Regularity And Mechanism Research On The Dynamic Change Of Reservoir Fractures

With the deepening development of the gas and oil field, people are increasing taking the researches on fractured reservoirs. Among them, correctly grasp the dynamic changes in cracks has already became the key point to protect a kind of oil and gas reservoirs and to maximize the oil and gas recovery. In exploitation process, the law of the existing research of the fracture with reservoir fluid pressure changes focuses on three areas:â‘ experimental study of permeability stress sensitivity in fractures;â‘¡researches on closure laws of fractures through theoretical calculation, which based on the convex model of fracture surface;â‘¢analysis of closure laws of fracture by means of finite element, which based on the elastic-plastic contact mechanics model.Present experimental study is macro-easy to achieve, numerical calculation to quantify the benefit of a particular rock in the specific circumstances of the closure pressure study law, but there are some uncertainties on clearly understanding of the mechanism; micro-convex model facilitate the mechanism researches, but superficially study and difficult in characterization and description of different types of the protruding surfaces result in the disparity between theoretical knowledge and practical application. In view of this, the paper takes the researches on change law and mechanism of fracture permeability and aperture with a combination of fracture stress sensitivity experiment and rock deformation experiment. Through experiment study and theoretical analysis we can obtain the following results:(1) The change law of fracture porosity and permeability under pressures is similar to the matrix porosity and permeability under the same stress conditions, that is, with the increase in pressure, fracture porosity and permeability decreased gradually, and at a relatively low pressure the change rate is larger; with the proceeding load, the change rate would slow down.(2) For matrix core, the change regularity of fracture porosity and permeability is much more complex due to the common effects of surface roughness, the initial opening of fracture, mineral composition of crack plane.(3) The deformation magnitude of fractured core from experiment behaves highly non-linear and curved upward, it means that fracture deformed larger under a lower normal pressure, the slope of curve is smaller and increases with the proceeding load. The non-linear characteristics at the beginning of the deformation curve mainly due to the damage of convex contact surface, and later as a result of non-linear elastic deformation owing to the increase of contact surface with increasing normal load. The relationship between fracture permeability and pressure is affected by the change law of crack deformations under different pressures.(4) Matching the data of fracture under different loads with semi-logarithmic equation, power function equations, exponential equations and an improved two indices equations, the matching result tells us that two indices is the best for test, and the correlation coefficient is above 0.99. So, two indices equations can effective characterize the relationship between closure magnitude and pressure.(5) This paper used sensitive coefficient of fracture opening to evaluate the sensitivity of fracture permeability and porosity according to the couple model of fracture opening and effective stress, and combined with the relationship between fracture permeability, porosity and fracture opening respectively. The result tells us that the sensitive coefficients of fracture opening, permeability and porosity are both controlled by the initial fracture opening, rock elastic modulus, Poisson's ratio and effective stress.(6) This paper conducted the deliverability equation of high-pressure gas wells and low-pressure gas wells which considered the stress sensitivity, according to the power function of fracture permeability and effective stress which take into account the effect of stress sensitivity on reservoir deliverability. The result shows that stress sensitivity seriously effects the deliverability of fractured gas reservoir, and the deliverability equation of low-pressure can better evaluate the effect of stress sensitivity on gas production.