岩石水和气体压裂破裂压力差异的理论和试验研究

发布时间：2019-05-31 16:33

【摘要】：随着我国经济的发展,对能源的需求量日益增大,且国内页岩气具有良好形成及富集的地质条件,因此页岩气的开采受到广泛的关注。页岩气藏储层具有典型的低孔、低渗特征,并随着埋深加大,物性变差,导致开采难度大,采收率低。但通过人工增透的方法改变储层的渗透性可以有效地提高采收率。水力压裂是人工增透的重要方法之一,但在实际实施过程中也发现了水力压裂的不足之处,压裂液的摩阻较大,易滞留在孔道的裂缝里,易产生水锁、贾敏效应,使储层渗透率降低;压裂结束后,压裂液反排困难,会对储层造成二次伤害,降低压裂效果;而且页岩矿物成分中含有较多的伊利石、高岭石、伊蒙混层等粘土矿物,这些矿物极易吸水膨胀,使裂隙闭合,采收率下降。最重要是我国页岩气丰富的地方大多又缺水,所以无水压裂技术得到大家的广泛关注,利用气体进行储层压裂改造有望取得进展。本文通过试验室试验、理论分析和数值模拟的方法系统地研究了利用水和气体压裂时裂缝启裂的差异,主要取得以下进展:(1)利用断裂力学、渗流力学和渗流应力耦合理论建立了基于水、气渗透差异的破裂压力准则模型。在气体渗流场计算中,考虑气体密度在空间上的变化对气体质量守恒方程的影响,推导出了新的气体渗流场计算方程。(2)设计了破裂压力的数值计算方法,并通过Comsol with Matlab编写了能够实现渗流边界随时间扩展的渗流应力耦合计算程序。(3)设计了不同水压和气压加载速率下红砂岩的水和气体压裂试验,试验发现在一定加载速率范围内加载速率对水和气体的破裂压力及试样的体积应变增量影响都较小,且气体压裂的破裂压力比水力压裂的破裂压力小40%左右;气体压裂后的试样破坏的更为严重,形成两条或三条主裂纹,裂缝会贯穿整个试样,且裂纹宽度较大,主裂纹周围有很多分岔裂纹;而水力压裂后的试样破坏的程度较小,只形成了两条对称裂纹,且裂纹没有贯穿整个试样,裂纹宽度较小。将数值计算出的水和气体的破裂压力和破裂时间与试验值比较,发现该计算模型可以较好地预测破裂压力与破裂时间。(4)利用数值计算的方法,模拟并分析水压和气压加载速率、储层的渗透率、孔壁裂缝长度和不同深度的地应力对水气压裂的影响,发现由于气体粘度小,易于渗透,所以在不同条件下气体压裂都有着明显的优势。
[Abstract]:With the development of economy in our country, the demand for energy is increasing day by day, and the shale gas in China has good geological conditions of formation and enrichment, so the exploitation of shale gas has received extensive attention. Shale gas reservoir has typical characteristics of low porosity and low permeability, and with the increase of buried depth, the physical properties become worse, which leads to the difficulty of exploitation and low recovery. However, the oil recovery can be effectively improved by changing the permeability of the reservoir by means of artificial penetration enhancement. Hydraulic fracturing is one of the important methods of artificial penetration enhancement, but in the actual implementation process, it has also found the shortcomings of hydraulic fracturing. The friction of fracturing fluid is large, it is easy to stay in the cracks in the channel, and it is easy to produce water lock and Jia Min effect. Reduce reservoir permeability; After fracturing, it is difficult to reverse the fracturing fluid, which will cause secondary damage to the reservoir and reduce the fracturing effect. Moreover, shale minerals contain more clay minerals such as Illite, Kaolinite, Yimeng mixed layer and so on, which are easy to absorb water and expand, closing fissures and decreasing oil recovery. The most important thing is that most of the places where shale gas is rich in China are lack of water, so the anhydrous fracturing technology has been widely concerned, and progress is expected to be made in reservoir fracturing transformation by using gas. In this paper, the difference of fracture initiation between water and gas fracturing is systematically studied by means of laboratory test, theoretical analysis and numerical simulation. The main progress is as follows: (1) using fracture mechanics, Based on the coupling theory of seepage mechanics and seepage stress, a fracture pressure criterion model based on the difference between water and gas permeability is established. In the calculation of gas seepage field, considering the influence of gas density on the conservation equation of gas mass, a new equation for calculating gas seepage field is derived. (2) the numerical calculation method of fracture pressure is designed. The coupling calculation program of seepage stress which can realize the expansion of seepage boundary with time is compiled by Comsol with Matlab. (3) the water and gas fracturing tests of red sandstone under different water pressure and pressure loading rates are designed. It is found that the loading rate has little effect on the fracture pressure of water and gas and the volume strain increment of the sample in a certain loading rate range, and the fracture pressure of gas fracturing is about 40% lower than that of hydraulic fracturing. After gas fracturing, the specimen is destroyed more seriously, forming two or three main cracks, the cracks will run through the whole sample, and the crack width is large, and there are many bifurcation cracks around the main cracks. However, the failure degree of the specimen after hydraulic fracturing is small, only two symmetrical cracks are formed, and the crack does not penetrate the whole sample, and the crack width is small. By comparing the fracture pressure and fracture time of water and gas calculated by numerical method with the experimental values, it is found that the model can predict the fracture pressure and fracture time well. (4) the numerical calculation method is used. The effects of water pressure and pressure loading rate, reservoir permeability, fracture length of pore wall and ground stress of different depths on water-gas fracturing are simulated and analyzed. It is found that the gas viscosity is small and easy to permeate. Therefore, gas fracturing has obvious advantages under different conditions.
【学位授予单位】：中国矿业大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TE377

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