Chemical-Mechanical Modeling Of Borehole Stability In Shale

Wellbore instability of shale is a complicate and worldwide problem, and manyresearch works have been done from 1940's about this issue. To 1990's, several mathmodels coupled with multi-fields have been developed, and accompany these models,the research went forward step by step. But, because the complexity ofmulti-fields-coupled problem, many parameters in these models can't be measured inlab, that decreased the reliability and applicability of models seriously. On the otherhand, the measuring method of membrane efficiency is not correct, because theflowing of fluid medium in shale is droved by the force coupled hydromechanics withchemical energy in the whole experimental run, but in fact, while calculating themembrane efficiency, it is taken grand that the flowing is droved by chemical energyonly, that is proved to be wrong by the experiment in this paper.It is important to developing the correct measuring method of membraneefficiency, and important to building up such a reliable and applicable boreholestability model that all parameters can be measured easily.The main matters studied in this paper and correlative results are follows:1. Deepened the theory of borehole stability coupled with multi-fields. A newborehole stability model coupled hydromechanics with chemical energy is developedfor shale, and the measuring methods of main parameters are introduced. Because allthe parameters in this model can be measured easily in lab, so the practicability of thismodel is increased .2. A new measuring method for membrane efficiency is developed in this paper.The flowing of fluid medium in shale is simulated by the model coupledhydromechanics with chemical energy developed in this paper, and then the membraneefficiency is calculated through inverting the parameter of model.3. The relation of strength and water content is studied in lab. Some conclusioncan be got that: cohesive force and angle of internal friction decrease with the watercontent's increasing. And further more, the relation of cohesive force and angle ofinternal friction with time and position are developed.4. An apparatus is developed to study the pore pressure propagation in shale coupled hydromechanics with chemical energy. The model's practicability andreliability are demonstrated by comparing the computational solution and themeasurements in lab.5. Affection of some sensitive parameters to borehole stability of shale areanalyzed. Some conclusion can be got that lower water activity of mud and highermembrane efficiency are benefit to borehole stability. The affection of cohesive forceand angle of internal friction are obvious and even vital.6. The effect of field application is introduced. Some works are done thatoptimized the drilling fluid, calculated advisable mud density. By the comparison ofthree kinds of drilling fluid, conclusions can be got that valchovite mud is the best andsilicate mud is the next and Polyacrylamide mud is the worst.Borehole stability of shale studies is a large system, this paper only improves theborehole stability model coupled hydromechanics with chemical energy and themeasuring method of membrane efficiency, there still have some issues need to deepenin future work.