The 0.3×10^-3μm² Reservoir Characteristics And Development Strategies In Ordos Basin

The reservoirs's permeability of being-developed and new-discovered regions in Ordos Basin are mainly less than 0.5×10-3μm2, so how to explore these oilfields effectively and economically is of utmost importance to the long-term development. There are some achievments bye lots of key tests, despite of that, it is still necessary to do more meticulous work because of the reservoirs' complexity.Based on the characteristics of low-permeability reservoir, the thesis first proposes and analyzes the necessity of reservoirs evaluation parameters, such as main flow throat radius, movable fluid saturation, and threshold pressure gradient; the parameter system also includes viscosity of underground crude oil and clay contents. Fuzzy comprehensive evaluation for low permeability reservoirs is established. According to the comprehensive value, there are four kinds of low-permeability reservoirs:the first two kinds have been effectively developed under present technical qualification, the third is in research now and the forth is out of recent technology. The 0.3×10-3μm2 reservoirs in Ordos Basin mainly refer to the ones with comprehensive value under 0.6, which belongs to the lower limit of the second and the third. All above makes it known which category of reservoirs in Changqing oilfield belongs to.The 0.3×10-3μm2 reservoirs in Ordos Basin locate in Ansai Delta, Yanchang Series, Northeast Depositional System, and the front of Xifeng Delta, Southwest Depositional System. Compared with 1×10-3μm2 reservoirs, the 0.3×10-3μm2 reservoirs have thinner kernels, stronger cementation, and are more poorly sorted. The channel diameter is almost the same with common low-permeability reservoirs', but its throat is smaller. Otherwise, the main flowable throat radius is less than 1μm, which means low seepage ability of that kind of reservoirs.It is more difficult to develop 0.3×10-3μm2 reservoirs because of the strong pressure sensitivity and non-linear flow effect. Because of the existence of non-linear flow, there are easy-flowing area, low-velocity and high-resistance area, and non-flow area around the well. The results of experiments and theoretical analysis demonstrate that the lower the permeability is, the stronger the non-linear effect is and the smaller the easy-flowing area is. Additionally, the pressure sensitivity effect, which is stronger than common low-permeability reservoirs', decreases the seepage ability around the well and influences the development result.The thesis evaluates the 0.3×10-3μm2 reservoirs' productivity, pressure system, and well pattern. There is a good linearity relationship between output in early stage and testing rate. The productivity is controlled by the thickness, size of permeability, well spacing, producing pressure drop, and scale of fracturing. In early stage, output is high and decreases quickly. Soon after water flooding response, water break through. The output and dimensionless oil productivity index fall down quickly, which means no ability to increase the output. So, the key points of effective development are the declining rate in early stage and lengthening the time of low-to-moderate water-cut stage. It is also very important to establish an effective displacement pressure system for effective development of 0.3×10-3μm2 reservoirs. Based on the fluid mechanics in porous medium, the thesis discusses how to evaluate the effective pressure system of fracturing effect in five-spot pattern and establishing strategy. The well pattern is controlled by the fracture development level. Generally, diamond shaped nine-spot pattern is fit for super low permeability reservoirs, and five-spot pattern is fit for natural fracture developed reservoirs. In addition, oil and water wells which are fractured at the same time, long axis which follows the direction of sand body lay and parallels the main direction of fracture development, all above mean the good development effect.0.3×10-3μm2 reservoirs should be fractured firstly, then advanced flooding. For the advanced flooding blocks, we should pay attention on the swept volume and advancing recovery efficiency. Finally, the thesis discusses the developing strategy from six aspects.