vertical and horizontal wells, respectively. Compared to prior empirical approaches, the RSM-2 model provided more precise and consistent predictions, with a narrower SEF distribution range and reduced uncertainty, enhancing its suitability for site screening and storage planning. The models were applied to two reservoirs in the Pohang Basin, Southeast Korea, and yielded storage capacity estimates within 5% of full-physics simulation results, confirming their validity and reliability. These models provide a cost-effective and time-efficient alternative to conventional simulations and are particularly advantageous in early-stage feasibility studies and large-scale CO2 storage assessments. Future work will expand these models to accommodate more complex reservoir conditions and enhance their generalizability across diverse geological settings.

ABSTRACT The pressure fluctuations in the wellbore caused by detonation during perforation operations in ultra-deep wells pose a serious threat to the safety of tubing strings and downhole tools. To effectively control the risk of accidents, a numerical model of annular pressure evolution during perforation detonation was established based on the fluid-solid coupling theory. Considering the influence of wellbore parameters such as the inner and outer diameters of the tubing string and its length, the study on the influence law of wellbore peak pressure during perforation detonation was conducted. The results show that the outer diameter of the tubing, the outer diameter of the perforation gun, and the length of the perforation interval are positively correlated with the peak detonation pressure, while the inner diameter of the casing, the length of the tubing, and the length of the rat hole are negatively correlated with the peak detonation pressure. Among these parameters, the outer diameter of the perforation gun has the most significant impact on the peak detonation pressure. The research results provide an effective method for accurately predicting the wellbore peak pressure.

ABSTRACT Deep underground rock formations are widely used for geological carbon dioxide (CO2) storage due to their large-scale, long-term capacity. However, geophysical and petrophysical complexities can lead to challenges such as gas migration and potential leaks, posing risks to groundwater and subsurface systems. Recent advancements increasingly integrate Artificial Intelligence (AI) and Machine Learning (ML) to mitigate these risks and enhance CO2 storage efficiency. This review explores ML applications in geological CO2 storage, highlighting recent advancements and their implications. ML has demonstrated effectiveness in enhancing CO2 storage efficiency. However, the complexities of geological storage necessitate further improvements in ML model applicability, particularly in real-world projects. Since ML models depend on the availability of data, ensuring high data integrity and quality is crucial. Moreover, CO2 storage projects involve significant risks and uncertainties, making advanced probabilistic ML models essential for quantifying uncertainties and mitigating associated risks. Lastly, integrating real-time monitoring systems with sensor data and ML algorithms can enhance anomaly detection, provide early warnings, and enable timely interventions. Addressing these challenges will strengthen the adoption of advanced ML techniques in geological CO2 storage, improving efficiency, safety, and reliability.

ABSTRACT Pipe sticking is one of the most common problems that arise during drilling activities. Despite attempts to mitigate this issue, the petroleum industry still suffers significant financial losses. During drilling wells in the southern Iraqi oilfields, cutting removal and wellbore instability are the biggest challenges that may commonly cause mechanical pipe sticking issues, which were investigated in this study. Two distinct models were established for the area of interest to avoid or minimize mechanical pipe sticking. Firstly, hole-cleaning model was created, minimal flow rate and cutting bed height desired for efficient borehole cutting transportation were computed. Secondly, employing a one-dimensional geomechanical model to specify the optimal mud weight and wellbore trajectory. The outcomes revealed that the major cause of the pipe sticking in Zubair formation was inadequate hole cleaning, i.e., the value of the minimum flow rate was greater than the actual flow rate. Additionally, improper mud weight (10.82 ppg) was utilized at an inclination of 34.12°. The research results recommended a mud weight of 12–12.8 ppg for an inclination less than 40° and an azimuth equal to 140° for stable drilling conditions. This study’s findings can improve drilling efficiency by suggesting cost-effective methods for planning adjacent directional wellbores.

ABSTRACT The anti-liquid tubing technology is one type of new gas production technique being applied. This paper develops a two-dimensional flow field model based on the dimensions of anti-liquid tubing and traditional tubing. Using computational fluid dynamics (CFD) simulations, the liquid discharge processes for both types of tubing are analysed. Research shows that the anti-liquid tubing string offers several benefits: it reduces pressure drop by isolating the liquid columns during well shut-in, shortens recovery time through gas lift during well reactivation, and enhances the tubing’s liquid-handling capacity during stable production. However, the installation of an anti-liquid valve reduces the flow space and increases the flow path within the tubing. As a result, gas wells take longer to fill when the valve is first used and experience higher frictional pressure drops during stable production. Based on both simulation results and experimental data, it is clear that this technology is suitable for stable or intermittent production in gas wells with liquid phase flow rates below 0.1 kg/s.

