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Summary Numerous technical papers have been published on the reservoir evaluation of tight sand gas in recent decades. It is believed that tight sand gas is characterized by low permeability, complex pore structure, abnormal pressure, and low gas saturation. Researchers have characterized tight gas in terms of, for example, statistics; and some of them are increasingly interested in the effect of fluid pressure and pore structure on gas saturation. However, there is still a lack of effective experimental and mechanism analysis. The focus of this paper is to investigate the effect of two factors on controlling the tight gas saturation. A method was proposed for determining the in-situ injection pressure of the DB 2 gas reservoir in the Kuqa area during the gas-accumulation period, and the result was 1.2 MPa. A polyetheretherketone carbon-fiber core holder (maximum temperature 180 °C, maximum confining pressure 30 MPa) was designed for the microcomputed-tomography (CT) simulation experiment. After that, four physical simulation experiments of different fluid pressures (0.1, 0.5, 1.5, and 5.0 MPa) were implemented using X-ray CT and the new core holder to study the effect of fluid pressure and pore structure on gas saturation. The results show that the fluid pressure and micropore connectivity are important factors for gas-saturation increase; the number of gas clusters (connected pores containing gas) increases with the increase of fluid pressure and the maximum volume of gas clusters increases with the increase of pore connectivity. The connected pores with the largest volume are an important source of gas-saturation contribution. The three types of modes (the blowing balloon mode, stitching mode, and composite mode) of microscopic gas-cluster formation are discussed. The blowing balloon mode is key to gas-saturation growth in the period of low fluid pressure or the early stage of the injection process. The stitching mode is key to gas-saturation growth in the period of high fluid pressure or the late stage of the injection process. The above research system investigates the quantitative control of fluid parameters (pressure) and reservoir parameters (pore structure) on gas saturation, which might change the situation where previous research was too focused on the pore-structure and fluid-pressure characterization, with a lack of quantitative control analysis of reservoir gas saturation. It provides a valuable reference and framework for the reader to conduct further quantitative research on tight gas saturation.

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Casing threaded joint wear is one of the main reasons that affect the sealing integrity and structural integrity of casing string. Due to the complex structural characteristics of casing threaded joint, there are few studies on mechanical properties of casing threaded joint under wear conditions. In this paper, to simulate the residual strength of casing threaded joint considering wear, a finite element model of BTC casing thread is established. The stress distribution of worn casing threaded joint is analysed under the effect of inner pressure, outer pressure and axial tension. The results indicate that under the effect of inner pressure and outer pressure, the maximum and average Mises stress increases with the increase of casing wear depth; under the effect of axial tension, with the increase of wear depth, the maximum Mises stress has a little change, but the average Mises stress increases significantly. Based on the stress analysis of thread under different wear depths, the decrease rate of bearing capacity of BTC thread is measured by the increase proportion of average stress after wear. Thread residual strength gets lower with the increase of wear depth, when wear depth reaches 4 mm, burst strength, collapse strength and tensile strength decrease by 14.7%, 25.9% and 20.7% respectively. Through the methods and results presented in this paper, the residual strength of casing threaded joint can be quantized, which can provide a theoretical foundation and technical basis for casing residual strength evaluation and casing strength check.

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