In acidizing operations, the acid flows selectively through large pores to create wormholes. Wormhole propagation has been studied by many experts at macroscopic scale. In this paper, the lattice Boltzmann model (LBM), which is a mesoscopic scale method, is adopted to simulate the flow, acid–rock reaction, and rock dissolution in porous media at mesoscopic scale. In this model, a new method based on nonequilibrium extrapolation is proposed to deal with the reactive boundary. On the basis of the model, extensive simulations are conducted on the propagation behavior of wormholes, and the factors influencing wormhole propagation are investigated systematically. The results show that the LBM is a reliable numerical technique to study chemical dissolution in porous media at mesoscopic scale, and that the new method of dealing with the reaction boundary performs well. The breakthrough time decreases with the increase of acid concentration, but acid concentration does not affect the ultimate dissolution pattern. As the reaction rate constant increases, shorter wormholes are created. A higher hydrogen ion diffusion coefficient will result in shorter but wider wormholes. These findings agree well with the previous experimental and theoretical analyses. This study demonstrates the mechanism of wormholing that the unstable growth of pores by the acid rock reaction makes the acid selectively flow through a few large pores which finally form wormholes.

页岩储层孔隙度和渗透率低,岩石脆性大,天然裂缝发育.大规模水力压裂过程中,页岩储层易发生张性破坏和剪切破坏相结合的复合破坏,形成复杂网状裂缝,而网状裂缝的展布形态关系到页岩压裂方案的设计和压后产能的评估.针对页岩储层网状裂缝扩展问题,基于流-固耦合方程和损伤力学原理,建立了二维网状裂缝扩展有限元模型.综合研究认为:水平主应力差增大,压裂裂缝分布长度增加,分布宽度降低,长宽比增大;压裂施工排量降低,压裂裂缝复杂程度降低,压裂裂缝分支数减少,分布长度增加,分布宽度降低,长宽比增大;水平井"多段分簇"压裂可能出现多裂缝干扰问题,部分压裂裂缝在延伸过程中止裂或者沟通两端射孔簇的压裂裂缝;水平井"多段分簇"压裂过程中,各个射孔簇形成的压裂裂缝分支长度差别较大,部分裂缝分支长度明显大于其他裂缝分支.

Due to the highly developed natural fractures and horizontal beddings in shale reservoirs, it is possible to generate complex volume fractures during hydraulic fracture treatment. In order to investigate the propagation process of complex fracture network, a three dimensional finite element model for volume fracture propagation of hydraulic fracture in shale reservoirs is built using fluid-solid coupling basic equations in porous medium and basic theory of damage mechanics. The numerical simulation results are in good accordance with laboratory experiment of shale fracture propagation, which prove the reliability of the numerical model. After a series of numerical simulations, some conclusions are drawn as follws: (1) The horizontal beddings can open and form horizontal fractures in hydraulic fracturing. They can intersect with vertical fractures and finally generate complex volume fracture network. (2) As the horizontal stress difference increases, the length of stimulated reservoir volume (SRV) can increase and the width can decrease, which means the ratio of length to width of SRV can increase. The length is the distribution distance of volume fracture along horizontal maximum principal insitu stress. The width is the distribution distance of volume fracture along horizontal minimum principal insitu stress. (3) As the pump rate of fracturing increases, the SRV length can decrease, width can increase, and the ratio of length to width of SRV can decrease accordingly. (4) When the residual tensile strength of the natural fracture becomes stronger, the width of SRV can decrease, the length and the ratio of length to width of SRV can increase. Research results can offer some references for hydraulic fracture design and operation of shale gas in China.

对塔河碳酸盐岩岩心进行了一系列围压、孔隙压力条件下的三轴压缩测试，分析结果发现：①低围压时，碳酸盐岩石表现出很强的脆性，抗压强度低；高围压时，碳酸盐岩石表现出强塑性，且可能出现塑性硬化。②碳酸盐岩的弹性模量随围压的增大而增大，抗压强度随孔隙压力的增大而降低。③孔隙压力增大，碳酸盐岩的内聚力增大，内摩擦角降低。

Multi-cluster staged fracking is a key technology to develop low permeability and unconventional reservoirs, with which, the stimulated reservoir volume (SRV) can be increased significantly, so as to enhance the oil or gas production and ultimate recovery. In order to study the interference between fractures of different clusters in multi-staged fractured horizontal wells, a three-dimension finite element model for simulating the propagation of fractures was established. The model was theoretically based on fluid-solid coupling equations in porous medium and damage mechanics, and zero-thickness cohesive element was adopted to simulate the rock damage during the initiation and propagation of artificial fractures. Factors affecting the inter-cluster fracture interference in multi-cluster staged fracking, including perforation cluster numbers, cluster spacing, reservoir and treatment parameters, were researched by a series of simulations. The results showed that the perforation cluster numbers and spacing are two primary factors. Based on the model, the cluster spacing of a horizontal well was optimized. As a result, the production of this well was distinctively improved compared with that of the adjacent well, which proves that the model is feasible and reliable in optimizing the design of multi-cluster staged fracking for horizontal wells.