ABSTRACT As oil and gas exploration gradually moves towards deep and ultra deep layers, deep granite buried hill reservoirs have shown enormous exploration potential. Due to the difference in pore space between granite buried hill reservoirs and conventional sandstone reservoirs, the damage mechanism of granite buried hill reservoir is complex, which seriously affects the exploration and development of such reservoirs. This paper takes X-1 well in the Qiongdongnan Basin as an example to collect rock core samples from its granite buried hill layer. The reservoir sensitivity experiments, drilling fluid performance evaluation experiments, and drilling fluid dynamic and static pollution experiments are carried out to explore the types of reservoir damage and clarify the mechanism of reservoir damage. The results show that sensitivity damage is mainly caused by stress sensitivity damage, which can result in a loss of permeability of over 80%. The application effect of solid free drilling fluid is the best in the metamorphic rock buried hill reservoir, with a dynamic experimental permeability loss rate of less than 30%. The damage of drilling fluid to the reservoir is not only the formation of filter cake by solid particles on the fracture surface, but also the retention of solid particles in the fracture. The research results have certain guiding significance for the optimization of reservoir protection technology in the exploration and development of granite buried hill reservoirs.

ABSTRACT The relationship between the holdup of the dispersed phase and the velocity at which drops move in the continuous phase, as well as the concept of characteristic velocity, has been valuable in the design and operation of liquid–liquid extraction (LLE) columns. At the flooding point, the maximum-phase throughput (Qd and Qc) and phase velocities can be reached. This point is known as the hydrodynamic loading limit, beyond which the phase flow rate becomes disorganized. Several significant research papers in the literature utilize pulsed or agitated extraction columns to investigate characteristic/slip velocities and flooding conditions. Numerous studies have examined various correlations, but there are inconsistencies in designing LLE columns accurately. The aim of this study is to examine existing literature comprehensively regarding hydrodynamic velocities and flooding characteristics in commercial pulsed and agitated extraction columns. Established correlations for predicting slip velocities, characteristic velocities, and flooding conditions are carefully evaluated. Furthermore, past studies on how key parameters affect hydrodynamic velocities and flooding characteristics are summarized in this manuscript.

ABSTRACT The efficiency of numerical simulation is of great importance for the development of tight oil reservoirs. In this paper, a nonlinear seepage model of tight oil reservoirs was presented, and the physical model of multi-fractured horizontal well is meshed by unstructured triangular grids, and the mathematical model is discretized by finite element method. Then, a fully implicit iterative solution scheme was derived. A novel algebraic multigrid method (AMG-CG) was proposed to solve the large sparse linear equation system during the iteration process. In order to demonstrate the computational efficiency of the novel algebraic multigrid method, a comparison was made between the novel method and others. The results reveal that the AMG-CG method has a faster calculation speed in solving linear equation systems. The calculation time of the AMG-CG method is 1029 s, the calculation time of the AMG method is 2029 s, the calculation time of the generalized minimum residual method (GMRES) method is 3709 s, the calculation time of the Gauss–Seidel (GS) method is 8115 s. The calculation speed of the novel method is almost 7.88 times faster than the Gauss–Seidel method, the novel method is more effective in the numerical simulation of tight oil.

ABSTRACT This study investigates the colorimetric measurement of pH 1–3 using both RGB (red, green, and blue) and CMY (cyan, magenta, and yellow) color systems to address reliable pH measurement methods in aqueous solutions. The experiment utilized pH indicator papers to measure solutions with pH values ranging from 1 to 3. Six hundred measurements were taken for each pH level, and the resulting color changes were analyzed using a free image analysis software provided by the Korean Agency for Technology and Standards. Key findings from the RGB analysis show that for pH 1, R values predominantly fell between 172 and 227, while at pH 3, R values significantly decreased to a range of 71 to 144. In the CMY analysis, cyan values were consistently zero across all pH levels, while magenta and yellow values exhibited varying distributions depending on the pH. The study revealed that the R value in the RGB system is particularly useful for distinguishing between pH 2 and 3, while the M value in the CMY system proved effective in differentiating between pH 1 and 2. These results demonstrate the potential of colorimetric analysis using both RGB and CMY systems as a simple, cost-effective, and reliable method for pH measurement.

ABSTRACT Early water breakthrough due to pressure depletion is often attributed to a strong water drive and must be established to avoid wrong reserves estimation and planning. In this study, we evaluated the Safsaf C reservoir, an undersaturated oil reservoir in North Africa that experienced early water breakthrough at 62 days after the commencement of oil production. To establish or disprove the presumption of it producing under a strong water drive, we critically analyzed the reservoir using the Material Balance Equation (MBE) coupled with the pot aquifer water influx model. The Campbell plot showed a weak aquifer, and the Solution plot confirmed aquifer presence. The pot aquifer plot gave the original aquifer in place an estimate of about 28.42 mmRB, which is about half the size of the estimated hydrocarbon pore volume of 51.15 mmRB. Water influx plots produced improved Oil initially in place (OIIP) estimates in the range of 11.50 to 12.20 mmSTB. The aquifer influx profiles show that water encroaches into the reservoir at a rate of 742 rbl/d. This approach is suitable when little is known about the aquifer and provides reliable estimates of OIIP and the degree of aquifer support for reservoirs bounded by small aquifers.