Experimental investigation of propagation mechanisms and fracture morphology for coalbed methane reservoirs

Fracture propagation mechanisms in coalbed methane(CBM) reservoirs are very complex due to the development of the internal cleat system. In this paper, the characteristics of initiation and propagation of hydraulic fractures in coal specimens at different angles between the face cleat and the maximum horizontal principal stress were investigated with hydraulic fracturing tests. The results indicate that the interactions between the hydraulic fractures and the cleat system have a major effect on fracture networks. "Step-like’’ fractures were formed in most experiments due to the existence of discontinuous butt cleats. The hydraulic fractures were more likely to divert or propagate along the butt cleat with an increase in the angles and a decrease in the horizontal principal stress difference. An increase in the injection rate and a decrease in the fracturing fluid viscosity were more conducive to fracture networks. In addition, the influence on fracture propagation of the residual coal fines in the wellbore was also studied. The existence of coal fines was an obstacle in fracturing, and no effective connection can be formed between fractures. The experimental investigation revealed the fracture propagation mechanisms and can provide guidance for hydraulic fracturing design of CBM reservoirs.

Discovery of the ancient Ordovician oil-bearing karst cave in Liuhuanggou,North Tarim Basin,and its significance

A typical ancient karst cave is found in the Yijianfang Formation of Ordovician at Liuhuanggou,North Tarim Basin.With an entrance of 4.8 m wide and 20 m high,the Liuhuanggou ancient karst cave extends to NW 315° and has been filled by sediments,which can be classified into three types according to their nature and origin:clastic sediments(gravels),chemical sediments(palaeotravertine),and mixed sediments.Among them,the Paleotravertine and gravels are most common.The palaeotravertine is laminated type and yellowish brown or black in color due to oil inside.When saturated with oil,the palaeotravertine is black.Under microscope,oil mainly accumulates in the cracks between calcite crystals.The filling period of the Liuhuanggou karst cave can be divided into three stages:mixed filling,chemical filling,and clastic(palaeotravertine breccia) filling.Numerous fractures are distributed on one side of the karst cave.The fractures can be as wide as 40 cm and over 40 m long.Fractures of different periods and sizes constitute a fracture network and are all filled with calcite veins.Oil can be found inside the fractures.In the chemical fillings,sulfur is also found,which can indicate the magma influence on karst caves.The existence of sulfur also confirms that the Liuhunggou karst cave was formed in ancient times rather than recently or nowadays.Different from normal sedimentary limestone,the ancient Liuhuanggou karst cave has a distinct isotope fractionation and the depletion of heavy carbon and oxygen isotope.Controlled by the northwest oriented vertical strike-slip shear fault,the karst cave is actually a dissolved vertical fault belt.

First record of ostracods from the Upper Ordovician red-coloured marine sandstones of the Tierekeawati Formation in Tarim Basin, NW China: implications on palaeoenvironment and palaeobiogeography

Ostracods are described for the first time from the red-coloured marine sandstones of Arisu section(Arisu red beds)of the Upper Ordovician Tierekeawati Formation in Kalpin area of northwestern Tarim Basin, Xinjiang Uygur Autonomous Region(Xinjiang), Northwest China. Twenty-two species belonging to thirteen genera are described and figured. The ostracod fauna suggests a probable Sandbian–Katian age for these beds. The palaeoecological assemblage of ostracod fauna implies the deposition in a nearshore-offshore environment during a regression when the Arisu red beds of the Tierekeawati Formation were laid down in the Tarim Basin. Many cosmopolitan and provincial genera were present in diversified ostracod fauna of the Arisu red beds, suggesting the possible biogeographic relationships among the Tarim, Tibet, and South China plates, as well as Europe and North America continents during the Late Ordovician. Ostracods experienced faunal exchanges between Laurentia and the Tarim Plate during the Late Ordovician Period.