Stimulation mode of complex oil and gas reservoirs in oil-rich sag during the secondary exploration: the case study on Jizhong Depression and Erlian Basin

TThe secondary exploration of oil-rich sags is the key to realize sustainable reserve increasing of old areas. However, there are more and more complex oil and gas reservoirs that are characterized by deep burial depth, low permeability or special lithology, so it is necessary to take some stimulation measures to get industrial oil and gas flow. The Jizhong Depression in the Bohai Bay Basin and the Erlian Basin are taken as the examples for study. First, it is technically difficult to stimulate thin heavy oil reservoirs of low permeability, which could be damaged easily, their mobility is poor and the fracture height cannot be controlled easily. To solve these technical difficulties, the fine stimulation pattern of “moderate scale, moderate sand ratio, whole-process antiswelling and efficient viscosity reducing” was established. Second, it is difficult to carry out in-depth acid fracturing stimulation in ultradeep buried-hill carbonate oil reservoirs with ultrahigh temperature and stress. To solve this technical difficulty, the ultrahigh temperature polymer fracturing fluid which has the resistance to high temperature 200 ℃ and shearing was prepared, the clean acid system with the excellent retarding effect and solid-liquid diversion capacity was optimized, and the volume acid fracturing stimulation pattern of “fracture detection in pad, solid-liquid diversion, multi-stage injection and closed acidizing” was established. And third, it is difficult to stimulate the low-permeability glutenite reservoirs with high clay content for they are sensitive and could be damaged and plugged with sand easily. To solve this problem, slick water and low-concentration fracturing fluid system was developed to reduce the damage rate from 38.6% to 13.4%, and the low-damage complex volume stimulation pattern of “compound fracturing, seal ball fracturing in pad and multi-layer separate fracturing” was established. The measures had been applied in Lixian slope, Niudong buried hill and Aer sag for 106 well times and the stimulation effect is over 150% higher than previously. It provides the effective support for the reserves and production increasing of new secondary exploration fields in oil-rich sags.