Abstract
The Ordos basin is the second largest basin in China, which is well known for its "three low" – low perm, low pressure, and low abundance (volume in place per km2). Deposited in deep-water lacustrine environment, Chang7 shale oil formation in Ordos basin has huge resources in place, which is expected to be the most important strategical succession resources in China. Through nearly 10 years of research and practice, Ordos shale oil has achieved scale-effective development and has established several million-ton production demonstration sites. Horizontal well dense-cluster volumetric fracturing, being one of the effective technologies applied in Ordos Chang7 shale oil development, its concept is closely related to fractured horizontal length optimization. During "fracturing-soaking-production" practices, optimal soak-in time, flowback control, and production management system have been established. This paper demonstrates the optimization process and the best practices on these aspects.
The method used to assess the optimal horizontal section is to study the relationship between the length of the horizontal section and the well production as well as the investment of a single horizontal well, which is based on the geological characteristics of the reservoir. The process to evaluate the best soaking period include laboratory seepage-imbibition measurement through NMR technology, the numerical simulation and onsite oil-water displacement process observation of the real production. In addition, a reasonable flowback strategy is established based on production data and problems caused by proppant production. The "flow-saturation pressure ratio", which is the ratio of flowing wellhead pressure of the single well over reservoir saturation pressure, is a key concept applied to optimize flowback rate and production control. During production stage, the production rate per unit horizontal length is optimized by water cut and flow-saturation pressure ratio.
