Abstract
Tight oil reservoir is commonly recognized to be difficult to supply the formation energy. Tight oil reservoir in Ordos basin is characterized to be continental sedimentation, strong heterogeneity, normal pressure (0.9), low reserve abundance, imperative to supply the displacing energy. CO2 have good injection and high displacing efficiency for extra-low permeability reservoir, which provides the reference for exploring the effective development mode in tight reservoir. Influencing by complex fracture and hydraulic fracturing, as CO2 flooding is conducted in tight oil reservoir, gas channeling phenomenon is very serious, displacing energy is difficult to be utilized. To enlarge CO2 sweep efficiency, how we can effectively control CO2 channeling becomes the first important issue in energy supplying development of normal pressure tight reservoir.
In the paper, a case of Honghe Chang 8 tight oil reservoir was carried out. Medium-large and medium-small scale fractures were artificial fabricated in natural crop cores according to G&G understanding. In different fracture openings models, two-stage blocking experiments with CO2 flooding were operated with high-strength starch gel and weak-strength ethylenediamine. Slug combination, volume and sequence were evaluated and by numerical simulation, blocking strength were defined to reflect the plugging effect in the experiment. Then build up well group numerical simulation model, characterizing the complex fracture network. Blocking sites and volume of plugging agents were analyzed and optimized. Two-stage blocking technique was determined in the fracture developed tight reservoir during CO2 flooding.
Research results indicate it is priority to control the gas channeling of medium-large scale fractures with sufficient volume of the starch gel. As CO2 flooding is performed, gas starts to break through, followed by small volume ethylenediamine to mitigate gas channeling. Numerical simulation shows that firstly fully controlling gas channeling from medium-large scale fracture along with principal stress, then injecting plugging agent from the injection well, being located in 1/3~1/2 of well space to treat medium-small scale fractures with the best production stimulation. Starch gel volume is determined by medium-large scale fractures pore volume(PV), while low concentration starch gel and ethylenediamine volume is optimized to be half of the medium-small scale fracture PV, in which PV of fracture permeability lower than 200×10-3μm2 determine ethylenediamine volume. Generally, plugging agent would be more than 1500m3 for a pair of I-P wells.
With low cost CO2 source, CO2 flooding is expected to be one of the important development modes for normal pressure tight oil reservoir. During CO2 immiscible flooding, gas channel controlling technique plays decisive roles in ensuring successful development. Research results are not only advisable for the Ordos normal pressure tight reservoir development, but also for the similar tight reservoir in the world.
