| "Wanfang liquid and thousand thousand sands" are often used in the fracturing process of shale gas Wells,but the flowback efficiency of shale reservoirs is very low.For example,the flowback efficiency in North America's shale reservoirs is about 30%,the flowback efficiency in Weiyuan block is about 28%,and that in Fuling block is less than 10%.The reason for this phenomenon is that shale reservoirs will suck a large amount of fracturing fluid by themselves during fracturing.At present,the understanding of the phenomenon and mechanism of "low backflow rate" in shale gas wells is not clear enough,and there is a lack of quantitative analysis and evaluation prediction of spontaneous imbibition ability of shale.Further research is needed to better guide the system arrangement of field fracturing and soaking operation.On the basis of previous studies on spontaneous imbibition of shale,the outcrop shale of Longmaxi Formation in Weiyuan block of Sichuan Province is taken as the main research object,and the outcrop shale of Zhaotong,Yunnan,Pengshui,Chongqing and downhole core of Fuling are taken as the comparative research object.The characteristics of shale in Weiyuan block are studied by means of mineral composition,TOC content,wettability,pore permeability parameter test,scanning electron microscopy and low pressure nitrogen adsorption method.On this basis,the theoretical model of shale spontaneous imbibition is established and the static spontaneous imbibition experiment was carried out in the laboratory.The spontaneous imbibition mechanism of shale gas well in Weiyuan block was comprehensively analyzed.The main understandings are as follows:(1)The reservoir characteristics of shale in Weiyuan block make it have strong spontaneous imbibition conditions.Horizontal bedding of target reservoir is well developed.Shale belongs to black carbonaceous graptolite shale with abundant clay mineral content(13.6%).Its initial water saturation is extremely low.Shale surface and pore ware mixed wetting.Its pore structure is complex and micro-nano pore is well developed.It is a typical low porosity(3.01%?6.93%)and ultra-low permeability(0.0017?0.0397 mD)reservoir;(2)The pore morphology of rock samples studied by low pressure nitrogen adsorption method is mainly circular and parallel plate holes.Parallel plate holes conform to the characteristics of clay pore morphology.Considering that shale has the characteristics of nonideal semi-permeable membrane,there is a large osmotic pressure between fluid and shale matrix,and the particularity of spontaneous imbibition force of shale in Weiyuan block is one of the mechanisms of its strong spontaneous imbibition ability.(3)Based on the traditional mathematical theory of capillary force spontaneous imbibition,considering the effects of boundary slip,clay osmotic pressure and capillary force in micro-nanopore flow,the water phase flow equations of circular and parallel plate holes and the theoretical models of shale spontaneous imbibition in two different forms were established.(4)Through static spontaneous imbibition experiments of different liquids in laboratory,the static spontaneous imbibition ability of shale was quantitatively analyzed.By analyzing the changes of porosity and permeability of rock samples before and after spontaneous imbibition,the influence degree of different liquid immersion on physical properties of shale was obtained.(5)The micro-pore characteristics of shale immersed in different liquids were analyzed by scanning electron microscopy.It was found that the induced micro-cracks were produced in the immersed samples.(6)By comparing static spontaneous imbibition experiment with spontaneous imbibition theoretical model,the validity of the theoretical model is verified.The total suction volume and return rate of the actual well in the target block are further evaluated and the reasonable soaking time is determined,which has a good guiding significance for field fracturing operation. |
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