The Characteristics Of Water Uptake And The Comparative Studies On Porosity Measurement In Organic-rich Shale Of Longmaxi Formation In Sichuan Basin

Using the shale core samples of Silurian Longmaxi Formation from Qianqian#1well in Chongqing southeast and the fresh outcrop shale samples of the Longmaxi Formation from Shuanghe town,Changning county,in Sichuan province,an experimental investigation of water uptake on shale was conducted.Measuring the saturated water uptake quantities of a series of shale cylinders with different diameters and lengths is used to build a linear relationship with their skeleton volumes.The slope value of this linear equation is called saturated water quantity per unit skeleton volume,represented with ?K? value.From this,a new method to measure the shale porosity is presented.In addition,by analyzing the water sorption curves of the shale cylinders,build a model for simulating the water uptake process of shale cylinders.By comparing the shale samples' water uptake porosity with the porosity from other method and exploring the main controlling factors of water uptake porosity changing,get some recognitions and conclusions like these:(1)The water uptake experiment can measure the porosity from micropores to macropores(with apertures larger than 350 nm)in shale.The porosity value is a statistical result from many shale cylinders with different sizes,so it is influenced slightly by the shale heterogeneity and can represent the porosity of the whole shale more accurately.(2)The 10 shale samples' water uptake porosity are in the range from 3.51% to 8.90%,and the porosity from nitrogen adsorption,helium gas expansion and nuclear magnetic resonance(NMR)are in the ranges of 2.38% ? 10.39%,4.55% ? 8.09% and 2.85% ? 5.19%,respectively.The water uptake porosity is usually greater than the nitrogen adsorption porosity,their difference is by 0.06% ? 1.96%;it is also greater than the NMR porosity,and the difference is by 1.72% ? 2.73%;it is slightly larger than the helium porosity,they differ by only 0.23% ? 0.81%,and there is a good consistence between them.More importantly,the water uptake experiment can discern the microfracture in the shale cylinder,which is an advantage that other methods without it.(3)When a series of shale cylinders with different diameters or lengthes from the same sample absorbed water to be saturated,all the water sorption curves converged and indicate that the pore connectivity of organic-rich shale of Longmaxi Formation in Sichuan basin is very well.The water uptake model which is seted up according to first-order kinetics equation,the empirical relationship between the bibulous rate constant ?k? and the shale cylinder diameter,can simulate water sorption curves of the shale cylinders nicely,meanwhile,they can forecast the durations of different diametral shale cylinders reaching a balanced water uptake,so it provide an empirical mode for researching the depth and duration of the fracturing fluid infiltrating to the matrix pores in shale.(4)The porosity of shale cylinders have a good positive correlation relationship with total organic carbon(TOC),suggesting that TOC mainly control the change of porosity.Meanwhile,some minerals like clay and quartz,control the formation of microfracture and inorganic pore in shale,so,they also have an important influence to shale porosity.