Water Injection Development and Stable Production Technology in Ansai Extra Low Permeability Oilfield
Authors: Li Tiancai 
Published: December 2007

Abstract
Ansai Oilfield has an average permeability of 0.49×10-3µm2 with the "three-low" feature of low permeability, low pressure and low production. And it belongs to extra low permeability geology and economical marginal oilfield. After hard development over ten years, Changqing Oilfield summarizes a full set of technical methods to develop this oilfield economically. Especially in the aspect of water injection development, with large amount of tests in lab and field, Changqing oilfield gives out a series of the stable production technical methods, and gets breakthrough in developing extra low permeability oilfield. This paper emphasizes on the sum-up of water injection and stable production technology for twelve years in this oilfield, also on the evaluation of injection technical policies under three drive types such as pore drive, pore-fracture drive and fracture drive, And on the choice of different injection policies for different exploitation stages and different reservoir types as well as different seepage flow property on single-well, well group and well block. All of this provides the reliable basis for economical and efficient development of extra low permeability oilfield.

Introduction
Ansai oilfield sites in the west of China, which was found in 1983 with the payzone of Chang 6 formation in Yanchang group of Triassic system. It is a huge integral oil field with reserves of hundreds million tons. As the main payzone, Chang 6 is found at a depth of 1000 to 1400m, and oil layer thickness of 11 to 14m, oil saturation of 55%, porosity of 11 to 15%, average air permeability of 1.29×10-3µm2 Kilinkenberg permeability of 0.49×10-3µm2, original formation pressure of 8.3 to 9.8MPa, saturation pressure of 6.0MPa, pressure coefficient of 0.7 to 0.8, wells can get the industrial productivity by fracturing, but no initial output will be obtained, so Ansai oilfield is characterized with extra-low permeability, low pressure and low production.
In order to develop the integral oil field with hundreds million tons reserves economically and effectively, well group, pilot and industrialized test areas were opened to the study of reservoirs, oil reservoir protection, drilling, completion, integral fracturing treatment, water injection and so on, which made the oil field into the large-scale industrial development since 1990. At present, the oilfield includes 1693 oil wells, which has the average daily individual-well oil production rate of 2.0t, the annual output of 102×104t, and it includes 458 water injection wells, which has the average daily individual-well water injection rate of 22m3, the annual water injection rate of 380×104m3.
Production Features of Different Stage
After Ansai oilfield was found in 1983, production tests were carried out on single-well group and well block in the former seven years. Thorough study was made in order to make production stable and developed recognition in geology on extra-low permeability reservoir after injecting water.

Development Stages of Natural Energy and Water Injection Test
The economic recovery ratio produced by natural energy in Ansai oilfield is 8%. The annual production decline ratio in early stage is as high as 20 to 40%. During Chang 6 production test, wells started to produce depending on natural energy in 1985, the original formation pressure was 8.8MPa and dropped 0.7MPa after 4 months, 4.23MPa after 943 days with the production in unit drawdown of only 653m3. There were four oil wells in the well group s6–71, in which the injection-production well spacing was 180 to 425m, they had produced for two and half years before injection, the daily oil output for each well decreased from 3.64 to 1.35t with the decline rate of 63%.
Development Stages of Natural Energy and Water Injection Test.
The economic recovery ratio produced by natural energy in Ansai oilfield is 8%. The annual production decline ratio in early stage is as high as 20 to 40%. During Chang 6 production test, wells started to produce depending on natural energy in 1985, the original formation pressure was 8.8MPa and dropped 0.7MPa after 4 months, 4.23MPa after 943 days with the production in unit drawdown of only 653m3. There were four oil wells in the well group s6–71, in which the injection-production well spacing was 180 to 425m, they had produced for two and half years before injection, the daily oil output for each well decreased from 3.64 to 1.35t with the decline rate of 63%.