Characterizing and Modeling of Ultra-Soft Clay Soil, Filter Cake and Drilling Mud

In this study, ultra-soft soils representing the deepwater seabed offshore, coastal soils, and onshore soils with filter cakes and drilling muds, was characterized using new non-destructive in-situ test methods and modeling of the behavior. The new test methods to characterize the ultra-soft soils included the two-probe electrical method and CIGMAT miniature penetrometer. The clay content in the ultra-soft soils, filter cakes and drilling muds investigated in this study varied from 2% to 10% by weight. The type of clays investigated include montmorillonite (bentonite) and kaolinite. The shear strength of the ultra-soft soils varied from 0.01 kPa to 0.30 kPa using the modified vane shear test. Electrical characterization of the ultra-soft soils identified the soil to be a resistive material. Several modifiers such as lime, polymer, sand, and cement were used to treat the ultra-soft soils. The effect of the modifiers on the shear strength, electrical resistivity, water content, density, and electrical impedance were investigated. The shear strength of the treated ultra-soft soil had the highest value of 6.8 kPa, a change in shear strength of 2167%, with 10% polymer treatment. Electrical resistivity was correlated with the solid content, shear strength, and water content for treated and untreated ultra-soft soils. Experimental, analytical, statistical, and finite element methods were used to model the stress-strain relationship of the ultra-soft soils. Filter cake formation and fluid loss occur concurrently during various engineering operations including during oil well drilling is influenced by the seepage and consolidation of the cake. A new coupling continuous function with time and depth variables was developed to represent the combined seepage-consolidation phenomenons during the filter cake formation under different pressures and temperatures. The new continuous function solution was compared with Terzaghi discrete consolidation solution and both solutions were verified using several experimental results. Currently, filter cake is modeled using the API method where the cake properties are assumed to be constant but the cake thickness varies with time. In the new kinetic model developed in this study, variations of fluid loss, porosity, permeability, relative solid content, and cake thickness with time have been included. The new kinetic model also takes into account the effects of both high pressure and high temperature. Also, the new kinetic model has a limit on the maximum amount of the fluid loss, however, the API method predicts the maximum fluid loss to be infinity. The prediction for both API and new kinetic models were verified using several high pressure and high temperature test results from the current study and reported literature. Drilling mud rheological behavior with and without contamination was investigated under different temperatures using the Herschel-Bulkley and hyperbolic models. Nonlinear models were used to investigate the combined effects of bentonite and salt contamination, and the changes in the temperature on the fundamental properties of the drilling mud such as yield and maximum shear stress, electrical resistivity and other hyperbolic model parameters. Nonlinear model showed that the bentonite content in the drilling mud had the highest effect in decreasing the electrical resistivity, yield and maximum shear stresses compared to salt contamination and temperature in the range of studied variables.