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The magnetorheological fluids (MRF), or magnetic field responsive fluids, are a combination of magnetically polarizable particles in a carrier fluid. This type of fluid has the ability to modify their rheological properties under the influence of a magnetic field. The generation of a tunable pressure drop could have the potential to controlling fluid losses while drilling and cementing in narrow operating windows and to provide a tunable fluid barrier that could work as a packer. Using the reduced form of the Navier-Stokes equation and a model to estimate the yield stress of the fluid based on the magnetic field strength, it is possible to determine the pressure drop caused by the fluid behavior.
Summary When drilling in an arid region through heavily fractured formations, it can be very challenging to manage drilling-fluid losses and at the same time maintain a downhole-pressure gradient that is compatible with the very-low geopressure gradient windows that are typically encountered in those drilling conditions. Nitrogen-enriched drilling muds may provide a good solution to both problems; however, the properties—such as density, rheology, specific-heat capacity, and thermal conductivity—of this type of drilling fluid are highly dependent on temperature and pressure, and in most cases those characteristics cannot be measured in situ, making it difficult to estimate the actual downhole-pressure conditions. The approach described in this paper consists of the reconstruction of the drilling-fluid-mix properties from the characteristics of its components and the incorporation of the resulting pressure- and temperature-dependent constitutive laws into a real-time multiphase- and multicomponent-drilling hydraulic model to estimate the downhole pressures along the drillstring and borehole as a function of the drilling parameters. Because of the uncertainty of some of the characteristics of the components of the drilling fluid as well as their actual proportion in the mix, the modeled values are only valid within a certain accuracy. Stochastic simulations are made during the estimation of the downhole pressures to ascertain the precision of the calculations. As a consequence, by comparing the obtained interval of confidence on the estimations with actual measurements, it is possible to evaluate whether the drilling conditions are normal or deteriorating. The validity and performance of the derived fluid-model extension are tested by use of a real-time data set recorded during the drilling of a well in the Erbril area of the Kurdish region of Iraq, by use of the wellsite information transfer standard markup language drilling-data-exchange protocol. The model results are reviewed and compared with the actual measurements recorded during the drilling operations. The potential sources of limitation, discrepancy, or error between the modeled and observed well and fluid behavior are discussed, along with potential explanations for the observed wellbore physics seen in the recorded-data feed.
The offshore drilling industry has been seeking to more realistically simulate gas-in-riser events, to optimize mitigation methods and justify the adoption of new pressure control equipment. This paper presents an improved model to describe gas in riser events with direct application to oil-based muds (OBM), with which most of the gas-in-riser events take place.
A mathematical model is developed and used to describe the behavior of gas influx migrating or being circulated out in a marine riser in an OBM. The release of dissolved gas in OBM is thus factored in and assumed instantaneous. Bubbly flow is assumed for the contaminated mud and simulations are performed for different choke openings as in Managed Pressure Drilling (MPD) applications and with different booster pump circulation rates. The contaminated mud section is discretized both in time and space to ensure that pressure and pressure dependent parameters are accurately quantified.
Results clearly show that the severity of the rapid unloading process can be overestimated when a water-based mud scenario is assumed. The severity of rapid unloading and the depth of the riser equilibrium point during the circulation of the influx out of the riser are seen to be of little dependence on the time at which the backpressure is initiated but with very high dependence on the magnitude of the backpressure and the circulation rates. Severity of rapid unloading increases with the circulation rate. If the gas is allowed to migrate without circulation, backpressure application would have a much greater effect on the unloading results.
The concept of riser equilibrium has been thus far developed in the context of water-based muds. This paper considers only OBMs where most of the events take place.
Once again, we find ourselves in a time of extreme challenges on many fronts in the arena of well construction, with corresponding needs for technological advancements. Anyone who has been around the drilling-and-completion world during the past several years can attest to the unique environment in which we operate today. Ever-increasing drilling depths and formation temperatures and pressures are combined with depletion of mature basins and unprecedented geopolitical uncertainty. The good news is that human innovation and problem solving continue to accelerate commensurate with these challenges. In this feature, we specifically highlight the persistent need for high-performance drilling fluids.
Abstract Most high temperature (HT) wells are drilled with oil or synthetic-based drilling fluids (O/SBM) for a variety of reasons. These O/SBM drilling fluids are naturally lubricious due to the hydrocarbon continuous phase, which also contributes to improved wellbore stability, as the fluids are relatively inert to the formations being drilled. These fluids also have acceptable temperature stability and drilling performance, which makes them suitable for several applications. Downhole losses with O/SBM can be costly and difficult to cure. Additional O/SBM can be mixed at the rig site, but this requires a supply of base oil to be available and the fluid can take time to prepare. The ideal scenario is to have a facility close to the rig location that can supply the high volumes of premixed O/SBM and base oil required. Exploratory wells are often drilled in remote locations with no convenient liquid mud plant close by to service the O/SBM requirements. Acquisition of good quality logging data from exploratory wells is crucial to understanding the field potential for commercial development. Some of the more sophisticated logging tools available in the industry are incompatible or difficult to run and interpret in an O/SBM environment. In such cases a water based drilling fluid (WBM) can be the solution. The logistic requirement for WBM is significantly lower than for O/SBM, as chemicals can be stored on location and water can be supplied from a nearby water well. WBM is much simpler to prepare than O/SBM, so WBM can be quickly prepared as required, and WBM downhole losses can often be cured more easily. Typical polymer-based WBM does not have high temperature stability, and is usually restricted to wells where the bottomhole temperature is less than 300°F. This paper will discuss the design, testing, and field application of a WBM for HT applications. To design a temperature stable HT-WBM fluid requires the use of drilling chemicals that can function adequately in a harsh environment. These wells required temperature tolerant polymers that provide an acceptable rheological profile and controlled fluid loss, so the wells can be safely drilled with no major complications. The HT-WBM was used to successfully drill, core, log, and case wells with bottomhole temperatures higher than 375°F.