A number of field and laboratory studies have been carried out to accurately predict the effect of drillstring rotation on downhole pressure and equivalent circulating density (ECD). Field studies indicated that drillstring rotation often results in an increased ECD. This is in contradiction with the results obtained from number of laboratory studies and other field studies. Consequently, there is no comprehensive model that accounts for the effect drillstring rotation on wellbore hydraulics. Recently, simple empirical models have been developed based on field measurements alone. Although these models can be very useful as they are based on field measurements, they have no physical basis and are limited to specific ranges of field parameters.This article presents results of field studies and theoretical analysis conducted on the effect of drillstring rotation on wellbore hydraulics. Field measurements during actual drilling operation were obtained from four different wells. Key drilling parameters such as flow rate, drillstring rotation speed, rate of penetration, ECD and return density, were recorded as a function of measured depth.Selected published field measurements were analyzed systematically using dimensional analysis techniques. After correlating different dimensionless groups, a new semi-empirical model was developed. The model was rigorously tested for its accuracy. Model predictions were compared with new field measurements and predictions of an existing model. Model predictions show good agreement with field measurements. The new model exhibits appreciably better accuracy than the existing one.The model developed in this investigation is relevant to manage ECD in slim holes, deepwater wells, and extended-reach wells where the increased wellbore length results in excessive pressure loss and limits the operating window for bottom hole pressure. In deepwater applications, ECD management becomes critical due to the narrow operating window between the pore and fracture pressure gradients.
Summary Surge and swab pressures have been known to cause formation fracture, lost circulation, and well-control problems. Accurate prediction of these pressures is crucially important in estimating the maximum tripping speeds to keep the wellbore pressure within specified limits of the pore and fracture pressures. It also plays a major role in running casings, particularly with narrow annular clearances. Existing surge/swab models are based on Bingham plastic (BP) and power-law (PL) fluid rheology models. However, in most cases, these models cannot adequately describe the flow behavior of drilling fluids. This paper presents a new steady-state model that can account for fluid and formation compressibility and pipe elasticity. For the closed-ended pipe, the model is cast into a simplified model to predict pressure surge in a more convenient way. The steady-state laminar-flow equation is solved for narrow slot geometry to approximate the flow in a concentric annulus with inner-pipe axial movement considering yield-PL (YPL) fluid. The YPL rheology model is usually preferred because it provides a better description of the flow behavior of most drilling fluids. The analytical solution yields accurate predictions, though not in convenient forms. Thus, a numerical scheme has been developed to obtain the solutions. After conducting an extensive parametric study, regression techniques were applied primarily to develop a simplified model (i.e., dimensionless correlation). The performance of the correlation has been tested by use of field and laboratory measurements. Comparisons of the model predictions with the measurements showed a satisfactory agreement. In most cases, the model makes better predictions in terms of closeness to the measurements because of the application of a more realistic rheology model. The correlation and model are useful for slimhole, deepwater, and extended-reach drilling applications.
Surge and swab pressures generated during well construction operations are critical. As thousands of wells are drilled every year, challenges associated with downhole pressure management have become more important for the oil industry. Inadequate estimation of surge and swab pressures can lead to a number of costly drilling problems such as lost circulation due to formation fracture, fluid influx resulting in kicks, breakdown of the formation at shoe due to limited kick tolerance or blowouts. An accurate surge pressure model is very important in planning drilling operations, mainly in wells with narrow safe pressure window, slimholes, low-clearance casings, deepwater, and extended-reach well applications.Existing surge/swab pressure models assume concentric wellbore. This assumption is hardly ever valid in horizontal and inclined wells with some degree of eccentricity. Ignoring the pressure reducing effect of eccentricity on surge and swab pressures may eventually lead to underestimating the tripping speeds, and thereby increase non-productive time and operation costs. Eccentricity between the wellbore and the drillpipe adds more complexity to surge and swab pressure calculations. Recent studies have indicated that surge pressure can be significantly reduced due to eccentricity. Although the numerical investigation shows encouraging results, understanding of pressure surges in wells with eccentric annular geometry is very limited.This paper presents the results of experimental investigations conducted to study the effects of eccentricity on surge and swab pressures. Experiments were performed in a test setup, which consists of fully transparent polycarbonate tubing, and inner pipe that moves axially using a speed controlled hoisting system. During the experiments, test fluid viscosity, trip speed, and eccentricity were varied. Experimental results were compared with predictions of existing models showing a satisfactory agreement. Results confirm that trip speed, fluid rheological properties, annular clearance and eccentricity significantly affect the surge pressure. In some cases, eccentricity can reduce surge and swab pressure by around 40%. Applying regression analysis, a generalized correlation has been developed to account for the reduction in surge pressure due to the eccentricity of the drillpipe.
For a successful drilling operation, downhole pressure or equivalent circulating density (ECD) control is very critical. Conventional drilling techniques require maintaining the bottomhole pressure between pore and fracture pressures. Specially, in deepwater wells, the margin between these pressures is very narrow. As a result, the bottomhole pressure and ECD must be predicted accurately and maintained within the narrow margin to avoid kicks and circulation losses. In the past years, a number of wellbore hydraulic studies have been conducted to predict the annular pressure losses. The effects of drillpipe rotation speed, borehole geometry, and pipe eccentricity on annular pressure loss have been investigated earlier. However, very limited studies have been carried to investigate the effect of tool-joint on the hydraulics. The presence of a tool-joint changes the annulus geometry between the drillpipe and casing/hole resulting in strong turbulence and fluid acceleration that generate additional viscous dissipation and pressure losses. This article presents the results of theoretical and experimental studies conducted to examine the hydraulic effects of rotating and non-rotating tool-joints. Two different tool-joint geometries were considered in the investigation. Tests were performed with water based fluids that have different rheological properties under both laminar and turbulent flow conditions. Results show substantial increase in pressure loss gradient around the tool-joints. The increase in pressure loss depends on fluid properties and flow geometries. The rotation of the pipe tends to slightly affect the pressure loss. Using dimensional analysis in conjunction with theoretical methods, new theoretical and semi-empirical models have been developed to account for the contribution of tool-joints to the total annular pressure loss. Models predictions show good agreement with previously reported laboratory experiments. The new models are very useful to predict downhole pressure in deepwater and extended reach wells where accuracy in hydraulic calculations is very important.
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