William Pao

CFD Method for Predicting Annular Pressure Losses and Cuttings Concentration in Eccentric Horizontal Wells

In oil and gas drilling operations, predictions of pressure losses and cuttings concentration in the annulus are very complex due to the combination of interacting drilling parameters. Past studies have proposed many empirical correlations to estimate pressure losses and cuttings concentration. However, these developed correlations are limited to their experimental data range and setup, and hence, they cannot be applicable to all cases. CFD methods have the advantages of handling complex multiphase flow problems, as well as, an unlimited number of physical and operational conditions. The present study employs the inhomogeneous (Eulerian-Eulerian) model to simulate a two-phase solid-fluid flow and predict pressure losses and cuttings concentration in eccentric horizontal annuli as a function of varying drilling parameters: fluid velocity, diameter ratio (ratio of inner pipe diameter to outer pipe diameter), inner pipe rotation speed, and fluid type. Experimental data for pressure losses and cuttings concentration from previous literature compared very well with simulation data, confirming the validity of the current model. The study shows how reliable CFD methods can replicate the actual, yet complex oil and gas drilling operations.

Design of Helical Capacitance Sensor for Holdup Measurement in Two-Phase Stratified Flow: A Sinusoidal Function Approach.

A 360° twisted helical capacitance sensor was developed for holdup measurement in horizontal two-phase stratified flow. Instead of suppressing nonlinear response, the sensor was optimized in such a way that a ‘sine-like’ function was displayed on top of the linear function. This concept of design had been implemented and verified in both software and hardware. A good agreement was achieved between the finite element model of proposed design and the approximation model (pure sinusoidal function), with a maximum difference of ±1.2%. In addition, the design parameters of the sensor were analysed and investigated. It was found that the error in symmetry of the sinusoidal function could be minimized by adjusting the pitch of helix. The experiments of air-water and oil-water stratified flows were carried out and validated the sinusoidal relationship with a maximum difference of ±1.2% and ±1.3% for the range of water holdup from 0.15 to 0.85. The proposed design concept therefore may pose a promising alternative for the optimization of capacitance sensor design.

Numerical investigation of gas separation in T-junction

T-junctions are commonly used in distributing two-phase flow by piping networks especially in oil and gas industries. Understanding the behavior of two-phase flow through a T-junction is very important as it has significant effect on the operation, maintenance and efficiency of the components downstream from the junction. The objective of this paper is to determine the effect of ratio of side arm to main arm diameters, initial inlet gas saturation and gas density variation on passive separation performance in T-junction. Via computational fluid dynamics tool, preliminary investigation found that separation efficiency is proportional to diameter ratio in between 0.5-0.75. Beyond diameter ratio 0.75, there is a flattening of separation efficiency. The change of fraction of gas taken off is inversely proportional to initial inlet gas saturation and the trend is almost inversely linear for diameter ratio 0.5. Beyond that, the relationship between initial inlet gas saturation and separation efficiency exhibits...

Modelling of pressure drop in eccentric narrow horizontal annuli with the presence of cuttings and rotating drillpipe

Accurate pressure drop estimation is vital in wellbore hydraulic design. The present study investigates the effects of diameter ratio, fluid velocity, fluid type, fluid rheology and drillpipe rotation speed on pressure drop in eccentric horizontal wellbore with the presence of cuttings using computational fluid dynamics (CFD). The drillpipe is positioned at eccentricity of 0.623 and rotates about its own axis at 80 rpm and 120 rpm. The diameter ratio varies from 0.64−0.90. Results show significant increase in pressure drop with increasing fluid velocity and diameter ratio. Increase in drillpipe rotation speed from 80 rpm to 120 rpm shows insignificant effect on pressure drop for both Newtonian and non-Newtonian fluids. Non-Newtonian fluid exhibited large pressure drop compared to Newtonian fluid especially at low fluid velocities. Fluid rheology effects are also discussed. Experimental data confirmed the validity of the current model setup with a mean percentage error of 0.84%.

Surrogate reservoir modeling-prediction of Bottom-Hole Flowing Pressure using Radial Basis Neural Network

Reservoir simulation provides information about the behaviour of a reservoir in various production and injection conditions. Reservoir simulator is used to predict the future behaviour and performance of a reservoir field. However, the heterogeneity of reservoir and uncertainty in the reservoir field cause some obstacles in selecting the best calculation of oil, water and gas components that lead to the production system in oil and gas. Due to intrinsic uncertainty in the reservoir simulation models, large number of computational resources such as simulation runs and long processing time are required to predict the properties in a reservoir. This paper presents an application of Surrogate Reservoir Model (SRM) for predicting the Bottom-Hole Flowing Pressure (BHFP) at different time step for an initially under-saturated reservoir. The developed SRM is based on Artificial Neural Network to regenerate the results of a numerical simulation model in considerable amount of time. The output of the reservoir simulation consists of oil production, gas rate, average reservoir pressure, saturation and BHFP etc. The proposed SRM adopted Radial Basis Neural Network to predict the BHFP based on the output data extracted from the Black Oil Applied Simulation Tool (BOAST). It is found that the developed SRM is capable in supporting fast track analysis, decision optimization and manage to generate the results in a shorter time as compared to the conventional reservoir model.

