Cleats are systematic, natural fractures in coal seams. They account for most of the permeability and much of the porosity of coalbed methane reservoirs and can have a significant effect on the success of hydraulic fracturing stimulation. Laboratory hydraulic fracturing experiments were conducted on coal blocks under true tri-axial stress to simulate fracturing stimulation of coal seams. Fractures were initiated by injecting a water gel with luminous yellow fluorescent dye into an open hole section of a wellbore. The impact of cleats on initiation and propagation of hydraulic fractures in coal seams is discussed. Three types of hydraulic fracture initiation and propagation pattern were observed in this study: 1) The hydraulic fracture initiated and then grew along the cleat. 2) The hydraulic fracture initiated along a butt cleat or a fracture (natural or induced by drilling) oriented roughly in the minimum horizontal stress direction, then turned to propagate along the first face cleat that it encountered or gradually turned towards the maximum horizontal stress direction. 3) The hydraulic fracture initiated perpendicular to the minimum stress and, when it encountered a face cleat, tended to propagate along the cleats if the extension direction does not deviate greatly (<20° as determined in this paper) from the maximum horizontal stress direction. When a coal seam is hydraulically fractured, the resulting fracture network is controlled by the combined effect of several factors: cleats determine the initiation and extension path of the fracture, the in-situ stress state dominates the main direction of the fracture zone and bedding planes impede fracture height growth.

Tight gas is one important unconventional hydrocarbon resource that is stored in tight sandstone, whose mechanical property greatly influences the tight gas production process and is commonly believed to be simply elastic when designing the stimulation plan. However, the experimental evidence provided in this work surprisingly shows that tight sandstone can deform in a viscoelastic way. Such an unexpected observation poses a challenge in accurately modelling the deformation process. We solve this problem by adopting the fractional Maxwell model to successfully derive the constitutive equation of tight sandstone, based on which not only all the experimental data can be interpreted quantitatively, but also reasonable and consistent predictions as to tight sandstone’s long-term deformation behaviour can be made. We then investigate the typicality of our results in China’s Changqing oilfield, which is one major centre of tight gas production and where the rock samples for experiments are obtained. It is estimated that a non-negligible portion of 18% tight sandstone samples in this area will probably display viscoelasticity. Finally, our work implies that the mechanical properties of other materials may also need further scrutiny to possibly uncover any unexpected behaviour, overlooking which may result in misleading predictions.

Background Hypokalemia (LK) was associated with peritonitis in peritoneal dialysis (PD) patients, while the role of its degree and duration have not been fully established. Here, we conducted a retrospective cohort study to identify the relationships of LK degree and duration with peritonitis in PD patients. Methods A total of 602 PD patients in our department from Jan 1^st, 2009 to Dec 31^st, 2019 entered the last analysis. Data were collected from their medical records. Serum potassium (SK) levels, degree of hypokalemia, and duration of hypokalemia were analyzed with peritonitis. The time association of hypokalemia and peritonitis was also analyzed. Results There were totally 320 (53.7%) and 123 (20.7%) patients who had ever suffered from LK and serious hypokalemia (SLK) in the cohort. Only 6.82% and 0.5% patients had LK and SLK at baseline, while the incidence rose and kept in 25‐32% and 5.5‐8.2% after PD. Both LK (HR 1.437, 95% CI 1.014‐2.038, p=0.042) and SLK (HR 2.021, 95% CI 1.429‐2.857, p<0.001) did correlate to peritonitis after adjusted analyses, while only SLK remained the significance at each follow‐up point. The LK / SLK duration were 6 (3‐12) and 6 (3‐6) months, and only longer SLK duration correlated to peritonitis after adjusted analyses. After categorized, those LK duration more than 6 months and SLK duration more than 3 months presented significant association with peritonitis. Of the patients suffered from both hypokalemia and peritonitis, 70.4% patients’ LK times were earlier than peritonitis time, while most SLK times (62.7%) were later. SLK also correlated to combined endpoint. Conclusions Hypokalemia degree and duration were tightly associated with peritonitis. Hypokalemia might be a causal factor of peritonitis, while peritonitis might also aggravate hypokalemia. We should manage SK as much as possible and avoid hypokalemia, especially serious hypokalemia in clinic practice.