页岩储层特殊性质对压裂液返排率和产能的影响

Study on the effect of gas-shale reservoir special properties on the fracturing fluid recovery efficiency and production performance

相对于常规储层,复杂的储层性质对页岩气井压后返排率高低和产能大小的影响更为显著。基于页岩气井早期生产返排特征,建立气水两相流数值模型,研究不同储层性质[超低含水饱和度、渗透率盲区(Permeability Jail)、润湿性和基质毛管力]对返排率和产能的影响。结果表明:在关井期间,储层"超低含水饱和度"现象导致压裂液大量渗吸,引起储层含水饱和度上升,最终使得初始开井产气量高,返排率低,且原始含水饱和度越低,影响越大;随着储层渗透率盲区程度增大,累产气量逐渐减小,强相渗盲区的储层生产3年仅有1.32×104 m^3的产气量。无相渗盲区(常规相渗曲线)的储层渗吸能力强,返排率极低(4.74%)。综合认为,弱相渗盲区储层的相渗曲线更适合页岩;储层基质水湿程度的降低会减弱渗吸置换能力,使得累产气量减小,返排率增加;储层基质毛管力的增大将导致渗吸作用加强,造成初始产气量升高,而在长期生产中,气相渗透率受到强渗吸引起的高含水饱和度的阻碍而降低,导致累产气量减少,并伴随着较低的返排率。

Compared with the conventional reservoir,the complicated property of gas-shale reservoir has a more significant influence on fracturing fluid recovery efficiency（RE） and production performance（PP）.A numerical simulation model about early gas-water two phase flow is established based on the typical data to study the effects of reservoir properties on RE and PP,such as subirreducible water saturation,relative per meability jail,wettability and matrix capillary force. The results,firstly,show that more fluidsareimbibed into the reservoir because of ＂subirreduciblewater saturation＂, resulting in the larger water saturation in- crement ,the higher initial gas production and the lower RE.However,the water saturation of matrix is still higher after 10 days shut-in if the original water saturation is higher at first causing less gas is produced in late stage.Secondly,the cumulative gas production（CGP） decreases with the severity of the ＂relative per meability jail＂, in which the CGP is only 1.32 × 10μm3 after 3 years production. Fewfluid is recovered （4.74%）if a conventional relative permeability is applied. Therefore, the weak relative permeability jail curve is more suitable for the tight reservoirs than other two types of relative permeability curves. What＇s more, the weak water wet formation reduces the imbibition and replacement leading to the decrease of CGP and the increase of RE. Finally,with the increase of capillary pressure,the higher gas production in early stage and the lower in late stage are revealed because of gas permeability impaired by high water saturation caused by the strong imbibition,resuhing in lower CGP and combined with lower RE.