Ergun Kuru

Effect of Elasticity During Viscoelastic Polymer Flooding: A Possible Mechanism of Increasing the Sweep Efficiency

It has been long believed that the viscoelasticity of polymer solution improves the displacement efficiency in polymer flood operations, but the individual effect of elasticity has not been clearly distilled for a single viscoelastic polymer. In this study, the effect of elasticity of polymer-based fluids on the microscopic sweep efficiency is investigated by injecting two polymer solutions with similar shear viscosity, but significantly different elastic characteristics. Blends of various grades of polyethylene oxide (PEO) with similar average molecular weight and different molecular weight distribution (MWD) were prepared by dissolving in deionized water. The polymer solutions exhibited identical shear viscosity, but different elasticity. A series of experiments were performed by injecting the polymer solutions through a sandpack saturated with mineral oil. The experiments were performed using a special core holder designed to simulate radial flow. Injection was done through a perforated injection line located at the centre of the cell and fluids were produced through two production lines located at the periphery. The experiments were conducted within a shear rate range of field applications. Because both polymer solutions had similar shear viscosity behaviour, but different elastic properties, it was possible to see the effect of elasticity on the sweep efficiency alone. Results of the polymer flooding experiments indicated that the sweep efficiency of a polymeric fluid could be effectively improved by adjusting the MWD of the solution at constant shear viscosity and polymer concentration. The polymer solution with higher elasticity exhibited considerably higher resistance to flow through porous media than the one with lower elasticity, resulting in higher sweep efficiency and lower residual oil saturation.

The Buckling Behavior of Pipes and Its Influence on the Axial Force Transfer in Directional Wells

An experimental setup was built at the University of Tulsa to study buckling and post-buckling behavior of pipes constrained in straight horizontal and curved wellbores. Experiments were conducted to investigate the axial force transfer with and without static internal pressure. Different stages of buckling phenomena and their relation to the axial force, the pipe diameter (1/4 and 3/8 in.) and the pipe end-support conditions have also been investigated. Experimental results have shown that the buckling load is a strong function of the pipe diameter and the pipe end-support conditions. Static internal pressure appears to have insignificant influence on the buckling behavior of pipes. A brief review of recently developed mathematical models to predict buckling behavior of pipes in inclined, curved, and horizontal sections of wellbore is also presented. Applications of the current theory are presented by using recently developed computer simulator. Results of the theoretical analysis have confirmed the versatility and effectiveness of computer simulator for better understanding and solving buckling related problems in the field.

Physico-Chemical Characterization of Aphron-Based Drilling Fluids

Colloidal gas aphron-based drilling fluids are designed to minimize formation damage by blocking the pores of the rock with microbubbles, which can later be removed easily when the well is open for production. Sizing colloidal gas aphron (CGA) bubbles in accordance with the rock pore size distribution is essential for effective sealing of the pores during drilling. The physical properties (i.e. viscosity, density, fluid loss, etc.) of the CGA-based drilling fluids also need to be understood in order to use these fluids more effectively. In this study, the physical properties of colloidal gas aphronbased drilling fluids are investigated. The results of rheology, API filtration loss and density measurement tests using various CGA-based drilling fluid formulations are presented. The effects of polymer and surfactant concentration, surfactant type, shear rate, mixing time and water quality on the CGA bubble size have been studied. Results of CGA bubble size characterization experiments are also reported. layer. The outer layer, which also supports the viscous layer, is hydrophobic outwards and hydrophilic inwards. Since this bubble is in contact with the bulk water, it is believed that there is another layer in which the surfactant molecules are hydrophobic inwards and hydrophilic outwards. This indicates that there is a region in between the aphron outer shell and the bulk phase layer where a hydrophobic globule will be comfortable and, therefore, oil can adhere to the gas aphron (3) . Aphrons are non-coalescing, can be recirculated, and are not affected by fine screen shale shakers. Downhole tools can be utilized with the aphronized drilling fluid system. Aphrons eliminate differential sticking by altering the near wellbore pressure drop (6) , which, in turn, reduces the need for costly downhole tools in low reservoir pressure applications (7) . One of the most important assets

Selecting Drilling Fluid Properties and Flow Rates For Effective Hole Cleaning in High-Angle and Horizontal Wells

This paper presents a method to determine optimum drilling fluid properties and flow rates to minimize cuttings bed height and circulation time in high angle and horizontal wells. The method uses empirical models relating the cuttings bed height and the bed erosion time to drilling fluid properties and flow rates. Bed erosion tests have been conducted using a cuttings transport facility available at the University of Tulsa. Cuttings bed height as a function of time has been investigated by using variable flow rates (200 - 400 gpm) and four different drilling fluid compositions. Experimental results were used together with a non-linear regression analysis program to establish a functional relationship among drilling fluid properties, flow rate, cuttings bed height and the time required to circulate the borehole clean. A numerical example is provided to explain the field application of the method. The sequential calculations involved in determining optimum combination of the Power Law viscosity parameters n and K, and the flow rate to minimize the cuttings bed height and circulation time are also given. Field implementation of the proposed empirical correlations and the new method can aid optimization of circulation practices before tripping, and so reduce the associated risk of non-productive time.

