Slick-water fracturing has been widely used in shale stimulation, which provides relatively long fracture length and large-scale complex fracture network without cross-linked gel. The shortcomings of slick-water fracturing are the proppant setting and bridging, which result in low sand concentration that compromises long-term fracture conductivity. A hybrid fracturing strategy is formulated with the combination of slick-water prepad followed with linear gelled, or cross-linked gel to balance the fracture network and long-term fracture conductivity. Compared to common hybrid fracturing, a modified hybrid fracturing is proposed by modified pumping schedules with alternating injection of slick-water, linear gelled fluid, or cross-linked gel, which is used for well stimulation in shale formations with high pressure and temperature.
In this paper, we present an in-house experimental device to simulate fluids and proppant flow in complex fracture network that contains two clusters, main fractures and several secondary fractures, which allows us to separately simulate hydraulic fractures and natural fractures. We have conducted a series of experiments on the slick-water fracs, the hybrid fracs, and the modified hybrid fracs. Compared with conventional hybrid fracs, the modified hybrid fracs with alternating injection of slick-water and cross-linked gel exhibit two advantages. (1) The modified hybrid fracs can form more flow channels in main fracture for viscous fingering, (2) the modified hybrid fracs enhance secondary fracture conductivity because slick-water pad between cross-linked gels reduces inflow rate of slurry into secondary fractures.
Based on the experimental results, an experimental method of fracture conductivity prediction is proposed with data regression method, which can be used for characters selection of pumping schedule in modified hybrid fracturing. Several shale well cases of the same shale gas field in Sichuan basin, Southern China, separately using the slick-water fracs, the hybrid fracturing and the modified hybrid fracturing are illustrated. The stimulated treatment pressure characteristics, long term well production performance of these three scenarios are discussed.
The modified hybrid fracturing has several advantages in shale stimulation, but there are also some difficulties in preventing sand screen out. This paper illustrates some basic experimental mechanisms of field applications of hybrid fracturing. In the future, we will present more field scenarios that are planned to carry out in other shale well pads.