160 oil fields have been put into development in Daqing, Shengli and other oil regions in China. The reservoir rocks of these oil fields are mainly continental sedimentation in origin characterized by multiple productive reservoirs and multiple-layers, with great difference in permeability between reservoirs and relatively insufficient natural energy supply. Reservoirs of this kind generally have a short period of flowing production, quick depletion of pressure and low primary recovery by means of natural energy. To improve the effect of oilfield development, water flooding from an initial stage of development for pressure maintenance and flowing well production has been adopted in most of oilfields. Such a system ensures that the annual crude output of China reaches one hundred million tonnes and over. However, the water cut of wells rises continuously and the capacity of flowing well production decreases gradually in the course of oilfield development. Since 1980, we have converted most of the flowing wells into the artificial lifting wells and at the same time large capacity pumps, hydraulic pumps and electric submersible pumps are gradually used to replace the conventional downhole pumps for production. Through field practice in recent years, the development result has been further improved so that crude output of our country not only remains stable but also Increases gradually year by year.
A continuous rise of water cut, an increase in borehole fluid gradient, a rise of flowing pressure causes a decrease of production pressure differential, resulting in a decline of the crude output. For example, in 1980, there were 3872 flowing wells in Daqing Oilfield with an overall water cut of 60%. The chokes of producing wells had been enlarged continuously but the producing pressure differential did not increase strikingly. Generally a rise in the producing pressure differential of 0.4 atm will occur for each water cut increase of 1%. Although the water flooding has been strengthened to restore the reservoir pressure, it could not match with the rising speed of flowing pressure. So, the producing pressure differential might not be increased and even further decreased in some cases.
There are two ways to increase the producing pressure differential at the water cut production stage. One way is to restore or increase the reservoir pressure and the other is to reduce the flowing pressure. The change of the mode of production from flowing to artificial lift is, in fact, to take the second way, i.e., water injection for pressure maintenance at a given level to ensure a sufficient supply of the fluid to the wells, while the flowing bottom hole pressure is greatly reduced by means of artificial lift techniques so that the aim of increasing the producing pressure differential to maintain a steady crude output is achieved.