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In drilling engineering, the rate of penetration (ROP) is a prevalent indicator to evaluate the energy efficiency of drilling operations. Nowadays, ROP prediction has become more critical since the production from deeper hydrocarbon resources is unprecedentedly escalating. So far, a wealth of theoretical and practical investigations has been conducted to develop ROP models; however, the existing models have not been adequately updated with the new technological advancements or geological restrictions. This research strives to integrate the latest advancements, restrictions, and future requirements in ROP prediction. To do this, the existing empirical and data-driven ROP models are elaborated and compared. From the conducted research, it was deduced that four uncontrollable factors, including the rock permeability, wellbore inclination, temperature, and rock hardness, have not been adequately considered in ROP models. Moreover, although data-driven ROP models deliver more accurate results than the empirical models, the determination of the number and type of the input parameters is still challenging. To tackle this issue, it is recommended to develop a formation-based classification system of input parameters for future investigations. This inclusive review can be adopted by the companies and engineers involved in drilling operations to update and reform their drilling strategies.

Permeability estimation from pressure-pulse decay method is complicated by two facts: (1) the decay curve often deviates from the single-exponential behavior in the early time period and (2) possible existence of gas adsorption. Both the two factors cause significant permeability error in most of pressure-pulse decay methods. In this paper, we first present a thorough analysis of pressure-pulse propagation process to reveal the mechanism behind the early time and later time behaviors of pressure decay curve. Inspired by the findings from these analyses, a new scaled pressure is proposed which can: (1) be easily used to distinguish the early time and later time data and (2) make the decay curves of all cases into a single 1:1 straight line for later time. A new data-proceeding method, which calculates the apparent porosity and permeability using the same set of measured data, is then developed. The new method could not only remove the effects of the adsorption on the permeability estimation, but also identify the apparent porosity as well as proper adsorption model and parameters. The proposed method is verified by comparing with true values and calculated values through numerical simulations that cover variations in typical rock properties (porosity, permeability, slippage, and adsorption) and the experiment configurations. It is found that the new method is accurate and reliable for all test cases, whereas the Brace’s and Cui’s approaches may cause permeability error in some cases. Finally, the new method has been successfully applied to real data measured in pressure-pulse decay experiments involving different types of rocks and gases.

Engineering geological investigation plays an important role in the preparation stage of engineering construction. In order to explore the hazards of geological problems, this paper selects a water supply project in South China as the research object. Based on the mobile GIS technology, the corrosiveness of environmental water and soil to the building is analyzed through geological investigation. According to the karst phenomenon found in the survey, water pressure and water injection tests are carried out to determine the permeability of the rock body. Combined with the statistical analysis of the existing research data, it is found that the permeability of fresh rock body is less than 5Lu, except for the permeability of sandstone and sand mudstone interlayer containing soluble rock which is more than 5Lu, whereas the permeability of strongly weathered and weakly weathered rock body is between 20~80Lu, and the permeability characteristics of the rock body have the structure of “shell-core binary”.

The effective stress coefficient for permeability is a significant index for characterizing the variation in permeability with effective stress. The realization of its accuracy is essential for studying the stress sensitivity of oil and gas reservoirs. The determination of the effective stress coefficient for permeability can be mainly evaluated using the cross-plotting or response surface method. Both methods preprocess experimental data and preset a specific function relation, resulting in deviation in the calculation results. To improve the calculation accuracy of the effective stress coefficient for permeability, a 3D surface fitting calculation method was proposed according to the linear effective stress law and continuity hypothesis. The statistical parameters of the aforementioned three methods were compared, and the results showed that the three-dimensional (3D) surface fitting method had the advantages of a high correlation coefficient, low root mean square error, and low residual error. The principal of using the 3D surface fitting method to calculate the effective stress coefficient of permeability was to evaluate the influence of two independent variables on a dependent variable by means of a 3D nonlinear regression. Therefore, the method could be applied to studying the relationship between other physical properties and effective stress.

The mechanical behavior and the influence of compaction banding on the hydraulic properties in soft porous rocks were studied. The tested rock was Calcarenite Tuffeau de Maastricht. In the frame of experimental investigations, triaxial and oedometric tests were conducted under dry and drained conditions. The results demonstrated that the rock is forming discrete compaction bands under high confining stresses and steep angle shear bands under low confining stresses. Permeability measurements during the oedometric and triaxial compression tests under drained conditions demonstrated that the axial permeability decreases with increasing axial strain. The maximum permeability decrease was three orders of magnitude for 40% of axial strain.

The permeability ( K ) of tight carbonate rocks is important to maximize the efficiency of hydrocarbon production and overall reservoir management. While such property is crucial for engineering design, conducting experimental tests to determine K can be both time-consuming and expensive. As such, reliable and high-fidelity models derived with soft computing techniques become useful for estimating K . Using a data set containing samples from 130 data points published in the literature, this work developed a sensitivity-driven Evolutionary Polynomial Regression (EPR) model to predict K . The model computes the permeability, log 10 K (mD), as a function of three explanatory variables: porosity, ϕ (−), formation factor, F (−), and the characteristic pore throat diameter, dPT (m). One unique feature of our approach is that it considers the physical meaning of the variables during the construction of the model. Verification of the methodology was carried out using split-sampling cross-validation. The developed model showed attributes such as parsimony (lower number of parameters and input variables), good predictive capability (accurate tracking observed log 10 K ), generalization ability (preserving physical meaning), and robustness (consistent performance under cross-validation). Sensitivity analysis revealed that the model can adequately simulate the increase in K with increasing ϕ and dPT , as well as its capacity to capture the non-linear relationship between log 10 K and F . Comparison of simulated K -values with results of models published in the literature, further validated the ability of our optimum EPR model structure. The proposed model shows potential as a promising method to estimate the permeability of tight carbonate rocks.

