Journal of Petroleum Science and Engineering最新文献

In view of the continuous occurrence of coal and gas outbursts (hereafter as ‘outbursts’), and the dynamic behavior and quantitative mechanism of water injection in coal seams preventing outbursts are not still unclear. In the study, the characterization of mechanical action and expansion energy release of gas initial desorption (GID) in coals with different moisture contents is revealed to clarify the influence of moisture on gas dynamic effect in coals. The results show that during the GID of gas-containing coals, the increased moisture content will decrease the pressure and momentum of gas from coals significantly. And the gas pressure reduction rate shows an increasing trend, with the decreasing reduction rate of gas momentum. Therefore, the ability of gas damaging coals with high moisture contents is weakened by reducing the degree of pressure-induced mechanical action on the coal surface and the impact intensity on the cracks in coals. Meanwhile, the gas-released cumulative expansion energy from the coals is significantly reduced, with the decreasing increase rate of the gas energy. Therefrom, the moisture in the coal masses synthetically weakens the dual effects of pressure attribute and expansion effect of gas decreasing the damage ability of gas to coals, which can prevent the further development of outburst preparation process. It is concluded that the correlation between moisture content and the initial expansion energy of released gas is linearly and negatively correlated. For moisture content with the every 1% increase in coal masses of Xuehu Coal Mine, the energy decreases by about 11% on average. Accordingly, the quantitative water injection in coal mining face is carried out to eliminate the local abnormal zone containing gas.

When a gas hydrate reservoir is depressurized for gas production, the production tendency and dissociation behavior may differ depending on conditions such as the bottom hole pressure and depressurization rate. Gas hydrate dissociation is a complex process that involves the transfer of materials and heat, and on-site analysis based on laboratory-scale results is critical. In the present study, a field-scale numerical analysis was performed to reflect the conditions of the Ulleung Basin in the East Sea of Korea. The dissociation behavior, which varies depending on the conditions in the gas hydrate-bearing sediment, was analyzed under various conditions of bottom hole pressure and depressurization rate. This study also identified the effects of depressurization conditions on gas hydrate saturation. As the bottom hole pressure decreased and the depressurization rate increased, the production rate and cumulative production of gas and water increased, and the radius of the pressure propagation effect at the beginning of production increased. In sediments with a gas hydrate saturation of ≥70%, the pressure propagation was unstable and the dissociation rate was low. These results can serve as preliminary data for the field production of gas hydrates in the Ulleung Basin.

Geomechanical effects monitoring on reservoir rock and fluid properties response during the oil production curve are essential to improve oil recovery in a petroleum field. Incorporating geomechanics to flow models become the mathematical formulation regarding well-test and reservoir engineering more realistic because geomechanical parameters, e.g., in situ and overburden stress, as well as Biot’s coefficient, play a fundamental role in pressure response. Hence, permeability stress-sensitive oil reservoirs are the scope of various research in the petroleum industry for minimizing formation damage during drilling, completion, and stimulation operations. In this context, mechanical formation damage control plays a key role in preventing early-permeability loss that may result in reservoir compaction and oil field disinvestments. This work develops a new analytical solution for the nonlinear hydraulic diffusivity equation (NHDE) with instantaneous point-source/sink effects in permeability effective stress-sensitive oil reservoirs. The proposed model considers Biot’s effective stress change in the permeability response, and a new deviation factor is derived from comparing the nonlinear effect concerning the constant permeability classical solution and a decoupled case available in the literature. The calibration methodology is performed using a numerical simulator named IMEX®, widely used in formation evaluation works, and the results presented high convergence. The findings of this study allowed us to notice the role of overburden stress, oil flow rate, deviation factor, and Biot’s coefficient in permeability change during production in the diagnostic plots. Thereby, the modeling developed in this paper becomes a useful and attractive tool for predicting and monitoring permeability loss, oil flow rate specification, and reservoir history matching.

