This paper explores unlocking major reserves from S1 and H8 reservoirs, which comprise unconventional gas reservoirs forming a network of barely tapped, complex meandering channels spread over 1,500 km2. These zones contain nearly half of the initial gas in place volume in Block Yan113-Yan133, yet present a great challenge for extraction. These secondary targets are further characterized by high lithic content, poor pore-throat structure, subnormal pressure gradients, low productivity, and serious water block issues.
Based on challenging reservoir characteristics, in 2017, alternative propped hydraulic fracture designs were tested in three trial wells attempting to ultimately achieve economic production from S1 and H8. One trial well used a conventional nitrogen-energized guar fracturing fluid but with a larger overall job size. A second trial well tested a cleaner hybrid fluid system, and a third trial well was pumped at a higher injection rate to increase stimulated reservoir volume. Unfortunately, no trial wells continuously produced at a stable gas rate, so the completion and fracturing design was further modified and tested.
It was hypothesized that the optimal solution was to create more fractures utilizing a minimal amount of low-or nondamaging fracturing fluid, and to successfully flow back after stimulation; thus, a CO2 self-crosslinking clean fluid system (CO2 foam) fracturing technique was introduced in 2018. When this technique was applied in three wells for single-stage frac stimulation in the S1 formation, encouraging results were achieved. This proved the feasibility and reliability of CO2 foam fracturing in this reservoir.
As the Yanbei project has progressed, CO2 foam fracturing is seen as the potential key to unlocking even more challenging scenarios, such as multilayer stimulation within the S1 or H8 formations. For example, well D, with dual S1 layers, achieved the best production to date following installation of a fit-for-purpose downhole mechanical completion system combined with the CO2 fracturing fluid. Comparing results of all S1-stimulated wells, all four wells treated with CO2 fluid have exceeded the production expectations, confirming the suitability of this system for the ultra-tight reservoirs of the Yanbei gas field.
As a result of our novel combined stimulation techniques, integrating fracturing fluids and the downhole completion system, a 300% production increment was achieved by the two-stage CO2 foam fracturing treatment in trial well D in the S1 and H8 formations. Further optimization of cost and operational efficiency will allow CO2 fracturing technology to significantly increase production and nearly double the gas initially in place (GIIP) volume in Yanbei field.