Utilizing Natural Gas Huff and Puff to Enhance Production in Heavy Oil Reservoir

Abstract L Block is a super-deep heavy oil reservoir with the degassed oil viscosity of 9,680∼12,000mPa.s @50☐, 2800∼3300 m deep with normal pressure system and 80∼95☐ in the reservoir. The GOR is 12m3/m3 and the initial bubble point pressure is only 4MPa. High oil viscosity and low flowability result in low production and rapid decline of primary development. This paper will review mechanisms, feasibility and pilot test of utilizing natural gas huff and puff to enhance production in L Block. Firstly, the paper will detail the lab experiments which include the conventional PVT and unconventional PVT. The conventional PVT shows that the natural gas dissolved in the oil can greatly decrease oil viscosity by 1∼2 orders of magnitude and enhance the flowability in one hand, in other hand, it can large the volume coefficient and add the elastic energy. The unconventional PVT shows that the heavy oil can effectively entrap the gas for more than several hours because of its high viscosity. There exists a pseudo bubble point pressure far lower than bubble point pressure in the manmade foamy oil, which is relative to the depressurization rate. Under the condition, the elastic energy could be maintained in a wider pressure scope than natural depletion without gas injection. Secondly, the paper will introduce a special experiment apparatus which can simulate the process of gas huff and puff in the reservoir. The huff and puff experiment shows that in this process the foamy oil can be seen and most of oil flows to the producer in pseudo single phase, which is one of the most important mechanisms of enhancing production. Finally, the paper will brief the pilot test of single well, which shows that the oil production is increased to 5∼6 m3/d from 1∼2m3/d by natural gas huff and puff and the stable production period is prolonged to 91days in the first cycle and 245 days in the second cycle from 5∼10 days before huff and puff.