Study On Fluid Mechanism And Developing Methods Of Fuyang Reservoirs In The Periphery Of Daqing Oilfield

The low and ultra-low permeability Fuyang oil layers in Daqing marginal oilfields are mainly developed by waterflooding. The uneven porosity, tiny throat, strong molecular force and capillary pressure on solid-liquid interface of Fuyang oil layers have caused large filtrational resistance, low displacement efficiency, rapid production decline and big difficulty in effective development. Therefore, based on the effect of microscopic pore structure on filtering flow in reservoirs and the influence of reservoir fractures on waterflooding effect, this paper makes deep study on non-Darcy flow mechanism and law in combination of physical experiments and field tests. The effective development strategy for Fuyang oil layers in Daqing marginal oilfields is put forward in this paper, and pretty good application results have been obtained by taking this measure. The major study content and results are showed as following:1. The microscopic pore structure characteristics, throat figures, rock particles sizes and their effects on filtering flow in Fuyang oil layers are analyzed by means of cast iron slice, electron microscope scan, constant rate mercury penetration and nuclear magnetic resonance. Fuyang oil layers are featured by deep buried depth, tiny and closely arrayed rock particles and high clay content. Its porosity type and throat type is dominated by reduced intergranular pore as well as laminar, curved lamellar and bundled tubes, respectively. Secondary growth of quartz and feldspar, along with filling of epigenetic calcite and carbonate saline minerals, both cause the further reduction of porosity, more complex distribution of throat and poor connectivity, where the filtering flow in the reservoir does not follow the conventional Darcy law while is feature by non-Darcy flow. As result, the conventional parameters including reservoir porosity, permeability, grading factor and uniformity coefficient can not precisely describe the inherent features of ultra-low permeability reservoirs in Daqing marginal oilfields, also it could not meet the needs of development. The throat characteristics are the parameters that with critical effect on fluid flow law, in which the size of throat radius and its distribution are deciding factors, especially.2. The development type, growth scale, distribution characteristics of fissures and their effect on waterflooding development are studied by means of field outcrop, core observation and well logging. Natural fractures are generally developed in Fuyang oil layers, where tectoclase mainly developed in tectonic axis, near faults and on big curvature structures. Explicit fractures are dominant by high dip angle shear stitches. The eastern fractures are mainly in approximately latitudinal trending, and the western fractures in approximate EN trending, but differences exist in different oilfields and blocks. Some minerals and matrix are sliced in micro-fractures, most of which have been filled by calcites wholly or partially. The existence of micro-fractures has double influences on oilfield development. On one hand, it increases reservoir percolation capacity and reduces water intake threshold pressure; on the other hand, explicit fractures increase intrastratal heterogeneity and interlayer contradiction. The open of fractures has sharp directivity, and the open order is relevant to injection pressure. The fracture system belongs to high-speed non-Darcy flow. Therefore, much attention should be paid on fractures and ground stress during waterflooding and effective measurements should be adopted based on relevant study3. The fluid flow mechanism of low permeability reservoir is studied through seepage experiment. Single phase fluid flow experiment of oil and gas further determines the air permeability limit for formation of non-Darcy single phase fluid flow. The air permeability limit for non-Darcy aqueous phase fluid flow and oil phase fluid flow is 1×10-3μm2 and 5×10-3μm2, respectively. Single phase flow experiments with different fluid viscosities show fluid viscosity is an important factor for non-Darcy flow in addition to pore throat features. The higher fluid viscosity is, the larger quasi-threshold pressure gradient will be. Stable-state oil-water seepage tests for rock samples with different permeability show that quasi-threshold pressure gradient for two-phase simultaneous flow is much higher than that of single phase fluid flow. The connection between quasi-threshold pressure gradient and water saturation assumes a parabolic shape. Water flooding experiments show that producing indexes will continuously reduce along with the advance of water displacement front to the producers. The period with low or medium water cut is featured by high degree of reserve recovery and injection efficient as well, and it is the main production period of the oilfield. Once water breakthrough exists in oil wells, the water cut will be very high and the producing indexes may sharply reduce, hence increasing the difficulty of boosting flowing back so as to maintain a stable production.4. Based on non-Darcy filtering flow theories and reservoir engineering fundamentals, the stationary and non-stationary flow models for fluids in ultra-low permeability matrix-fissure and the non-stationary 2 phase flowing mathematic models for matrix-fissure coupled system are built. In the former research method, it is assumed that the fluids flow in the fissures is accordant with the Darcy flow, while according to the new model, it is assumed that the flow in the fissures is high velocity non-Darcy flow, which is more accordant with the actual flow state. Moreover, with the considerations of field application, the matrix-fissure coupled flow potential models under different development well patterns are established. Foi the first time, the model can systematically demonstrate the combination and coupling relations between the fracturing well pattern mode and fissure system, provides the development well pattern optimization, development fracturing design and flow potentia prediction with theoretical basis, increases the accuracy of development indexes prediction foi ultra-low permeability reservoirs and the establishment of development plans, and improves the research methods for oil reservoir engineering.5. According to the research on the filtering flow mechanisms of ultra-low permeability reservoirs and considering both form the change of the flow regime of the fluids in the reservoirs and the establishment of effective driving, the effective development strategy combining well pattern optimization with the fracturing for development, is proposed, namely enlarging the well spacing, reducing the well line distance, enlarging fracturing scale anc optimizing the fissures parameters for the development design of rectangular pattern, and this becomes the most important and effective development technique in the development o Fuyang oil layers in Daqing marginal oilfields. With the application of this technique, a new design method for well pattern optimization is set up, changing the invariable inverted 9-spo pattern water flooding mode existed for many years and effectively employing most part o reserves difficult to be produced before.