Gao Li

An Anisotropic Strength Model for Layered Rocks Considering Planes of Weakness

List of Symbols b Inclination angle between planes of weakness and the major principal stress VR Volume of layered rock Vm Volume of matrix Vf Volume of weakness planes cR b ð Þ, /R b ð Þ Cohesion and internal friction angle of layered rock at angle b cm, /m Cohesion and friction angle of matrix cf , /f Cohesion and friction angle of weakness plane lR b ð Þ Friction coefficient (tan/R) of layered rock at angle b rf b ð Þ Strength reduction coefficient of anisotropic planes at angle b rc b ð Þ Uniaxial compressive strength of layered rock at angle b q Inclination angle of minimum shear strength c, d Inclination angle of minimum cohesion and friction angle Rc Degree of anisotropy a, b, c1,2, /1,2, b’s, d’s, ĉ, l̂ Constants

Weakening laws of rock uniaxial compressive strength with consideration of water content and rock porosity

Laboratory compressive experiments are regarded as the most accurate method to obtain the strength parameters of rock, and the rocks used for experiments are often dry. However, in geotechnical engineering, the rock masses are often under different hydraulic environments because of which rock strength decreases by varying degrees; consequently, there are considerable differences between laboratory results and engineering practices. Research on the quantitative relationships between rock strength and water content would contribute to the application of laboratory results to engineering practice and can become the basis for engineering rock mass stability analysis and forecasting. On the basis of 14 different groups of rockxa0strength data with different water contents, statistical and numerical methods are used to obtain the least squares fit relationship between rock strength and water content, and the fitting results demonstrate that sandstone strength and water content have a negative exponential relationship. Meanwhile, through parameter analysis, we obtain the physical meaning of each parameter in the three-parameter exponential fitting curves, thus establishing the laws that govern the weakening of rock uniaxial compressive strength. The newly established model takes the rock porosity and water content into account, and the rock strength at a certain water content can be computed according to the experimentally obtained rock strength in dry and completely water-saturated conditions, as well as the rock porosity. The model has been validated by two case studies using comparative analysis between theoretical values and experimental data.

Modified Hoek–Brown failure criterion for anisotropic rocks

The direction-dependent strength behavior of anisotropic rocks is a common characteristic. It is essential to develop the model that can describe the nonlinearity as well as the anisotropy in the triaxial strength behavior of the rocks. In the present study, a modified Hoek–Brown failure criterion was developed to describe the triaxial strength behavior of the rocks by the using of anisotropic index (

Apparatus for testing rock drillability under stratum condition

alpha_{beta }

Micro-flow kinetics research on water invasion in tight sandstone reservoirs

αβ). The procedures for determining the failure criterion parameters are introduced. In the model, the estimation of the uniaxial compressive strength at a given angle β was required, so an empirical equation for predicting the variation of the uniaxial compressive strength of anisotropic rocks is presented. The performance of the proposed equation was tested by using a triaxial test database compiled by Singh (Rock Mech Rock Eng 48(4):1387–1405, 2015). The variation of the uniaxial compressive strength was then incorporated into the modified Hoek–Brown failure criterion. Based on the analysis of the extensive triaxial test database, it was possible to accurately predict the strength of anisotropic rocks using limited test data. It is concluded that the modified Hoek–Brown failure criterion has good ability in predicting the strength of anisotropic rocks.

Wellbore stability analysis based on a new strength criterion

In this work, a methodology for characterizing reservoir pore pressure and permeability during underbalanced drilling of horizontal wells was presented. The methodology utilizes a transient multiphase wellbore flow model that is extended with a transient well influx analytical model during underbalanced drilling of horizontal wells. The effects of the density behavior of drilling fluid and wellbore heat transfer are considered in our wellbore flow model. Based on Kneissl’s methodology, an improved method with a different testing procedure was used to estimate the reservoir pore pressure by introducing fluctuations in the bottom hole pressure. To acquire timely basic data for reservoir characterization, a dedicated fully automated control real-time data monitoring system was established. The methodology is applied to a realistic case, and the results indicate that the estimated reservoir pore pressure and permeability fit well to the truth values from well test after drilling. The results also show that the real-time data monitoring system is operational and can provide accurate and complete data set in real time for reservoir characterization. The methodology can handle reservoir characterization during underbalanced drilling of horizontal wells.