| Shale gas is the focus of unconventional gas exploration and development.Hydraulic fracturing is the key technology to develop these reservoirs.In the practice of shale fracturing,it is found that the flowback rate of shale fracturing fluid is lower than that of conventional reservoir,and the higher production and the lower fracturing fluid.The mechanism is that fracturing fluid enters shale result in a greater improvement in shale structure and physical properties,and fracturing fluid enters into the shale by spontaneous imbibition.At present,the domestic and foreign research on shale imbibition less.Therefore,this paper mainly studies the shale imbibition ability and its influencing factors,and understanding the imbibition laws of shale preliminary,providing guidance for shale fracturing and lay the foundation for the establishment of shale imbibition theory.In this paper,the shale of the Longma River Formation is taken as the research object,and the imbibition experiment is taken as the main means,and the imbibition theory is used to analyze.Firstly,the rock characteristics,such as pore form,pore size distribution,pore type,mineral composition and wettability,were analyzed experimentally.Based on the understanding of the characteristics of the rock,two kinds of imbibition methods,which do not consider the confining pressure and the influence of the confining pressure of the shale,are established.The effects of shale properties,imbibition liquid type and method on the imbibition ability were analyzed.Based on the imbibition model of Hu and so on,a imbibition curve feature prediction method is established.Based on the experiment,the effects of imbibition on microscopic pore structure,microcrack and physical properties were analyzed.Finally,according to the experimental understanding,the influence of shale imbibition on the flowback of fracturing fluid is analyzed by combining the shale characteristics of different blocks and the actual flowback data of fracturing fluid,and the technological measures to improve the imbibition ability of shale are put forward.Through the experimental study of this paper,the following main conclusions and understanding are obtained:(1)Two imbibition experimental devices with no confining pressure and confining pressure are designed,and the imbibition evaluation method is proposed.(2)The effects of shale properties on the imbibition ability were studied.It was found that the imbibition ability was parallel bedding>vertical bedding,a large number of initial microcracks>no initial microcracks;the smaller the sample size,The higher the imbibition capacity of the rock sample is,the higher the imbibition capacity of the rock sample is.The imbibition capacity of each liquid is distilled water>16%KCl solution>broken glue>linear glue>8%KCl solution>slick water>2%KC1 Solution>kerosene,and downstream imbibition>countercurrent imbibition.(3)The imbibition curve is mainly divided into "S" type("S" curve),"L" type(straight line)and "A" type(curved curve).The relationship between pore size distribution,time index and self-priming curve was established.(4)Based on the qualitative and quantitative analysis of the influence of imbibition on micro-porosity of shale.The higher the imbibition ability,the greater the percentage of shale mass loss and the larger porosity and permeability;The results show that the stronger the imbibition ability,the less the microfractures time in the imbibition process.The induced microfractures time is defined,and the stronger the imbibition ability is,the smaller the induced microfractures time is.(5)Combined with imbibition experiments and flowback results of field,found that the higher the shale imbibition ability,the lower the flowback rate,the higher the production.The better the shale imbibition ability of the montmorillonite or imidone layer,the natural fracture development and the lower stratigraphic skeleton stress,and the higher the glue ratio,the remodeling volume and the closed time will contribute to imbibition of shale. |
