Organic-rich Shale Alkali Solution Erodes Wellbore Instability Mechanism

Wellbore instability in rich organic shale is a common problem during development of shale gas reservoir, seriously restricting the shale gas in well drilling and completion process. At present, the use of oil base drilling fluid controlling wellbore instability. However, according to statistics, the wellbore instability problem is still serious. In order to keep the good rheological property of drilling fluid and the activity of various treatment agents, the oil based drilling fluids are usually alkaline in order to absorb the hydrogen sulfide during the process of the exploitation. Previous research has confirmed the lye will erosion siliceous clay minerals and quartz, feldspar and other minerals. Therefore, it is necessary to in-depth study of shale gas in well drilling and completion process of alkali erosion effect and borehole wall loss of the relationship between the stability and for the study of new strong inhibition of anti-sloughing water-based drilling fluid to provide new ideas.In Sichuan Basin Longmaxi group organic rich shale as the research object, of Longmaxi group of organic rich shale potential borehole losing stability factors were analyzed. To carry out the alkali erosion changes in mechanical strength, alkali erosion crack expansion, alkali erosion clay minerals, alkali erosion experiments of hydration expansion. In dynamic theory as the foundation, explores the factors of influence of lye and shale rate; through carry out hot shale mud soak experiment, summarized the high pH drilling fluid under the action of rich organic shale borehole instability mode and put forward the control method.To deepen the understanding of Sichuan basin Longmaxi organic rich shale potential borehole instability factors. Longmaxi organic rich shale clay mineral content is about 35%, quartz and feldspar mineral content is about 50%. A large number of micro cracks are developed between the edges of the clastic particles and the mica sheets. The development of a large number of nano-micro pore, the average pore throat diameter of 5.87nm, mainly in the nanometer-grade pores as the main. A lot of nano-micro pores and micro cracks for lye into the reservoir seepage channel, siliceous clay minerals and quartz, feldspar and other minerals can react with alkali solution, namely has the potential alkali erosion wellbore instability factors.It is Revealed that the alkali erosion mechanism of rock mechanics of shale. Lye in shale reservoir, shale fractured wall surface, the tip of the clay minerals of erosion on the one hand, resulting in crack extension, on the other hand, the alkali enhanced shale swelling capacity and the resulting hydration expansion force to achieve the degree of expansive shale, also contributed to the crack expansion, communication. Alkali erosion and hydration expansion leads to the cracks in the shale expansion, the formation fracture network and shale cutting and macro performance shale rock strength is decreased.The dynamics equation was established by erosion and alkali activation energy equation. Kinetic equation shows that alkaline pH value is higher and higher, greater reaction rate, activation energy equation shows that as the temperature increases the reaction rate increased. According to the curve of reaction rate and temperature, the reaction is divided into slow reaction stage and rapid reaction stage, and the temperature is higher than 70. Longmaxi formation temperature 82 DEG C, the drilling fluid pH value is 11. Therefore, the reaction between the drilling fluid and shale in periods of rapid reaction, the reaction rate greatly.Summarized and described the borehole instability mechanism under the effect of high value drilling fluid pH. Clear lye erosion process, raised the possibility of KC1 weakened lye erosion and experiments. Experimental results show that potassium has the ability to weaken the erosion of lye. In the clear lye erosion mechanism on the basis of geological parameters to obtain, on-site engineering process design, drilling fluid system design three summarizes the wellbore instability lye erosion control method.