Abstract :The key for effective development of shale gas reservoir with nano- darcy permeability in North America is maximizing stimulated reservoir volume ,which in term of technology system ,is called “volume stimulation technology ”.It emphasizes “breaking up ”reservoir to get the largest contact area between fracture surface and reservoir matrix ,future achieving three dimensions stimulation of the reservoir .In this study ,volume stimulation technology is defined in “narrow ”and “broad ”sense according to different characteristics of shale and tight gas reservoirs. The “narrow –sense ”definition is derived from three elements including object (shale ),technology (horizontal well drilling &fracturing ),and verification (micro seismic fracture diagnosis ).The “broad-sense ”definition refers to multi-interval horizontal well and multi-layer vertical well fracturing techniques for tight oil and gas reservoir .Dsepite their different objects of reservoir ,two techniques have the same ultimate goal .The core theory of volume stimulation technique is proposed as follows :”breaking up ”the reservoirs to form complex fracture networks for artificial permeation ,enabling porous flow from the matrix to the fracture in the shortest distance ,and maximally reducing the driving pressure of oil and gas flow in the matrix .Further ,the core reservoir conditions for volume stimulation are put forward ,including obvious brittleness ,development of natural fractueres and bedding ,and small difference between the maximum and minimum stresses .Here the brittleness index measures the speed (or difficulty )of transient changes (i.e., fracture formation and expansion )prior to the breaking of the rocks .The key optimization design for volume stimulation include ‘reversal design ’,multi-cluster perforation ,and optimization of hole number and fracture spacing .Field test demonstrates that multi-cluster perforation ensures the effective opening of each cluster with an optimal hole number of 40 to 50.The relationship between optimal hole number and output volume can be obtained ,and the smaller fracture spacing ,the easier fracture reorientation . Additionally ,theoretical models and calculation results of perforation optimization ,optimal fracture spacing for achieving stress interference ,and matrix segmentation are presented .The above theory and optimization design of volume stimulation technology have great guidance value for improving the post-fracturing performance of unconventional reservoir .
Key words :volume stimulation; “break up ”reservoir ; the shortest distance in porous flow ; minimum driving pressure gradient ; multi-cluster perforation ; stress interference ; reversal design