Water-gas ratio characteristics and development concepts for water-producing gas reservoirs.

Water production from gas wells is a key factor affecting the effectiveness of gas-reservoir development, and it poses serious challenges in terms of increasing the degree of recovery during the waterless production stage and reducing the impact of water production on gas-reservoir development in the middle and later periods. Thus, gas reservoirs must be efficiently exploited on the basis of identifying gas-water layers accurately, defining gas-water relationships, and understanding gas-water production performance. Accordingly, this study analyzes the production characteristics in gas reservoirs with different gas-water relationships, and it summarizes the rules that determine water-gas ratios. The results reveal that the water-gas ratio increases rapidly in the early stage of water production, but after a period of time, it enters a relatively stable state in which it is almost a fixed value. According to the material balance equation, the theoretically calculated water-gas ratio is fully consistent with the production rules for an entire confined gas reservoir. This shows that the reality of gas-well-water production must be faced, and that the development of water-bearing gas reservoirs must accommodate gas and water co-production. The gas-water relationship, water body scale, and reservoir heterogeneity determine the time of water breakthrough and the water-gas ratio. Therefore, we should change the traditional "water fear" concept in gas-field development, aim for an overall improvement in recovery, face up to the fact that gas wells produce water, and coordinate the development of multi-wells for entire gas reservoirs, all of which will achieve the ultimate goal of improved gas recovery.

Experimental and numerical study on gas production decline trend under ultralong-production-cycle from shale gas wells.

It is of engineering interest to explore recovered shale gas composition and its effects on total gas production trend over a long-term extraction period. However, there are previous experimental studies mostly focused on short term development for small scaled cores, which is less convincing to mimic reservoir-scaled shale production process. In addition, the previous production models mostly failed to account for comprehensive gas nonlinear effects. As a result, in this paper, to illustrate the full-life-cycle production decline phenomenon for shale gas reservoir, dynamic physical simulation was performed for more than 3433 days to simulate shale gas transport out of the formations over a relatively long production period. Moreover, a five-region seepage mathematical model was then developed and was subsequently validated by the experimental results and shale well production data. Our findings show that for physical simulation, both the pressure and production declined steadily at an annual rate of less than 5%, and 67% of the total gas in the core was recovered. These test data supported earlier finding that shale gas is of low flow ability and slow pressure decline in the shale matrices. The production model indicated that free gas accounts for the majority of recovered shale gas at the initial stage. Based on a shale gas well example, free gas extraction makes up 90% of produced total gas. The adsorbed gas constitutes a primary gas source during the later stage. Adsorbed gas contributes more than 50% of the gas produced in the seventh year. The 20-year-cumulative adsorbed gas makes up 21% of the EUR for a single shale gas well. The results of this study can provide a reference for optimizing production systems and adjusting development techniques for shale gas wells throughout the combinations of mathematical modeling and experimental approaches.

Multiple Gas Seepage Mechanisms and Production Development Research for Shale Gas Reservoirs from Experimental Techniques and Theoretical Models.

Shale gas seepage theory provides a scientific basis for dynamically analyzing the physical gas flow processes involved in shale gas extraction and for estimating shale gas production. Conventional experimental techniques and theoretical methods applied in seepage research are unable to accurately illustrate shale gas mass transfer processes at the micro- and nanoscale. In view of these scientific issues, the knowledge of seepage mechanisms and production development design was improved from the perspective of experimental techniques and theoretical models in the paper. First, multiple techniques (e.g., focused ion beam scanning electron microscopy and a combination of mercury intrusion porosimetry and adsorption measurement techniques) were integrated to characterize the micro- and nanopore distribution in shales. Then, molecular dynamics simulations were carried out to analyze the microscale distribution of gas molecules in nanopores. In addition, an upscaled gas flow model for the shale matrix was developed based on molecular dynamics simulations. Finally, the coupled flow and productivity models were set up according to a long-term production physical simulation to identify the production patterns for adsorbed and free gas. The research results show that micropores (diameter: <2 nm) and mesopores (diameter: 2-50 nm) account for more than 70% of all the pores in shales and that they are the primary space hosting adsorbed gas. Molecular simulations reveal that microscopic adsorption layers in organic matter nanopores can be as thick as 0.7 nm and that desorption and diffusion are the main mechanisms behind the migration of gas molecules. An apparent permeability model that comprehensively accounts for adsorption, diffusion, and seepage was developed to address the deficiency of Darcy's law in characterizing gas flowability in shale reservoirs. The productivity model results for a certain gas well show that the production in the first three years accounts for more than 50% of its estimated ultimate recovery and that adsorbed gas contributes more to the annual production than free gas in the eighth year. These research results provide theoretical and technical support for improving the theoretical understanding of shale gas seepage and optimizing shale gas extraction techniques in China.

[Clinical value of combined detection of serum tumor markers in lung cancer diagnosis].

OBJECTIVE: To investigate the clinical significance of a combination of several serum tumor markers in the diagnosis of lung cancer. METHODS: Serum CEA, CA125, CA199, CYFRA21-1 and NSE were measured with RIA, chromatometrychemoluminescence, ELISA and biochemoluminescence methods respectively in 340 patients with lung cancers at different TNM stages, 120 patients with benign lung diseases, and 45 healthy people. The sensitivities, specificities and accuracies of different combination of those markers for the diagnosis of lung cancers were compared. RESULTS: CYFRA21-1 had the highest sensitivity and accuracy (60.0% and 70.3%) and CA199 had the highest specificity (99.4%) for detecting lung cancers. CYFRA21-1 had the highest sensitivity (79.6%) for detecting squamous carcinoma. CEA had the highest sensitivity of (75.7%) for detecting adenocarcinoma. NSE had the highest sensitivity (76.2%) for detecting small cell lung cancers (SCLC). The combination of several serum tumor markers had higher sensitivities than the single marker for the diagnosis of lung cancers. With two markers, the combination of CYFRA21-1 and NSE had the highest sensitivity and accuracy (82.9% and 83.4%), while the combination of CA125 and CA199 had the highest specificity (94.5%). With three markers, the combination of CEA, NSE and CYFRA21-1 had the highest sensitivity and accuracy (89.1% and 85.1%), while the combination of CEA, CA125 and CA199 had the highest specificity (86.7%). The combination of CEA, CA125, CA199, CYFRA21-1 and NSE produced the best value, with a sensitivity of 93.8%, a specificity of 71.5%, and an accuracy of 86.5%. CONCLUSION: Serum CEA, CA125, CA199, CYFRA21-1 and NSE are helpful markers for the diagnosis of lung cancer. The combination of the markers can improve the sensitivity and accuracy of the diagnosis.