Key Technology of Volumetric Fracturing in Vertical Wells of Hugely Thick Shale Oil Reservoirs in the Fengcheng Formation of the Mahu Sag

Abstract

Shale oil reservoirs in the Fengcheng Formation of the Mahu Sag are characterized by a large buried depth of sandbodies, huge thickness, overall oil-bearing properties, dense matrix, and enrichment in metal ions. As a result, fracturing is prone to be insufficient in vertical extension and unable to cover the reservoir, with low fracture complexity, high sanding risks, and an incompatibility of conventional guar gum fracturing fluid. Based on a detailed description of reservoir mechanical properties and an evaluation of the main controlling factors of fracture networks, a calculation model of fracability index was built to fully exploit the exploration potential of the shale oil in this area. In light of the vertical fracture propagation, the perforation clusters and cluster spacing were determined to develop a fine vertical layered approach. In accordance with the development of natural fractures, the optimized volumetric fracturing technology was upgraded, and the net pressure in fracture was increased by construction with high pumping rate. Multi-scale filling was realized with the combination of proppants of different sizes, and a gradual pumping rate increase was employed to ensure safe sand adding. Further, a series of temperature-resistant and salt-resistant polymer fracturing fluids were developed. As a result, the key technology was developed for volumetric fracturing in vertical wells of hugely thick shale oil reservoirs in the Fengcheng Formation of the Mahu Sag. This technology has been successfully applied in Well MY1 in the shale oil reservoirs of the Fengcheng Formation, with a high-yield oil flow of 50 m3 per day after stimulation treatment. This started a new and promising prospect of unconventional reservoir exploration in the Mahu Sag, which is of great significance in guiding the subsequent shale oil development in this area.