Wellbore Fluids Model Provides Basis for Drilling Optimization
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Numerous publications have addressed the problems inherent to calculating well hydraulics in slimhole applications Accurate grasp of well hydraulics is of increased importance under slimhole conditions. Most models are valid in a controlled environment, but fail to fulfill the expectations under field conditions. Factors influencing the hydraulic behavior of the well have to be critically examined, and the accuracy of these factors entering into the calculations has to be quantified together with their relative impact on the results. This work has compared several "established" approaches with field data from five deep slimhole wells, and critically reviewed limitations and possible advances in slimhole hydraulics' calculations. Analyzing field data showed phenomena that are not considered in existing simulations. This work tries to define these phenomena and evaluate their impact on future simulations. These factors influencing pressure loss and ECD development are listed and quantified, and examples from well data are presented. A successful development of simulators of such complex systems is demanding a new way of thinking, away from the deterministic towards a probabilistic view of the problem. Introduction Controlling wellbore hydraulics has always been one of the primary - if not the main - concerns in slimhole drilling technology acceptance. While it was of minor importance for the mining industry - having utilized slimhole concepts for decades in penetrating hard, crystalline rocks - the oil and gas drilling community quickly learned, that experiencing 90 % of the total pressure losses in the annulus rather than inside the drillstring created unprecedented difficulties. Equivalent circulating densities (ECD's) of up to 3 or even 5 ppg higher than the static mud weight all but eliminated slimhole applicability from many possible areas. "Connection gas" had already previously been a known phenomenon among oilfield drillers, however, a 30–40 % reduction in bottom hole pressure whenever stopping circulation or a 10–20 % reduction when simply slowing down rotary RPM accounted for enough hazardous incidents to give slimhole drilling an "unsafe and experimental" reputation. The changed behavior of fluid flowing under special conditions, as observed in slimholes, has already been recognized by authors decades ago. As slimhole drilling showed the potential to drill in a more economic way than conventional drilling, the increased interest in this type of technology led to the need to predict the pressure losses in the wellbore more accurately. Consequently it has been proven in numerous applications all over the world that slimhole wellbores can be controlled safely and that slimhole wells can be drilled with at least the same degree of control - if not even better - than compared to conventional wells. These changed requirements regarding pressure loss calculations resulted in numerous algorithms to predict pressure losses in slimhole wells. P. 393
Numerous publications have addressed the problems inherent to calculating well hydraulics in slimhole applications Accurate grasp of well hydraulics is of increased importance under slimhole conditions. Most models are valid in a controlled environment, but fail to fulfill the expectations under field conditions. Factors influencing the hydraulic behavior of the well have to be critically examined, and the accuracy of these factors entering into the calculations has to be quantified together with their relative impact on the results. This work has compared several "established" approaches with field data from five deep slimhole wells, and critically reviewed limitations and possible advances in slimhole hydraulics' calculations. Analyzing field data showed phenomena that are not considered in existing simulations. This work tries to define these phenomena and evaluate their impact on future simulations. These factors influencing pressure loss and ECD development are listed and quantified, and examples from well data are presented. A successful development of simulators of such complex systems is demanding a new way of thinking, away from the deterministic towards a probabilistic view of the problem. Introduction Controlling wellbore hydraulics has always been one of the primary - if not the main - concerns in slimhole drilling technology acceptance. While it was of minor importance for the mining industry - having utilized slimhole concepts for decades in penetrating hard, crystalline rocks - the oil and gas drilling community quickly learned, that experiencing 90 % of the total pressure losses in the annulus rather than inside the drillstring created unprecedented difficulties. Equivalent circulating densities (ECD's) of up to 3 or even 5 ppg higher than the static mud weight all but eliminated slimhole applicability from many possible areas. "Connection gas" had already previously been a known phenomenon among oilfield drillers, however, a 30–40 % reduction in bottom hole pressure whenever stopping circulation or a 10–20 % reduction when simply slowing down rotary RPM accounted for enough hazardous incidents to give slimhole drilling an "unsafe and experimental" reputation. The changed behavior of fluid flowing under special conditions, as observed in slimholes, has already been recognized by authors decades ago. As slimhole drilling showed the potential to drill in a more economic way than conventional drilling, the increased interest in this type of technology led to the need to predict the pressure losses in the wellbore more accurately. Consequently it has been proven in numerous applications all over the world that slimhole wellbores can be controlled safely and that slimhole wells can be drilled with at least the same degree of control - if not even better - than compared to conventional wells. These changed requirements regarding pressure loss calculations resulted in numerous algorithms to predict pressure losses in slimhole wells. P. 393
Drilltronics: An Integrated System for Real-Time Optimization of the Drilling Process
Drilltronics is the project name of a new and innovative system for drilling automation and simulation. The concept uses all available drilling data (surface and downhole) in real time to optimize the drilling process. If a sensor is failing (PWD for example) then advanced models mirroring the drilling process will calculate the parameter missing (pressure at the PWD sensor position). The system is composed of the following elements:Software modelling with algorithms that reflect the wellbore behaviour and its interaction with the drilling equipment. Currently under development are transient flow and heat transfer models, a drillstring torque and drag model, weight on bit optimization, and stick/slip analysis.The models are driven in real time by drilling data that are logged at high acquisition rate. The system is continuously calibrated through analysing the data with advanced filtering techniques.Real-time diagnosis of the drilling process is obtained from comparing measured data with model predictions. This makes up a basis for continuous adjustments to the drilling strategy.An integrated drilling simulator is developed by linking the modules together, and combining it with an ROP model. This can be used for pre-planning, sensitivity analysis during drilling, post-well analysis, and training objectives.Selected, critical sub-operations will be automated for fast and reliable reaction, e.g. automatic control of the drawwork based on dynamic surge and swab calculations, and automatic detection and first action after easily recognised symptoms on hole problems like pack off or stuck pipe. In order to realize the full potential of Drilltronics, surface and downhole drilling data must be available in real time. The drilling equipment will need to be computer controlled with an interface that supports automatic responses to the model's analysis. Introduction Other systems The oil industry has made significant progress in developing new facilities to improve the drilling operation and its technology. Examples are real-time formation evaluation, directional control while drilling, improved bottom hole assembly components, and improved drilling fluids. In the last few years, the request for real-time applications to improve performance and, at the same time, maintain safety for personnel and environments has increased related research activities. A historical view of the driller's role from the early start of the oil industry at the beginning of the twentieth century until today is given in [1]. The driller's role will need to evolve from one of basic drilling mechanics into that of real time drilling supervisor. This was emphasized when Havrevold and Hytten presented their Analysis-While-Drilling (AWD) in 1991 [2], one of the first real-time applications for drilling operations. At this time the increasing amount of available data from Measurement-While-Drilling tools (MWD) not only gave valuable information on many drilling conditions, but also enforced the need for a more efficient data handling. Based on relatively simple prediction models, the AWD concept included a real-time application with modules handling crucial parts of drilling operations such as standpipe pressure analysis and tripping optimization.
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