Stability of Microbubble-Based Drilling Fluids Under Downhole Conditions

Colloidal gas aphrons (CGA) have the unique ability to form a bridge in the pores of reservoirs, which stops fluid invasion. Sizing microbubbles in accordance with the rock pore size distribution is imperative for effective sealing during drilling. The effects of time, temperature and pressure on the stability and size of the microbubbles needs to be better understood in order to design a fluid that will sufficiently block the pores of the formation for extended periods. In this study, the effects of time, pressure and temperature on the size of microbubbles and the stability of microbubble (CGA)based drilling fluids were investigated. The change in the CGA diameter with time was determined by using a microscopic imaging technique. Effects of base fluid viscosity and surfactant concentration on the size and stability of the microbubbles were also investigated.

Mathematical Modeling of PDC Bit Drilling Process Based on a Single-Cutter Mechanics

An analytical development of a new mechanistic drilling model for polycrystalline diamond compact (PDC) bits is presented. The derivation accounts for static balance of forces acting on a single PDC cutter and is based on assumed similarity between bit and cutter. The model is fully explicit with physical meanings given to all constants and functions. Three equations constitute the mathematical model: torque, drilling rate, and bit life. The equations comprise cutter’s geometry, rock properties drilling parameters, and four empirical constants. The constants are used to match the model to a PDC drilling process. Also presented are qualitative and predictive verifications of the model. Qualitative verification shows that the model’s response to drilling process variables is similar to the behavior of full-size PDC bits. However, accuracy of the model’s predictions of PDC bit performance is limited primarily by imprecision of bit-dull evaluation. The verification study is based upon the reported laboratory drilling and field drilling tests as well as field data collected by the authors.

Numerical modelling of cuttings transport with foam in horizontal wells

In this study, a 1D unsteady-state two-phase mechanistic model of cuttings transport with foam in vertical wells has been developed. The model is solved numerically to predict the optimum foam flow rate and rheological properties to maximize the cuttings transport efficiency in vertical wells. Comparisons of model predictions with the field test data have shown that model predictions are in close agreement with the field test results. The numerical solution allows analyzing the effects of borehole geometry, drilling rate, foam rheological properties, gas and liquid flowing rates, and reservoir fluid influx on the cuttings transport efficiency when drilling vertical wells. Results of sensitivity analysis study are presented.

Experimental Evaluation of the Lateral Contact Force in Horizontal Wells

In horizontal and extended reach drilling, a large frictional drag may occur. If the pipe buckles laterally or into a helical shape, additional lateral contact force, LCF, is developed between the pipe and the wellbore wall, increasing the drag force. This paper presents the results of an experimental study of the lateral contact force between the drill pipe and the wellbore wall, for helical pipe configuration. Comparison of the experimental results with the current analytical models is also presented. A horizontal well was simulated using a 2-in-dia hole, 86-ft long, and three different sizes of pipe. Two different techniques were used to measure the lateral contact force. Results from both techniques seem to be in good agreement. The comparison with the current analytical models shows that higher values are predicted. The results will find application in directional drilling, horizontal drilling, and coiled tubing operations.

Elastoplastic Modelling of Sand Production Using Fracture Energy Regularization Method

This paper extends the capacity of the current sand production models by eliminating the influence of artificial conditions and numerical mesh on localization and deformation response in the sanding model. Past studies indicate strong size effects when using classical elastoplastic models. To rectify this deficiency, a fracture energy regularization method is implemented in the numerical model. The model incorporates both the geomechanical aspects (e.g. rock elastoplastic deformation and rock disaggregation), as well as the transport aspects (e.g. the role of seepage on rock deformation and solid release). The model employs a Mohr-Coulomb flow theory of elastoplasticity with friction hardening/cohesion softening. Emphasis is given on calibration procedure and validation of the enriched model through back analysis of triaxial and uniaxial compression tests. Next, the model is used to compare the numerical predictions with laboratory data on sand production. The comparison incorporates the stress and deformation, as well as the sand volume. The calibration study shows that friction hardening and cohesion softening can satisfactorily reproduce numerically the weak sandstone response to various loading conditions. Further, computation results of strain softening material illustrates that a fracture energy regularization strategy enables the model to exhibit mesh invariance of the energy dissipation.

Effect of Tool Joints on Contact Force and Axial-Force Transfer in Horizontal Wellbores

An experimental study has been conducted to investigate the effect of tool joints on the buckling/post-buckling behavior of drillpipes constrained in straight horizontal wellbores. Buckling/post-buckling behavior of drillpipes has traditionally been investigated with continuous pipes. To our knowledge, this is the first time the effect of tool joints is included in such a study. The U. of Tulsa Drilling Research Projects experimental buckling facility has been used to carry out the desired experiments. Axial loads at both ends of the pipe and contact forces at the tool joints were measured. Changes in the drillpipe configuration were also investigated visually as the axial load increases. Some of the new findings of this study can be summarized as follows. Sequential occurrence of buckling/post-buckling configuration of jointed pipe is similar to that of continuous pipes, reported previously by various investigators. In other words, in both cases, the pipes buckle first laterally and then helically as the axial compressive load increased. The presence of tool joints does not affect the critical lateral (sinusoidal) buckling load significantly. However, it increases the critical load, causing helical pipe configuration (helical buckling) of approximately 20%. The use of tool joints improved the efficiency of the axial load transfer by approximately 40%. The results of this study will help to improve the design of operational parameters for drilling with jointed pipes as well as with coiled tubing (CT). In particular, improved axial-load transfer performance would allow drillers to use a higher weight on bit and, consequently, faster and possibly less costly drilling.