Understanding hydraulic fracture propagation behavior in tight sandstone–coal interbedded formations: an experimentalinvestigation

Whether hydraulic fractures could connect multiple gas zones in the vertical plane is the key to fracturing treatment to jointly exploit coalbed methane and tight sandstone gas through integrative hydraulic fracturing in tight sandstone–coal interbedded formations. Laboratory true triaxial hydraulic fracturing experiments were conducted on layered specimens with di erent combination types of natural sandstone and coal to simulate the propagation behavior of hydraulic fractures. The effects of the fracture initiation position, fracturing fluid viscosity and injection rate were discussed. The results showed that di erent fracture morphologies could be found. When initiating from coal seams, three patterns of fracture initiation and propagation were obtained:(1) The main hydraulic fracture initiated and propagated along the natural fractures and then diverged due to the effects of in situ stress and formed secondary fractures.(2) The hydraulic fracture initiated and propagated in the direction of the maximum horizontal stress.(3) Multiple fractures initiated and propagated at the same time. With the same fracturing fluid viscosity and injection rate, the hydraulic fractures initiating in sandstones had greater chances than those in coal seams to penetrate interfaces and enter neighboring layers. Excessively small or large fracturing fluid viscosity and injection rate would do harm to the vertical extension height of the induced fracture and improvement of the stimulated reservoir volume. Compared with operation parameters(fracturing fluid viscosity and injection rate), the natural weak planes in coals were considered to be the key factor that a ected the fracture propagation path. The experimental results would make some contributions to the development of tight sandstone–coal interbedded reservoirs.

Association between serum cystatin C level and hemodynamically significant aortic stenosis: a prospective cohort study

BACKGROUND Cystatin C(CysC)is a cysteine protease inhibitor involved in proteins catabolism and plays an essential role in human vascular pathophysiology.CysC may also increase the risk of aortic stenosis(AS),but limited studies have reported on this association.This study aimed to investigate if elevated serum CysC levels are associated with hemodynamically significant AS.METHODS Serum CysC levels were estimated in 4,791 participants,samples were collected in 1990−1992.The study popula-tion was divided into quintile groups.Follow-up continued in 2011-2013 when participants returned for echocardiography exam-ination.Incidence of aortic valve disease(AVD)was ascertained by Doppler echocardiography through the end of 2013.AVD defined in hemodynamic progression was assessed and classified as aortic sclerosis,mild stenosis,and moderate-to-severe sten-osis.RESULTS Overall,a total of 4,791 participants(mean age:54.8±5.0 years,females:57.6%,blacks:8.2%)were included in this study.During a follow-up of 21 years,we identified 736 cases(15.4%)of aortic sclerosis,194 cases(4.0%)of mild stenosis,and 42 cases(0.7%)of moderate-to-severe stenosis.Compared with serum CysC levels within individual quintile groups,the odds ratio(OR)was per standard deviation associated with an increased incidence of AVD(OR=1.15,95%CI:1.05−1.26,P=0.002).CONCLUSIONS In this large population-based study,an increased serum CysC levels is independently associated with the incidence of hemodynamically significant AS.However,this association appears not to extend to patients with extremely high serum CysC levels and necessitate further investigation.