Background The dynamic mechanical properties and permeability evolution of deep rocks under coupled osmotic-mechanical conditions are vital for evaluating the stability of surrounding rock in deep rock engineering and further improving deep mining efficiency. However, there is currently no valid experimental system to measure both the dynamic mechanical response and the permeability evolution of deep rocks. Objective In this study, a novel experimental system is developed for determining dynamic compressive properties and permeability evolution of deep rocks subjected to coupled differential pore pressure and confinement. Methods The experimental system is composed of a dynamic loading system, an in-situ stress system, a differential pore pressure system, and a data acquisition system. The differential pore pressure system is introduced in the dynamic loading system, and the validation of the proposed system is verified by checking the stress wave propagation in the bars and the dynamic force balance on the two loading ends of specimens. It indicates that the differential pore pressure device added to the dynamic loading system barely influences the measurement of the dynamic behaviors of rocks. A homogenous green sandstone (GS) is employed to verify the feasibility and reliability of the proposed system. Dynamic compressive strength, permeability evolution, and failure mode of GS under cyclic dynamic impact loading in combination with coupled osmotic-confining pressure are explored using the proposed system. Results The stress–strain curves change with the increase of impact number, and the cyclic impacts deteriorate the dynamic compressive strength of GS. The permeability of GS first increases and then decreases with the impact number. The differential pore pressure enhanced the permeability of GS under the same impact cycle. The main fracture mode of the GS specimen is mainly compressive-shear fracture in combination with a tensile fracture in the middle of the specimen due to the coupling effect of the reflected stress wave and the osmotic-confining pressure. Conclusions The proposed experimental system is valid and effective to measure and observe the dynamic compressive behaviors and permeability evolution of rocks under coupled osmotic-mechanical conditions.

Geothermal energy is a clean and environmentally friendly energy source that can be used sustainably; however, attention towards geothermal energy has been intermittent during the last 40 years as a function of the crisis of oil price. However, geothermal reinjection and clogging has been a challenge limiting geothermal development and utilization. In China, widely distributed sandstone geothermal reservoirs have reduced production due to technical constraints such as excessive reinjection pressure and blockage. In this paper, we took the Binzhou sandstone geothermal field in North China as an example and conducted displacement experiments under different temperature and flow rate conditions by collecting in situ geothermal fluid and core rock to obtain changes in sandstone permeability. By comparing the variation in geochemical and mineral composition of geothermal fluids and cores before and after the experiments, combined with a water–rock interaction simulation, we investigated the reasons for the changes in permeability and porosity. The results show that high temperature and low flow rate have relatively minimal displacement pressure, and a flow rate of 1.0 mL/min at 45 °C shows a minimal effect on permeability, while 1.0 mL/min at 55 °C and 0.5 mL/min at 45 °C show a minimal effect on porosity. Flow rate is the main factor controlling permeability, while temperature demonstrated a relatively minor effect. The shift in permeability and porosity is mainly caused by the precipitation of quartz and the conversion of albite to montmorillonite. The injection of fluids at 55 °C may have dissolved additional minerals with a minimal change in porosity. However, the permeability reduction at 55 °C is greater than that at 45 °C, indicating that the blockage, which led to the permeability reduction, contains multiple causes, such as chemical and physical blockages. From the laboratory studies, we recommended that reinjected geothermal water be cooled or kept below the reservoir temperature before reinjection and at moderate flow conditions.

Fractures in rocks often provide preferential fluid migration pathways and their geometrical properties are the main factors influencing the permeability of the rock mass. By taking into account the complex geometries of rock fractures, including tortuous features, highly variable fracture apertures, rough fracture wall surfaces and complex fracture intersections, a highly effective approach is proposed in this work to investigate the behaviour of fluid flows in laboratory-scale fractured rocks. The computed tomography (CT) scanning was used to capture the micro-structures of non-planar fractures in a sandstone specimen. An image processing method was then developed to extract the three-dimensional fracture network. The fractures and fracture network were represented using an equivalent pipe network model, taking into consideration the complex geometries mentioned above. The nonlinear flow is incorporated into the model through the consideration of a friction factor in the pipe flow method (PFM). The absolute difference of the derived permeability between PFM and finite volume method (FVM) is 3.45%, but the FVM needs 15 times more CPU computation time. Therefore, the proposed approach is better than FVM in terms of computational efficiency. In addition, the use of the friction factor was demonstrated to be effective and efficient to model nonlinear flow within the fracture network, where the flow nonlinearity is caused by high flow velocity and the formation of eddies in certain parts of the fracture network, leading to a decrease in the overall apparent permeability and an increase in the flow tortuosity. The proposed method was further validated against flow test data covering a wide range of linear and nonlinear flow regimes.