Experiment Research On Weak Gel Profile Modification In Low Permeability Fracture Reservoir

In order to develop the low permeability reservoir, which matrix-fracture dual media characteristics reservoir property is poor and nature fracture is developed, fracturing is used to obtain the oil production and economic benefit. Because of the developed fracturing, water flooding problem is serious that the oil well water cut increases rapidly, and invalid water injection is serious and even sudden water flooding appears in oil well. Therefore, some measure should be taken for fracture to achieve stable oil production and water controlled.The average temperature of XF oilfield is 60 ℃, total dissoloved solid(TDS) of formation water is 60345 mg/L. What’s more fracture developed in the reservoir, which maximum fracture width is 2 mm, and most between 0.2 to 1.0 mm, causing part of oil well water cut rising quickly and appearing sudden water flooding phenomenon. Now, XF oilfield has been carried out large scale weak gel profile control to solve water problem and has got many important achievement. But there are still some significantly problems of water logging in oil well, and pilot is not ideal. The method of evaluation on profile control agent model in laboratory is often used in simple physical model porous medium like porous medium model or artificial fracture model which exist some problems that the degree of similarity to reservoir is low, and the parameters of fracture are difficult to quantify, and failed to reflect the improvement of matrix-fracture dual media respectively. Therefore, this thesis firstly studies the performance of weak gel profile control system, including the effect of shearing, salinity, aging time and the swelling on the properties of weak gel materials properties. Secondly, new physical model is designed and invented, based on similarity criterion. The new low permeability fracture model has many functions, including the variable fracture parameters, fracture and matrix flowing respectively in one model, simple operation, reusing, and high degree of simulation. We study the fracture parameters including fracture width, fracture area and fracture shape influence weak gel strength by using the new model and realize the main reason of low recovery by water flooding and its corresponding improvement method.The weak gel system performance evaluation results show that: polymer HPAM, the main agent of weak gel system, molecular weight and apparent viscosity will decrease slowly with the polymer concentration increasing by shearing; the apparent viscosity of weak gel is higher than 28000 mPa·s after aging for 180 d without shearing, but dehydration is occurring in weak gel after 25 d which deteriorate weak gel strength and stability by shearing and salinity; weak gel swell and its strength decreasing.The weak gel system displacement experiment results in the fracture physical model show that:Weak gel plugging strength decreases with the increasing of fracture width, and determined the critical fracture width is 0.69 mm. Weak gel has different plugging mechanism in different fracture width. When fracture width is less than critical width of 0.69 mm, the weak gel plugging strength is high because of weak gel viscosity and elasticity. But when fracture width is more than critical width, weak gel plugging strength mainly rely on their own internal friction or viscosity to increase the resistance of water flow, so as to improve the injection pressure, but limited. When the fracture width is 0.43 mm, the weak gel plugging strength and fracture area is proportional to the relationship that the unit area pressure is the same. Weak gel plugging strength is poor in straight fracture but the more complex of the fracture shape, the better ability to plug. The relationship between weak gel and fracture is complex, and the plugging strength is determined by weak gel adhesion and elasticity. Polymer is prone to adsorption on the surface of the medium in static condition and the weak gel formed with high adhesion and high strength plugging. But the ability of plugging decline range reaches as high as 35 times without adhesion. Weak gel plugging strength is reduced with the increasing of the fracture width, and the flow fraction in matrix is decreased and the dead oil area caused by no sweeping is increasing at the same time. After weak gel plugging, the sweep efficiency of subsequent water decreases from 100 % to 67.84 %, and the water flooding recovery decreases from 43.91 % to 29.28 %, because of water flowing in matrix by pass in wide fracture. With the increasing of matrix permeability, the matrix flow fraction is increased in 0.43 mm fracture where the weak gel plugging strength is high. The reason of recovery increasing is the low oil displacement efficiency in matrix permeability from 0.1 to 1.0×10-3 μm2 rather than sweep efficiency that is close to 100 %. The weak gel and pre-gel particle composite profile control system can effectively improve the plugging strength in wide fracture, reducing the possibility of water flow by pass, and making the recovery increasing from 29.28 % to 42.35 % in 0.98 mm fracture.