Numerical simulation of two-phase flow regime in horizontal pipeline and its validation

Purpose: The purpose of this paper is to investigate oil-gas slug formation in horizontal straight pipe and its associated pressure gradient, slug liquid holdup and slug frequency. Design/methodology/approach: The abrupt change in gas/liquid velocities, which causes transition of flow patterns, was analyzed using incompressible volume of fluid method to capture the dynamic gas-liquid interface. The validity of present model and its methodology was validated using Baker�s flow regime chart for 3.15 inches diameter horizontal pipe and with existing experimental data to ensure its correctness. Findings: The present paper proposes simplified correlations for liquid holdup and slug frequency by comparison with numerous existing models. The paper also identified correlations that can be used in operational oil and gas industry and several outlier models that may not be applicable. Research limitations/implications: The correlation may be limited to the range of material properties used in this paper. Practical implications: Numerically derived liquid holdup and holdup frequency agreed reasonably with the experimentally derived correlations. Social implications: The models could be used to design pipeline and piping systems for oil and gas production. Originality/value: The paper simulated all the seven flow regimes with superior results compared to existing methodology. New correlations derived numerically are compared to published experimental correlations to understand the difference between models. © 2018, Emerald Publishing Limited.

Numerical Validation of Two-Phase Slug Flow and its Liquid Holdup Correlation in Horizontal Pipeline

The transition of one flow regime into another is a very common phenomena in pipeline networks, which can be potentially hazardous for the structural integrity of the pipeline. Literature review showed that there is almost no reported detail investigation of transitional flow whereby the fluid constituents change from one regime to another especially slug transition. Most of the open research papers focused on slug flow regime and its liquid holdup in horizontal pipelines and channels have been carried out on experimental test rigs. The objective of this study is to explore oil-gasoil vapor slug transition and its liquid holdup in a 3.15 inch diameter horizontal straight pipe. The abrupt change in gas/liquid velocities, which causes transition of flow patterns is analyzed using incompressible Volume of Fluid (VOF) method, along with Piecewise Linear Interface Construction (PLIC) technique to capture the sharp front of segregated gas-liquid interface. Slug liquid holdup derived from the present numerical model is compared to existing experimental correlations in the literature.

Numerical analyses for improved terminal velocity of deep water torpedo anchor

Purpose This research aims to investigate the effects of manipulation of a torpedo’s geometries to attain higher terminal velocity. The parameters of interest include geometric changes of the original design, as well as sea water properties that reflect water depth in South China Sea. Design/methodology/approach The research make use of computational fluid dynamics (CFD) software, FLUENT, to solve viscous incompressible Navier–Stokes equations with two equations k-epsilon turbulent model. The calculated drag coefficient is subsequently used to calculate the maximum attainable terminal velocity of the torpedo. Findings It was found that the terminal velocity can be improved by sharper tip angle, greater aspect ratio, greater diameter ratio and optimum rear angle at 30°. Sensitivity of drag coefficient toward each of the parameters is established in this paper. Originality/value The paper, in addition to verifying the importance of aspect ratio, has also established the tip angle, diameter ratio and rear angle of the torpedo as important geometric aspects that could be tuned to improve its terminal velocity.

Surrogate Reservoir Model for Average Reservoir Pressure

Reservoir models comprise of number of grid blocks, multiple layers and production/injection wells. Reservoir models also contain a number of different characteristics in its structure. Such as gas-oil ratio, water/oil saturation, permeability and volume factor, etc. As characteristics of the reservoir model increases, then also the time to calculate the reservoir outputs increase exponentially. This situation leads to a very cumbersome process. And this situation is considered as a very cumbersome process. There are other alternatives available, such as artificial neural network (ANN) techniques, which are utilized to manufacture the surrogate reservoir model (SRM) for multiphase flow. In this paper, Black Oil Applied Simulation Tool (BOAST) is utilized to gather the reservoir characteristics to build the spatio-temporal database. The spatio-temporal database is used to train the SRM. SRM is used to predict or calculate the result of Average Reservoir Pressure in a short amount of time as compared to the black oil simulator. It is also proposed that SRM is based on Radial Basis Neural Network (Radil Basis NN) to enhance the simulation process.

Sinusoid based calibration for holdup measurement in two-phase flow using helical capacitance sensor

A helical capacitance sensor was designed to measure the holdup of two-phase stratified flow in horizontal pipe. The structure of 360° twisted helical capacitance sensor had been designed, analyzed and optimized using finite element method. A sinusoidal function was observed as the relationship between the capacitance readings and holdup. As the nonhomogeneity in the distribution of capacitance sensitivity field cannot be completely eliminated, a helical capacitance sensor was designed in such a way that a sinusoidal relationship was generated. The numerical readings and the approximation model obtained a very close agreement, where the mean absolute difference was about ±1%. Besides, the effects of geometric parameters on the capacitance values were analyzed and investigated, where it was found that the symmetry of sinusoidal function can be modeled by adjusting the pitch of helix.