基于离散裂缝模型的页岩油储层压裂渗吸数值模拟

Numerical simulation of fractured imbibition in a shale oil reservoir based on the discrete fracture model

对于含黏土矿物较高的页岩油储层,地层水的矿化度可高达4.786×10^(3) mol·m^(-3),压裂过程中与注入的低矿化度压裂液形成的渗透压作用显著.为探究渗透压对渗吸的影响作用,建立了综合考虑渗透压和毛管力渗吸作用的油水两相二维离散裂缝网络模型,开展了页岩油储层压裂液泵注和关井阶段渗透压、毛管力、关井时间、盐浓度、膜效率、分支缝面积占比等对渗吸的影响规律研究.结果表明:①滤失主要由压力差、毛管力和渗透压3种机制驱动,其中压力差是滤失的关键控制机制;②关井时间对压裂液的渗吸作用影响较大,关井50 d时,前15 d渗吸量可达到总渗吸量的80%,且关井压力扩散会波及到两侧压裂段;③与压力扩散相比,渗透压达到平衡的时间较长,对于地层水矿化度为4.786×10^(3) mol·m^(-3)的情况,裂缝附近的矿化度达到600 mol·m^(-3)左右所需关井时间为50 d;④由于压力差是渗吸主要驱动力,页岩膜效率对渗透压力扩散影响微弱,页岩膜效率30%与5%相比渗吸量仅增加4%;⑤对于密切割压裂,关井后,含水饱和度受小间距水力裂缝控制,分支缝对渗吸含水饱和度的影响有限.

When a shale oil reservoir contains a mass of clay minerals,the salinity of formation water can reach up to 4.786×10^(3) mol·m^(-3) and the formation water and low salinity fracturing fluid create significant osmotic pressure during the fracturing process.To investigate the effect of osmotic pressure on the imbibition effect,a two-dimensional,oil-water,two-phase,discrete fracture network model was established.This model comprehensively considers osmotic pressure and capillary force.Additionally,a series of studies were carried out to explore the influence of osmotic pressure,capillary force,shut-in time,salt concentration,membrane efficiency,and the proportion of branch fracture area on the imbibition effect in shale oil reservoirs during fracturing fluid pumping and shut-in.The results show that:(1)Filtration is mainly influenced by pressure difference,capillary force,and osmotic pressure,and pressure difference is the key control mechanism of filtration.(2)The shut-in time has a great influence on the imbibition effect of fracturing fluid.The imbibition amount in the first 15 d can reach 80%of the total imbibition amount when the well is shut in for 50 d,leading to the shut-in pressure spreading to the fracturing interval on either side.(3)Osmotic pressure takes longer to reach equilibrium than diffusion pressure.Osmotic pressure takes 50 d to shut in the well and make the salinity near the fracture reach 600 mol·m^(-3) when the salinity of local layer water is 4.786×10^(3) mol·m^(-3).(4)As pressure difference is the main factor that affects the imbibition effect and the effect of shale film efficiency on seepage pressure diffusion is weak,the extent of imbibition increases by only 4%when the shale film efficiency increases from 5%to 30%.(5)Water saturation is controlled using hydraulic fractures through small spacing during shut-in,and the influence of branch fractures on water saturation is limited in intensive volume fracturing to horizontal wells.