Water Producing Mechanism Of Horizontal Well For Ultra-low Permeability Reservoirs

Horizontal well is one of the most effective methods to develop ultra-low permeability reservoirs because it can increase flow area between reservoir and well. However, water producing in horizontal well is one of the most serious problems in the use of this modern technology. Therefore, it is vital to understand the mechanism of water producing, especially in the use of fracturing horizontal well. This paper systematically studied original distributions of oil and water, and waterflood features both in the micro-and macro-scale of the ultra-low permeability reservoirs rock cores taken from the Chang 8 formation in the Ordos basin using the fluorescence microscopy and nuclear magnetic resonance(NMR) techniques. Meanwhile, effect of reservoir heterogeneity on water producing was also studied in this paper. A novel method studying mechanism of water producing was finally established in this paper. The main research findings are summarized as follows:1. The micro-pore structure and original distributions of hydrocarbon fluid in ultra-low permeability reservoir was qualitatively and quantratively studied using NMR and fluorescence microscopy techniques. The (?) results indicated that reservoir rock pore-throat is fine but the micro-p(?) better sorting. Original distribution of water and oil is obviously non-uniform. The original water saturation is quite high and there is a certain amount of movable water in the pore space. Consequently, flow capacity of the crude oil is extremely less than that of the formation water. Microscopic image is able to explain the trend of NMR T2 spectrum. Combination of the above two techniques is an effective method to study the micro-flow mechanism in porous media.2. Waterflood experiments discovered that oil can be produced from rock matrix pore space by water flooding and imbibition. The higher the rock permeability, the higher the recovery efficiency by water flooding action. Pressure gradient has a great influence on the water effective permeability at residual oil saturation. But it little effect on oil recovery efficiency of the high permeability reservoir. Most of the oil was produced by imbibition in fractured reservoir rocks. It was also found that waterflood pressure gradient has a great effect both on the oil recovery efficiency and water effective permeability at residual oil saturation.3. The features of water cut increasing were comparatively studied both in homogeneous and heterogeneity reservoir using the parallel rock plug waterflood experiments. There are three regions in the water cut curve of the homogeneous reservoir named water free oil production stage, water cut rising stage and high water cut stage, respectively. Those factors including reservoir permeability, original oil saturation and oil displacement efficiency all have effect on the water cut curve. Unlike the homogeneous reservoir, the water cut rising process of the heterogeneity reservoir can be divided to four stages. They are free water oil production stage, water cut rising stage, high water saturation rising stage and high water cut stage. The features of water cut curve were influenced by the average permeability and permeability ratios of the heterogeneity reservoir. Additionally, the features of water cut rising were also affected by fractures in the fracture reservoir. The water cut rising process can be divided to three stages, named stable water oil production stage, water cut rising stage and high water cut stage when oil bearing formations are connected with aquifers. The water cut rising process was simultaneously influenced by the original water distribution in oil bearing formation and the permeability ratio between oil bearing formation and aquifer. Generally, water cut at the early stage is high when oil bearing formations are connected with aquifers.4. A novel experimental method was established in this research during which the rock cores are used to simulated the depletion development process. The research results suggested that the water cut was influenced by permeability ratio between oil bearing formation and aquifer as well as the permeability between oil bearing formations. It was also discovered that the water cut is stable during the early depletion development. But the water cut will change with the decrease of reservoir pore pressure due to reservoir rock permeability stress-sensitivity and insufficient fluid supply.