By the expansion of production from source-related unconventional petroleum resources, accurate approximation of Total Organic Carbon (TOC) through well logs has become progressively important. Accordingly, recent studies have mainly focused on increasing the precision of TOC estimation by using different types of AI techniques and/or optimizing algorithms. Along with utilizing these approaches, this study emphasized on removing an unaddressed source of error inherited from lithological heterogeneity with the same goal. Like organic matter quantity, lithological variations within a source interval also induce well log responses, which may interfere with the training process of Artificial Intelligence (AI) techniques. In the present research, the effect of lithological variations on the performance of TOC estimators was evaluated by employing Adaptive Neuro Fuzzy Inference System (ANFIS) and Multilayer Perceptron network (MLP). Firstly, ANFIS and MLP models were constructed and trained using a database containing different lithologies (original models). Then, a new methodology was developed based on modeling the relationship between log data and TOC values for each type of lithology (litho-based method). The result showed that the litho-based method estimates more reliable and accurate TOC values, proving the adverse effect of lithological variations on the original models. Furthermore, the litho-based ANFIS models provide the most promising results. Since metaheuristic algorithms are usually employed to optimize AI techniques, Genetic Algorithm (GA) and Particle Swarm Optimization (PSO) were also implemented into the original models (hybrid models). Accuracy of TOC values estimated by the hybrid models is slightly higher than those derived from the original models. However, these hybrid approaches are not as efficient as the litho-based method. Applicability of the proposed approach was guaranteed by performing it over Pabdeh source rocks in a well of SW Iran. Based on its high efficiency, the newly developed litho-based method can be used as a powerful tool to reliably evaluate unconventional hydrocarbon resources, as well as the potential of the conventional petroleum sources. Moreover, it can be utilized, instead of/along with traditional optimization approaches, to approximate other geochemical factors as well as petrophysical parameters from log data.

In-situ mining of lignite requires dehydration, pyrolysis, gasification, and other stages. The injected fluid, dehydrated water, and pyrolysis products are transported in the developing pores and fractures. The internal structure and properties of lignite change significantly under the joint action of temperature and fluid pressure. In this study, X-ray computed tomography (X-CT) was used to scan lignite samples in the temperature range of 25°C–450 °C. Grayscale images and three-dimensional reconstruction images of the internal structure were obtained to investigate the evolution of the internal pore structure during lignite pyrolysis. It is found that the porosity of lignite increased as the temperature rose from 25 °C to 250 °C. The porosity was 6.54% at 250 °C. At 350 °C, the porosity decreased to 2.45% due to channel blockage and softening of the coal. At 450 °C, the pyrolysis of the lignite organic matter resulted in numerous large and interconnected honeycomb pore clusters. At this temperature, the porosity was 16.02%. X-CT and nuclear magnetic resonance enabled detailed and quantitative characterization of the internal structure of lignite. The research results provide theoretical and technical information on the evolution of migration channels in lignite for the potential improvement of in-situ pyrolysis and gasification efficiency of in-situ lignite mining.

Current flow models for fault-karst reservoirs are mostly described as a single fault formation, which cannot be applied in recent-developed multibranched fault-karst reservoirs. This paper established a novel analytical model to investigate pressure response behavior of a horizontal well in multibranched fault-karst reservoirs. The model is able to describe the influence of the physical properties and spatial structure of fracture-cave system on pressure transient response. The flow model considers different flow behaviors in each region, which includes Darcy flow (gravity included) in fault-fracture, large-scale storage flow in karst-cave, and Poiseuille-law-based horizontal laminar flow in the horizontal wellbore, respectively. These assumptions enable the model to match complex situations in multibranched fault-karst reservoirs. Then, the model was retrograded to compare with a single fault-karst reservoir model to verify its accuracy. Further, the solutions were graphed on log-log plots, and we discussed the effect of fluids mobility, formation storability, and structure characteristics (e.g., length, angle, depth, distance) of fracture-cave branches on transient pressure responses. Results show that (a) the number of fracture-cave branches in a reservoir can be directly observed by counting the number of V-shaped appearances on the pressure derivative curve. (b) The exact shut-in time when V-shape appears is affected by volume and distance between two neighboring fracture-cave branches. (c)The characteristics of the V-shape are affected by fluid mobility, formation storability, and length of fracture region. (d) The slope of the pressure derivative curve in the boundary-dominated flow regime can be used to evaluate the gravity effect. (e) The pressure response behavior exhibits a near-well effect when a horizontal well commingled production in the multibranched fault-karst reservoir. Finally, we applied our model and resulting observations to analyze pressure build-up data tested from SHB Oilfield, which demonstrated a workflow to identify the number of fault-karst branches and also to estimate reservoir properties.

The effective flushing of a drilling fluid filter cake is crucial for improving the zone isolation of cement sheaths during cementing operations, some studies have researched the flushing law of filter cake in circular wellbore, however, only a few have studied the flushing law and mechanism of elliptical wellbore and the irregularity of elliptical wellbore may lead to the increase of the non-uniformity of filter cake that may result in different flushing results than the circular wellbore. In this study, a filter cake formation and flushing experimental system is developed for circular and elliptical wellbore, and the formation and flushing experiments of drilling fluid filter cake in circular and elliptical wellbore are conducted, the flushing law and mechanism of drilling fluid filter cake in circular and elliptical wellbore are obtained by combining the computational fluid dynamics (CFD) simulation calculation results with the experimental results. The results revealed that the filter cake flushing efficiency of the same drilling fluid and displacement were primarily affected by the thickness of the initial drilling fluid filter cake, shear strength, wellbore shape, wall shear stress and eccentricity, the initial thickness and shear strength were higher, the filter cake flushing efficiency was lower, the wall shear stress of the flushing fluid was higher, the flushing efficiency of the filter cake was higher, and the change in wellbore shape and eccentricity led to an uneven distribution of annular velocity and reduced the flushing efficiency of the filter cake. The method and conclusion enable to gain an understanding for improving the flushing efficiency of the drilling fluid filter cake and forms a basis for the design of flushing fluids in cementing operations.

