Foam is commonly used for UBD (underbalanced drilling) because of its low variable densities; which make adjustment of its densities possible in order to keep control of the circulating bottomhole pressures, and because of its high effective viscosities; which give a superior cuttings lifting and transport ability. Foam is also used to transport formation fluids that enter the borehole while underbalance-drilling. This paper is intended to propose an improved computer program in the VISUAL BASIC language that can be used to better simulate the foam hydraulics for vertical wells. The developed model considers foam as a Non-Newtonian power-law fluid in spite of the dispute that still persists among the researchers. Results of the proposed model revealed that the foam flow is affected by the injection pressures, injection flow rates, bottomhole temperatures, ROP (Rate of Penetration), cutting sizes and densities, fluid influxes and the surface back-pressures. The model also proposes a try-and-error procedure to initially determine the best selections of the injection pressures and the injection rates for both liquid and gas. The model evaluation was tested by running the program on two field wells drilled underbalanced with foam in the Middle East with an absolute average error of less than 2.6 % for the first well and with an absolute average error of less than 10.9 % for the second well.

Casing While Drilling (CWD) technique allows the operator to simultaneously drill and case a wellbore section in only one run without the necessity to pull the drill string out of the hole. This innovative technique comes into play in time when oil and gas well drilling faces a lot of challenges such as excessive lost circulation, formation fluid influx, formation instability, etc. CWD is implemented into today drilling activities in order to safely and economically deal with these obstructive challenges. In addition to eliminating/minimizing the effects of these challenges, it remarkably improves the entire economics of the drilling project. Researches and developments are continuously on their ways to further improve the performance of the technique in order to minimize its limitations. A lot of case studies all over the world have been analyzed and their related practical lessons have been stated. Based on a study carried out on five shallow wells and seven deep wells drilled between 2007 and 2008 in the region of Western Desert, lost circulation and stuck pipe represented more than 80% of the drilling problems. The study revealed that CWD could be an alternative approach in the region to eliminate/minimize the aforesaid problems as was the case in numerous fields in the world. Therefore, the potential applicability of CWD technique in the region has been widely discussed. A proposed drilling program that suits CWD has been given. Special reference has been also given to casing string design, mud program, hydraulics optimization, bit selection criteria, working conditions and operational parameters such as WOB and RPM.

Foam underbalanced drilling is the use of the entrapped gas bubbles within the continuous liquid fluid as the drilling medium to overcome one the most challenging drilling problems: lost circulation, regardless of its various reasons. It is also used to improve the reservoir productivity by preventing the formation damage and the skin factor. The complexity of foam composition as a drilling fluid is by the fact that it is affected by both constituent ingredients: liquid and gas as well as the downhole conditions. The combination between compressible gas and incompressible liquid in addition to the intrusion of annular cuttings and influx fluids necessitates better understanding of the mechanism of the hydrodynamic flow of the three-phase. One of the first fundamental steps in achieving that is to intensively conduct the experimental and analytical studies of foam rheology. A hydraulic model, based on the recent experimental and analytical conclusions, was developed not only for better studying foam rheology, but also for analyzing the exhaustive foam drilling operation from the injection lines till the cuttings removal on the surface. In order to simplify the use of the complex mathematical equations and iterative procedures, the programming language VISUAL BASIC has been used and implemented for the modeling. For the rheological study, results concluded the dependence of foam hydraulics, primarily, on rheology. The results also revealed the influence of the rheological properties by drilling parameters such as injection pressure, liquid and gas rates, rate of penetration (ROP), and surface back-pressure. If the increase of injection pressure acts to decrease the flow power index (n) and Reynolds number (NRe), it, in contrast, increases the flow consistency index (k) and friction factor (FF). Gas injection opposes the effects of the injection pressure, whereas liquid injection has variable effects depending on the nature and amount of the addition.

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In this paper, the emphasis is put on the effect of the surface back pressure on selected foam rheological properties. A hydraulic model for foam drilling in vertical well has been developed using Visual Basic program to thoroughly analyze foam hydrodynamics and to study the effect of the back pressure on these selected rheological properties such annular pressure, foam quality, density, effective viscosity, power and consistency indices and cuttings concentration along the entire annular section. The model intends to simulate and predict foam rheology and characteristic parameters for foam drilling in vertical wells. The model was evaluated and validated by running its program on two foam-drilled wells in the Middle East. The developed model was also compared with two other models: Valco-Economides’ model; and Sporker’s model. The average errors of the developed model accuracy were 2.59% and 10.85% for the first and second wells, respectively. The model accuracy was slightly better than that of Valco-Economides’ one, but less than that of the Sporker’s one.