A Model Study of Fluid Filtrate Diffusion into the Shale Membrane

ABSTRACT: We have proposed and studied a well-bore Stability model (a modified Failure Criterion), taking into account shale problems, effect of temperature, vibration of drill-string, drilling fluid jet impact, osmotic pressure, dynamic pore pressure ahead of the bit, and capillary pressure. In our laboratory, we conducted a series of tests on the effect of caustic soda on the surface tension, resistivity, and capillary pressure of the pore fluid. In addition, we generated a computer program in order to solve two dimensional diffusivity and dynamic pore pressure equations using Darcy's laws simultaneously. Moreover, in this model we included the effects of vibration of drill-string and drilling fluid et impact on diffusion processes of filtrate into the shale membrane. Results show that the amount of invasion of drilling fluid into the drilled formation immediately ahead of drill bit is significant. Our study of the model shows that the higher than usual filtrate diffusion into the shale membrane occurs when there is not enough time for the mud cake to be formed at the rock-bit interface and the formation immediately ahead of the drill bit is pressurized by the sum of several pressures. The sources of these pressures are circulating mud, bit jet, weight-on-bit, and bit torque. While diffusion of drilling fluid occurs, caustic soda (NaOH) in the drilling fluid filtrate mixes with the pore fluid in the pore space. Depending on the concentration of caustic soda in the drilling fluid filtrate and permeability of the formation, the above mentioned mixture causes a tremendous reduction of surface tension and capillary pressure of the pore fluid in the pore space. This phenomenon enhances rapid diffusion of fluid filtrate into the shale membrane. Results also show that diffusion of the drilling fluid filtrate in the formation during dynamic and static periods is mainly controlled by osmotic pressure and adsorption of the drilling fluid's components on the rock grains.1. INTRODUCTIONLiterature Review. Before presenting the proposed model, the authors would like to focus on the previous models which have been studied mainly in recent years, with concentration on well-bore stability in shaly formations. Our proposed model will also be explained in more detail and the results will be discussed in the later sections. After studying the nature of shale and its properties, Apande (1980)¹ presented his model. Theoretically, he had worked on a model proposed by Hayatdavoudi which would connect some of the shale properties such as capillary and swelling pressures to well-bore stability considerations. Although his model did not include most of the other parameters like capillary pressure, the effect of temperature change, the effect of vibration of drill-string and pulsation of bit jet hydraulic, his model is still more realistic as compared with other works. His approach to the problem had a solid foundation. Huang et al. (1998)² presented a model based on considerations that the pore pressure change and the onset of swelling pressure are two main factors in well-bore instability issue.