Movable fluid content and permeability are important reference factors for reservoir quality evaluation and recovery enhancement. In this study, based on multiple experimental results, 10 typical samples from a tight sandstone gas reservoir in the coal measure strata of the Shanxi Formation along the southeastern margin of the Ordos Basin were divided into three lithofacies to discuss the factors influencing movable fluid content and permeability. The results show that the fluid has a strong seepage capacity and a high degree of mobility in relatively large pore throats. The relatively large pores in the study area are secondary dissolved pores of various origins. High quartz and feldspar contents are conducive to the formation of secondary pores, while the presence of carbonate minerals and clay minerals play an inhibitory role. The pore throat size range of 0.05–0.1 μm is the critical interval for the conversion of bound fluid to movable fluid. The movable fluid saturation and movable fluid porosity are affected by submicron- and micron-scale pore throats of >0.1 μm, while the permeability is controlled by micron-scale pore throats sizes of >1 μm. The volumetric proportion of the relatively large pore throats is influenced by the mineralogical composition of the rock, the size of the pore throats, and the degree of sorting, which further control the amount of moveable fluid and its percolation capacity. The highest movable fluid content and permeability appear in the massive gravel-bearing coarse to medium sandstone lithofacies (Lm) with a high proportion of submicron- and micron-scale pore throats, whereas the lowest occurs in parallel bedding or ripple laminations，medium to fine sandstone lithofacies (Lpr) with a high proportion of nano-scale pore throats. The lithofacies with cross bedding and medium sandstone (Lc) is also dominated by nano-scale pore throats, which shows the characteristics of low movable fluid content and medium permeability due to the retention of some micron-scale pore throats. This study describes the mobility of fluids with different pore throat sizes in detail and determines the pore throat size range corresponding to the transition from bound fluid to movable fluid, which can provide a reference for the evaluation of movable fluid seepage in other regions.

Seismic inversion is a significant technique for estimating petroleum reservoir parameters. The low frequency component of the initial model represents the geological background information, which plays an important role in the seismic inversion. It is challenging to precisely depict the actual geological model in seismic inversion because of the inherent velocity-depth ambiguity. Therefore, the initial model which is closer to genuine geological backdrop is essential. We propose a workflow which estimates a fusion initial model based on data fusion algorithms. It is well known that seismic facies analysis can provide more low-frequency information about the geological background. For example, the boundaries of sedimentary bodies can be represented by seismic facies classification data. We utilize a combination of the seismic facies classification data and well curves interpolation initial models to accurately invert the special geological body with the support of a feature-level fusion algorithm. Then, a practical pre-stack seismic inversion method is implemented, and a field data example further demonstrates its applicability and steadiness in seismic inversion.

Pore-scale forces have significant effects on the macroscopic behavior of multi-phase flow through porous media. We develop a robust and accurate accelerated process-based method for the computation of relative permeability from direct simulations of pore-scale two-phase flow on micro-computed tomography images. In the pressure drop calculation, we take advantage of an existing analysis that establishes a relationship between pore-scale forces and Darcy-scale pressure drops using an energy-conservation approach. We establish a thermodynamically consistent approximation of Darcy-scale viscous pressure drops as the rate of energy dissipation per unit flow rate of each flowing phase for the first time within the context of a free-energy lattice Boltzmann method (LBM). In addition, we propose and test a new computationally efficient partial-mirror periodic boundary condition for a fully coupled visco-capillary pore-scale flow simulator based on a free-energy LBM. The new boundary condition is imposed only in the main flow direction and significantly reduces the computational cost of the process-based relative-permeability computation protocol at a small compromise on accuracy.

We first compute primary-drainage and subsequent imbibition relative permeability curves for a reservoir sandstone rock sample. We use this real-reservoir dataset to validate the pressure drop computation method and the partial-mirror periodic boundary condition. We then simulate the entire drainage and imbibition cycle using an extensively studied Berea sandstone dataset. We quantitatively demonstrate that pore-scale direct numerical simulation-based relative permeability curves computed with our novel process-based method agree well with experimental steady-state relative permeability measurements. We also quantitatively demonstrate that the new partial-mirror periodic boundary condition accelerates the relative permeability computations 4 to 13 times.