Emulsion stability directly affects the effect of surfactants in driving oil production. In this paper, the influence of crude oil components and the molecular structure of surfactants on the stability of emulsions is studied. First, emulsion samples with different asphaltene contents are prepared and passed. The emulsion time strength stability was characterized, and its mechanism of action was analyzed by molecular simulation methods. It was found that asphaltenes directly affect the strength of the oil-water interface membrane. The higher the asphaltene content, the stronger the emulsion stability. Then the emulsion containing different molecular structure surfactants was prepared, it was found that the more unstable bonds on the hydrophilic groups of nonionic surfactants, the weaker the strength of the interfacial membrane, and the worse the stability of the emulsion, and the cationic surfactants. The binding ability of different hydrophilic group ions to water molecules is different, and the thickness of the water adsorption layer is different. The order of the binding ability of ions to water is: K⁺ > Na⁺ > NH4⁺. Graphical Abstract

Hydraulic fracturing is the most preferred oil well stimulation technology worldwide. In fractured oil wells, toxic gases (hydrogen sulfide and carbon monoxide) present a problem when the produced water is reclaimed as base fluid for fracturing. These gases can cause equipment corrosion, environment pollution, and adversely affect the operational personnel’s health. Extant research focuses on toxic gases produced at high temperatures (>200 °C). Therefore, this paper examines the causes and critical conditions for toxic gas formation in fractured wells in conventional reservoirs at low temperatures (<160 °C) through a series of laboratory experiments that include bacterial culture and heating experiments. The results show that for temperature in the range 40–60 °C, the presence of sulfate-reducing bacteria in the produced water is the main cause of hydrogen sulfide production in fractured wells. However, when the temperature is higher than 102 °C, the sulfur compounds (potassium persulfate and sodium sulfite) in the fracturing fluid and crude oil can produce hydrogen sulfide by thermochemical sulfate reduction. In addition, when the temperature is higher than 75 °C, oxygenated compounds (aldehydes) in crude oil can produce carbon monoxide by decarbonylation. Both processes can occur simultaneously when the temperature is higher than 102 °C. This paper is of great significance in controlling the formation of toxic gases in fractured wells.

Well pattern infilling optimization is a crucial measure to enhance gas recovery, especially in tight gas reservoirs with low permeability and small-scale sand bodies. Traditional methods of determining well pattern density rely on qualitative analysis from the perspective of gas blocks. However, these methods are challenging to apply to sand bodies with different properties, and there have been no studies conducted on infilling time, which significantly impacts production increment. In response to this situation, this paper establishes a series of evaluation indexes and proposes a numerical simulation of economic well pattern density and infilling time based on real parameters obtained from the Linxing–Shenfu gas field. To quantitatively determine the economic well pattern density and infilling time based on fine characterization of sand bodies, a sensitivity analysis is conducted, considering various permeability levels and reserve abundance. Two intersection charts of economic well pattern density and infilling time, relating to reserve abundance and permeability, are then drawn. Furthermore, a real well site is selected as an example, and the infilling effect analysis confirms the reliability of the charts. The new method of determining economic well pattern density and infilling time presented in this article can provide theoretical support for the economic and efficient development of the Linxing–Shenfu gas field, serving as a useful reference for the beneficial development of similar gas reservoirs.