Heterocyclic Mannich bases, N -(3-oxo-3-phenylpro-pyl)thiazol-2-aminium chloride (DTZA) and N -(3-oxo-3-phenylpropyl)-1 H -pyrazol-3-aminium chloride (DPZA), were developed for the corrosion inhibition of N80 steel in a 15 wt % lactic acid solution. Weight loss measurements, electrochemical techniques, surface characterization, and theoretical calculations were combined to investigate their anticorrosion performance and mechanism. The results showed that DTZA exhibited a satisfactory inhibitor efficiency of 97.56% with a dosage of 0.15% at 363 K, while DPZA achieved only 58.3% under the same conditions. Adsorptions of both inhibitors on the metal surface followed the Langmuir model with physical and chemical adsorptions. Based on X-ray photoelectronic spectroscopy (XPS) analysis, DFT calculations, and molecular dynamics (MD) simulations, stronger interactions between DTZA and iron than those in the case of DPZA were revealed, leading to the formation of a compact protective film on the metal surface, which is attributed to the presence of a thiazole ring in the DTZA chemical structure.

Accurate stress field calculation of the casing-cement-stratum system is crucial for evaluating wellbore integrity. Previous models treated in-situ stress as boundary pressure loads, leading to unrealistic infinite displacements at infinity. This study presents a three-dimensional (3D) analytical solution for the stress field within the casing-cement-stratum system in inclined wells, considering in-situ stress and hydrostatic stress in cement as the initial stress state and taking into account stress components related to the axial direction. Assuming a plane strain condition and superimposing the in-plane plane strain problem, elastic uni-axial stress problem and anti-plane shear problem, a 3D analytical solution is obtained. Comparisons with previous models indicate that the existing model overestimates the absolute values of stress components and failure potential of casing and cement in both 2D and 3D scenarios. The presence of initial stress in cement greatly increases the absolute value of the compressive stress state but decreases the failure potential in cement, which has not been well studied. Additionally, a low Young’s modulus and high initial stress state of the cement benefits the cement’s integrity since the maximum Mises stress significantly decreases. The new 3D analytical solution can provide a benchmark for 3D numerical simulation and quick assessment for wellbore integrity.

Nowadays, with the breakthrough of hydraulic fracturing in marine shale gas play, the annual production reached 20 billion cubic meter in the year of 2020. However, it is not enough to satisfy the demand of ever-increasing economy of China. So that other kinds of lacustrine shale gas must be developed economically. However, there are many challenges in hydraulic fracturing such as confined fracture height penetration as a result of stratified lithology, less complexity as a result of bigger horizontal stress difference (>10 MPa) and higher clay content (>50%), higher stress sensitivity and clay expansion so as to cause higher fracture conductivity damage ratio. Accordingly, new strategies and countermeasures were put forward as followings: 1) maximizing fracture height penetration vertically through the stratified lithology by virtue of higher viscosity gel in pre-pad injection and using directional perforation technique;2) adopting acid plugging or acidic slick water, using temporary plugging agent within fractures and decreasing distance between adjacent clusters as well, and 3) developing longer effective clay stabilizer, mechanism study and control of fracture face compaction and flowing back (imbibition under high pressure), and increasing proppant weight or intensity as soon as possible, etc. Furthermore, above research achievements were applied and tested in more than 8 pilot wells, and the results indicated 100% fracturing treatment success, on the other hand, the vertical fracture height growth and complexity were both improved a lot (>30%, >20%, respectively) than ever before, and 4 wells obtained an expected performance, which brings a beam of light in economically develop lacustrine shale gas play in the near future. Therefore, it has a great significance in the study and implement of hydraulic fracturing stimulation in the similar shale gas all over the world. Introduction With the success of horizontal well staged fracturing technique in marine shale gas play in China in the past decade, its most recent development mainly focusing on densely distributed perforation clusters, high proppant intensity as well as temporary plugging either with balls in horizontal wellbore or granular agents in main fractures (Tang et al., 2011). However, when we adopted above strategies in lacustrine shale gas play, it did not always work. For example, using higher and higher pumping rate is feasible in marine shale gas wells while using the same technique in the lacustrine ones, the fracture face compaction as a result of high clay content may play a key role in reducing permeability dramatically so as to prevent shale gas flowing from rock matrix to fractures after fracturing treatment job (Dennis D, 2016). In other words, high pumping rate may cause high bottom hole pressure and high compaction effect on surface of fractures correspondingly. Therefore, the positive effect of high pumping rate may be offset greatly by its negative one.

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