Study On The Influence Of Hydration On The Stability Of Hard And Brittle Shale

In recent years, with the increasing of demand for oil and gas resources, major oil companies have also increased shale gas exploration and development efforts. However, the problem of wellbore stability of brittle shale formations have restricted the development of deep oil and gas resources, especially the hydration influence for the wellbore stability of brittle shale. Therefore, in order to reveal the mechanism of wellbore instability of brittle shale formation, select the brittle shale from target area as the research object for experiment, including composition of mineral experiment, experiment of specific surface area, cationic exchange capacity test, XRD, SEM and triaxial compression test, with witch to study the influence of hydration on strength characteristics of the brittle shale from research work area. Based on the results from above, the chemical and mechanical of brittle shale can be conclusion, and analyze the reason of nature of brittle shale that resulting in the wellbore instability. And establish the collapse pressure calculation model of vertical wells based on the properties of brittle shale. And obtain the influence of collapse pressure with the hydration. And the results show:(1) The main mineral of brittle shale of target area is quartz and clay minerals, and illites-smectite mixed layer and illites are the top2minerals. Through there is no smectite appears in the brittle shale, it can show water-based expansion with the illites-smectite mixed layer based. Meanwhile, invasion to micro-fractures and micro structure can be obtained with polarizing high power microscope.(2) The influence of hardness and strength of brittle shale with hydration can be obtained through rock mechanics experiments. The results show that the hydration could reduce the strength and hardness of brittle shale, the decline range become weak with degree goes of hydration.(3) It can reflect the influence of brittle shale with hydration by the method of acoustic test and resistivity test. It shows that the acoustic and resistivity become smaller with time of hydration, which can reflect the changes of brittle shale internal structure with hydration. Combine with the results of microstructure observation, which shows shale micro-cracks appears with hydration and meanwhile it force the hydration.(4) Combine the rock strength parameters gained from triaxial pressure test with hydration and properties, mechanical of brittle shale, established vertical well borehole collapse pressure calculation model which could be used to analyze the influence of collapse pressure with hydration.The results show that the brittle shale is full of macro-cracks and bedding. Keeping mud with Strong blocking performance and the ability to control water loss when drilling is the key to avoid the downhole accident.