This paper concludes the analysis of the results from two sets of experimental tests performed at PETROBRAS real scale test facility aiming the evaluation of solids return times in aerated fluid drilling. It reviews important results on effects such as liquid and gas injection rates and particle diameter from air-water experiments1 and extend the discussion by adding the effect of the viscosity of liquid phase and annular back pressure on the transport capacity of solids in a vertical well with an aerated polymer-based drilling fluid. Results indicate that the gas has a major effect in accelerating the liquid phase, which would be responsible for carrying the particles to the surface. The trend was confirmed in the experiments with polymer-based mud, where lesser return times were associated with the fluids with higher liquid-phase viscosity. The concept of actual liquid velocity coupled with a procedure for particle sedimentation velocity calculation in non-Newtonian fluids adequately reproduced the experimental results. Background and Objectives Optimizing gas and liquid flow rates in light weight fluid drilling design is a complex task which involves knowledge on two-phase flow hydraulics. A lot of effort has been spent in the prediction of the impact of gas and liquid flow rates on bottom hole ECDs (Rommetveit et al.[2]). Naturally, a desired ECD in the bottom can be achieved by several combinations of liquid and gas flow rates. The decision of which values to use will depend on downhole motor requirements and hole cleaning criteria. These parameters usually define the operational window to work while drilling a certain well. Suppliers generally specify the required minimum and maximum liquid flow rates for driving downhole motors. On the other hand, flow rate requirements for adequate hole cleaning using aerated fluids poses as a much more challenging task. Compared to the widespread implementation of the various underbalanced drilling techniques worldwide, very little has been done in investigating hole cleaning in light weight fluid drilling. The most challenging drilling scenarios are the highly inclided and horizontal wells. Some important work can be cited, such as the attempt from Vieira et al.[3], that working in low pressure and temperature pilot-scale surface facility, developed empirical correlations to estimate minimum liquid and gas flow rates requirements for proper hole cleaning in inclined and horizontal air-water drilling. Li and Walker[4] applied concepts developed to estimate hole cleaning time and wiper-trips velocity for conventional overbalanced conditions to underbalanced drilling while working in very similar conditions as Vieira[3]. More recent comprehensive modeling efforts can be found in the work of Doan et al.[5] and Zhou et al.[6]. This last work offers data from a well instrumented pilot scale facility, including pressures up to 500 psig and temperatures up to 80°C, but with small gas-liquid ratios. Although academia and part of industry already turned their attention to more complex situations, the truth is that many questions regarding optimum flow rates design still exist while drilling vertical wells, which are still the typical candidate of aerated mud drilling. Under this scenario, Guo et al.[7] proposed a simplified model for liquid gas flow rate prediction which would provide a given solids concentration in the annulus. Adewumi et al.8 performed pilot scale experimental studies for air/solids flow. A major problem is that scaling down techniques seems limited in representing adequately the phenomena involved in the three phase flow. Primary field experience indicates that fluid effective velocities of 120 and 150 ft/min would clean vertical and directional wells, respectively. The minimum velocity requirements for hole cleaning depends on several aspects, including fluid and solids properties, wellpath, etc. Consequently, in many cases, the velocities normally used in the field may be much greater than necessary, resulting in high drilling costs. This was the motivation for the development of an experimental program on PETROBRAS real scale test facility, aiming the determination of solids return time for different conditions.
This paper summarizes the results of two sets of experimental tests performed at PETROBRAS real scale test facility aiming the evaluation of solids return times in aerated fluid drilling. The effect of the following parameters was studied: liquid and gas injection rates, particle diameter and depth. Results indicate that the gas has a major effect in accelerating the liquid phase, which would be responsible for carrying the particles to the surface. The concept of effective liquid velocity coupled with an adequate procedure for particle sedimentation velocity calculation reproduced the experimental results adequately.
A joint project involving PETROBRAS and Federal University of Rio de Janeiro, investigated the effects of an uncontrolled underwater blowout using two distinct approaches:theoretical modeling, andexperimental study. A water tank was built and accurate instruments were installed to read relevant parameters. At last after an extensive experimental and development effort, a Windows TM based software was released, including the most significant results. Based on the results of the research project, this paper presents and analyzes some case studies. The points of interest are on the effect of bubble plumes on buoyancy, the dynamic interaction between the produced gas and sea water. the formation of gas hydrates, and the effects on the oceanic environment. Introduction Bubble plumes occur when a gas blowout is discharged below the sea surface. The produced gas rises and flows to the surface, forming an up side down cone from bottom to surface. In addition, the upward gas movement induces seawater motion, producing streams. Better understanding and evaluation of the effects of bubble plumes phenomenon require an detailed study of the gaswater mixture flow and the definition of the main physical parameters affecting its behavior. Thus, experimental tests and mathematical modeling were performed aiming at the development of a computer program to serve as an engineering tool for bubble plume analysis. The developed software was used to analyze a variety of operational scenarios, defined by the combination of input parameters, such as: water depth, blowout gas mass rate, well head diameter and sea surface temperature. Six cases were chosen among all the analyzed scenarios to be presented in this paper. The most relevant results obtained from this analysis include:the well head diameter and sea surface temperature do not cause significant influence on the mechanics of bubble plume;formation of gas hydrates in deep water scenarios was identified as a factor that significantly influences the dynamic of bubble plumes;as presented before by Milgram, over not very shallow waters the buoyancy reduction are not enough to cause problems for floating vessels; anddue to the induced streams, bubble plumes can be very harmful to oceanic environment. specially in deep waters. Program Output The program output encompasses the following variables:bubble plume radius along water depth, from sea bottom to surface;bubble rising velocity along water depth;void fraction along bubble plume vertical center-line;gas volumetric flow rate along bubble plume;divergent wave velocity on sea surface as a function of radial distance from surface bubble center;reach of descending streams (flowing from sea surface downward) induced by bubble rising. as a function of radial distance from surface bubble center;mass flow of gas from sea floor to surface, affected by hydrates' formation on deep water;buoyancy of bubbles along water depth;momentum of bubble plume andsea water density reduction along bubble plume center line from sea bottom to surface. The seven first functions were chosen to be presented. Variable (10) is presented together with (3), near sea surface, since they are closely related. Results The scenarios were defined with the combination of the following variables:well head diameter (203 and 762 mm);sea surface temperature (10 and 30 C);water depths (50, 400 and 1200 m); andblowout gas mass flow rate (10 and 70 Kg/s). With the purpose of studying hydrates formation in deep water, water depths of 1000, 1500 and 2000 m were also tested for both blowout mass flow rates.
This paper summarizes the results of two sets of experimental tests performed at PETROBRAS real scale test facility aiming the evaluation of solids return times in aerated fluid drilling. The effect of the following parameters was studied: liquid and gas injection rates, particle diameter and depth. Results indicate that the gas has a major effect in accelerating the liquid phase, which would be responsible for carrying the particles to the surface. The concept of effective liquid velocity coupled with an adequate procedure for particle sedimentation velocity calculation reproduced the experimental results adequately.
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