Summary Liquid loading seriously affects gas wells production and even causes gas wells abandonment. Many researchers still focus on correcting a critical liquid-loading flow rate to alleviate these problems. However, they still cannot reasonably be explained. Gas flow rate is higher than the critical liquid-loading flow rate, but liquid loading can still occur. Therefore, until an accurate critical fluid-loading flow rate is discovered, we should monitor the fluid-loading phenomenon to prevent it from affecting production gas wells’ performance. In this work, a fracture liquid-loading monitoring (FLLM) model is proposed and solved for the timely monitoring of fracture liquid-loading (FLL) positions and volume. The Newman product and Green function methods are used to develop and solve the FLLM model. The fracture is discretized into 2nxnz grids to describe an FLL volume and position. The numerical simulation method is used to verify the accuracy of the FLLM model. As a result, four innovative flow regimes, including fracture cavity liquid-loading flow, fracture root liquid-loading flow, transitional flow considering fracture cavity liquid-loading flow, and transitional flow considering fracture root liquid-loading flow, are identified on the pressure response curves. The pressure response of the same gas well at different times is well matched by the model in this paper, and the obtained parameters are more reasonable. The FLLM model can correct for magnified permeability, shortened half-length, and magnified wellbore storage coefficient. In conclusion, the FLLM model is established to monitor FLL, and alert engineers to remove liquid loading on time to prevent water from suddenly rushing into a wellbore and causing gas wells abandonment.
Pore structures with rich nanopores and permeability in tight gas reservoirs are poorly understood up to date. Advanced techniques are needed to be employed to accurately characterize pore structures, especially tiny pores which include micron and nanopores. In this study, various experimental techniques such as scanning electron microscopy (SEM), nuclear magnetic resonance (NMR) , nitrogen adsorption method, and NMR cryoporometry (NMRC) are combined to interrogate the complex pore systems of the tight gas reservoir in the Linxing formation, Ordos Basin, China. Results show that tight gas sandstones are primarily comprised of residual interparticle and clay-dominated pores. Clay and quartz are two dominate minerals while pyrite occupies a nontrivial amount as well. The permeability of tight gas sandstones is very low, exhibiting an extremely poor positive correlation with porosity. While pore types and relative pore contents are more influential factors on the permeability, accurate characterization of pore size distribution is critical for the permeability of tight gas sandstones. Therefore, complementary characterization methods are carried out, indicating that neither small pores with (around peak 1 in NMR distribution) nor large pores with (around peak 3 in NMR distribution) control the permeability by analyzing the connectivity of the pores in various size ranges, but rather pores averaging approximately (around peak 2 in NMR distribution) have sufficient connectivity to host and transmit hydrocarbons. The pore size of tight gas sandstones is dominated by the clay-rich mineral assemblage. The study shows that the NMRC technique can be a very promising method, especially when referred to as a promising “roadmap” on how to interrogate tight formations such as the tight gas sands or even shale especially for the nanopore characterization.
The usage of direct current (DC) voltage has enormous potential for oil fields due to the effect of wettability alteration. However, the unclear mechanism of the wettability alteration has limited the application of this technology to oil fields. In this study, chemical and physical methods including contact angle tests, Fourier-transform infrared spectroscopy (FTIR) measurements, and atomic force microscope (AFM) experiments were combined to investigate the wettability alteration mechanism for tight sandstones subjected to DC voltage treatment. From the view of a chemical factor, FTIR results show that DC voltage decreases the number of Si-O-Si, C-O-C, C-O, and COOH groups, while it also increases the number of C═O and OH groups. The changes in molecular groups further improve the water-wetting property of tight sandstones. On the other hand, in a physical way, AFM results indicate that DC voltage improves the roughness of the rock surface. At the same time, the wetting state transfers from the Cassie-Baxter to the Wenzel. This increases the contact area of the solid-liquid interface. The augment of roughness and the transfer of the wetting state improve the water-wetting property of tight sandstones. By comparing the influences of both chemical and physical factors on wettability, it is concluded that although roughness indeed affects the wettability, chemical factors play a dominant role in determining the wettability. Achievements in this study can help researchers and engineers better understand the mechanism of wettability alteration and further accelerate the development of tight sandstones with DC voltage-related technology.
Slickwater is generally applied in the domain of coalbed methane (CBM) stimulation. However, the residual slickwater after fracturing operations may adversely affect the production of CBM. In this paper, nuclear magnetic resonance (NMR) measurements are used to compare and analyze the differences of functional groups on the coal surface in different regions and provide the basis for the construction of molecular simulation models. Then, different concentrations of slickwater were made, and coal samples from different regions were soaked. Isothermal adsorption experiments and molecular simulations were applied to explore the effect of slickwater on methane adsorption properties of coal from different regions. The NMR results show that the amount of naphthalene functional groups in coal samples from Dongtan coal mine (DT) is more than that in coal samples from Houwenjialiang coal mine (HW), which has a stronger attraction to methane. The isothermal adsorption experiment results show that the methane adsorption capacity of the coal sample from HW is stronger than that of the coal sample from DT. When the concentration of slickwater is 0.5%, the methane adsorption capacity of DT increases to 3/2 of raw coal, and the methane adsorption capacity of HW declines to 1/3 of raw coal. The retention of slickwater will have different effects on coal samples in different areas. Comparing the experimental and simulation results, we can see that polyacrylamide can adsorb methane, thereby increasing the specific surface area of the local "slickwater–coal" system due to its porous coating structure, resulting in an increase in the methane adsorption capacity. The negative effect of slickwater on the methane adsorption properties is because slickwater changes the pore, resulting in the reduction of effective surface area and finally damages the adsorption amount of methane.
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