Microseismic mapping of horizontal well multi-stage fracturing of shale reservoirs has proven that the geometry of fracture is not planar as assumed/observed in conventional reservoirs, but rather a complex connected fracture network. The network is assumed to be constructed by three basic fracture types (Horizontal and vertical fractures parallel to, and vertical fracture perpendicular to flow direction - Type I, Type II, and Type III fractures respectively). A new fracture test cell is designed and constructed for investigating propped fracture network conductivity similar to the API standard fracture conductivity test cell that is presently used by the oil and gas industry.
In the experimental investigation, nitrogen gas is selected as fluid for safety reasons and also its physical properties are similar to methane gas. Marble is selected as a substitute for shale in constructing the fracture network in laboratory tests because of its similarity with shale in both permeability and wettability. Shale samples cannot be easily obtained and shale slices are difficult to prepare. A lightweight (1.5 g/cc), small size proppant (40-70 mesh) is selected because of its wide use in shale gas development and ease of transportation in the fracture network. The fracture widths of the three fracture types are assumed to be equal in the same propped fracture network. Twenty-two groups of successful experiments are selected to calibrate equipment, investigate the influence of actual fracture width, number of each fracture type, and closure stress on propped fracture network conductivity by selecting various combinations of fracture patterns, confining pressures, and proppant concentrations. Darcy's law is employed in calculating propped fracture network conductivity by introducing concepts of equivalent fracture width, equivalent proppant concentration, and equivalent fracture conductivity.
The experimental results showed that the modified API fracture conductivity test cell was calibrated well against the API standard cell. The deviations were in the range of 4.4% to 8.1%. The value of equivalent propped fracture conductivity measured by the modified API fracture conductivity test cell is acceptable. It was observed that the propped fracture network conductivity (1) decreases with increasing closure stress, and reached a limiting value when closure stress exceeded a critical value and (2) increases with increasing actual fracture width (proppant concentration). Furthermore, increasing number of Type-I and II fractures will increase propped fracture network conductivity, and Type-II fracture is more dominant than Type-I fracture, but Type-III fracture does not affect propped fracture network conductivity as significant as other two fracture types do.