The Ordos basin, located in northwest China, has large known hydrocarbon reserves that represent unique reservoir challenges and require specific operational, logistical, and technical expertise. One of the immediate objectives for PetroChina Changqing is to exploit these low-permeability oil and gas sandstones of the Yanchang formation, and this is done through both vertical and horizontal wells types that include single-stage and multistage hydraulic fracturing treatments. Historically, individual well productivity has been impacted by complex reservoir dynamics, uncertain subsurface properties, and identifying those areas where critical geological and reservoir factors exist in order to generate substantial production. However, most hydraulic fracturing treatments result in low production rates due to reservoir pressure being below the hydrostatic pressure gradient. Therefore, principal fracturing challenges include trying to engineer a highly conductive flow path that depends on fluid viscosity and stability to build geometry and transport proppant toward the fracture tip, while rapidly returning to the base-fluid viscosity to be flowed back. Since reduced fracture conductivity and length were detrimental to well performance, a hydraulic fracturing technique that incorporates the creation of channels within the fracture was deployed. This technology optimized reservoir inflow performance through a specified engineering workflow that requires specific equipment and pumping controls with fluid and fiber technologies. These channels created infinite-conductivity paths for fluid to flow from the reservoir to the wellbore, resulting in higher well productivity. The deployment and application of this technology included 65 treatments performed on five vertical and five horizontal wells in oil blocks and two vertical wells in a gas block. Production performance for all oil wells treated with the flow-channel fracturing technique had initial production index figures being 2.4 times the field average and those wells continue to be upper performers when compared to 180-day accumulated production figures. For the gas wells, 4 to 5 times initial gas production rate was achieved when compared to the average offset wells. In all well cases, channel fracturing increased operational efficiency while resulting in no screenouts.

With the rapid development of the world economy, the exploration and development of unconventional oil and gas resources have become a new hot spot. As an important subset of unconventional reservoirs, ultra-low permeability(ULP) reservoirs have been greatly concerned. Studies suggest that the ultra-low permeability reservoir belongs to the typical tight reservoir. The current low-permeability reservoir classification criteria available in the literature cannot meet the actual needs of ultra-low permeability reservoir development. Through researches, reservoir characterization parameters of effective porosity, mainstream throat radius, movable fluid saturation and starting pressure gradient are screened out to build quaternary classification coefficient. A new standard of classification and evaluation of ultra-low permeability reservoirs is established, and ultra-low permeability reservoirs are divided into three categories. Thus it provides a theoretical basis for speeding up the effective development of ultra-low permeability reservoirs. Since the beginning of the 21st century, the world economy has entered a new cycle of development. The demand for oil and gas resources in the countries soared. Facing the huge demand for energy, productivity construction and production rate are relatively insufficient throughout the world. Unconventional oil and gas resources are beginning to receive more attention. Ultra-low permeability reservoirs, as important unconventional oil and gas reservoirs, have huge potential for exploration and development, but the current classification of reservoir evaluation criteria cannot meet the needs of ultra-low permeability reservoir exploration and development. In order to effectively guide the exploration and development practice of ultra-low permeability reservoirs, it is of great significance to launch researches on ultra-low permeability reservoir characterization parameter system, and to establish a new classification and evaluation criteria.

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