A numerical approach for pressure transient analysis of a vertical well with complex fractures

A new well test model for a vertical fractured well is developed based on a discrete-fracture model in which the fractures are discretized as one dimensional（1-D） entities.The model overcomes the weakness of complex meshing,a large number of grids, and instability in conventional stripe-fracture models. Then, the discrete-fracture model is implemented using a hybrid element finite-element method.Triangular elements are used for matrix and line elements for the fractures. The finite element formulation is validated by comparing with the semi-analytical solution of a single vertical fractured well. The accuracy of the approach is shown through several examples with different fracture apertures,fracture conductivity, and fracture amount. Results from the discrete-fracture model agree reasonably well with the stripefracture model and the analytic solutions. The advantages of the discrete-fracture model are presented in mesh generation, computational improvement, and abilities to handle complex fractures like wedge-shaped fractures and fractures with branches. Analytical results show that the number of grids in the discrete-fracture model is 10 % less than stripefracture model, and computational efficiency increases by about 50 %. The more fractures there are, the more the computational efficiency increases.

Characterization of Organic-Rich Shales for Petroleum Exploration & Exploitation: A Review-Part 1: Bulk Properties, Multi-Scale Geometry and Gas Adsorption

Shales, the most abundant of sedimentary rocks, are valued as the source-rocks and seals to porous petroleum reservoirs. Over the past-twenty years, organic-rich shales have also emerged as valuable petroleum systems （reservoir, seal, and source rocks contained in the same for- mation）. As such they have become primary targets for petroleum exploration and exploitation. This Part 1 of a three-part review addresses the bulk properties, multi-scale geometry and gas adsorption characteristics of these diverse and complex rocks. Shales display extremely low permeability, and their porosity is also low, but multi-scale. Characterizing the geometry and interconnectivity of the pore-structure frameworks with the natural-fracture networks within shales is essential for establish- ing their petroleum exploitation potential. Organic-rich shales typically contain two distinct types of porosity： matrix porosity and fracture porosity. In addition to inter-granular porosity, the matrix po- rosity includes two types of mineral-hosted porosity： inorganic-mineral-hosted porosity （1P）; and, organic-matter-hosted （within the kerogen） porosity （OP）. Whereas, the fracture porosity and per- meability is crucial for petroleum production from shales, it is within the OP where, typically, much of the in-situ oil and gas resources resides, and from where it needs to be mobilized. OP increases signifi- cantly as shales become more thermally mature （i.e., within the gas generation zones）, and plays a key role in the ultimate recovery from shale-gas systems. Shales＇ methane sorption capacities （MSC） tends to be positively correlated with their total organic carbon content （TOC）, thermal maturation, and mi- cropore volume. Clay minerals also significantly influence key physical properties of shale related to fluid flow （permeability） and response to stress （fracability） that determine their prospectivity for pe- troleum exploitation. Clay minerals can also adsorb gas, some much better than others. The surface area of the pore structure of shales can be positively or negatively correlated with TOC content, de- pending upon mineralogy and thermal maturity, and can influence its gas adsorption capacity. Part 2 of this three-part review considers, in a separate article, the geochemistry and thermal maturity cha- racteristics of shale; whereas Part 3, addresses the geomechanical attributes of shales, including their complex wettability, adsorption, water imbibition and ＂fracability＂ characteristics. The objectives of this Part 1 of the review is to identify important distinguishing characteristics related to the bulk properties of the most-prospective, petroleum-rich shales.