This paper investigates the effects of a down-hole anti-stall tool (AST) in deviated wells on the drilling performance of a rotary drilling system. Deviated wells typically induce frictional contact between the drill-string and the borehole, which affects the drill-string dynamics. In order to study the influence of such frictional effects on the effectiveness of the AST in improving the rate-of-penetration and drilling efficiency, a model-based approach is proposed. A dynamic model with coupled axial and torsional dynamics of a drilling system including the down-hole tool in an inclined well is constructed. Furthermore, the frictional contact between the drill-string and the borehole is modelled by a set-valued spatial Coulomb friction law affecting both the axial and torsional dynamics. These dynamics are described by state-dependent delay differential inclusions. Numerical analysis of this model shows that the rate-ofpenetration and drilling efficiency increases by inclusion of the AST, both in the case with and without spatial Coulomb friction. Furthermore, a parametric design study of the AST in different inclined drilling scenarios is performed. This study reveals a design for the AST,
Summary In this paper, we investigate the effect of a downhole passive regulator [anti-stall tool (AST)] on the dynamics of rotary drilling systems in interbedded formations. Drilling in interbedded formations can significantly affect the vibrational signature of these systems and the associated drilling performance. Hence, models to assess the impact of drilling in such formations are needed. Here, we construct a dynamic model of the drillstring system which incorporates the bit/rock interface laws for the transitional phase of bit motion between rock layers with distinct mechanical properties. In the model, the axial and torsional dynamics of the drillstring systems are coupled by these interface laws and cast in the form of discontinuous (state-dependent) delay differential equations. Next, we include the AST in the dynamic model to enable the assessment of the influence of this downhole tool on drilling performance, in particular in terms of rate-of-penetration (ROP) and drilling efficiency. Furthermore, we also investigate the mechanical specific energy (MSE) and steady-state power losses at the bit (due to frictional torque) for different operational conditions and rock layer thicknesses. The analysis reveals that an increased drilling efficiency and lower MSE are obtained by incorporating the downhole tool in the drillstring, resulting in a higher ROP and a lower frictional contact between the bit and rock in interbedded formations.
Over the last years, the use of autonomous solutions for balancing the loading on the drill-bit has increased annually. By 2021, downhole tools for this purpose have been used for more than 1,500 wells and these become possibly the fastest growing trend in drilling. Polycrystaline Diamond Compact (PDC) drill-bits represent a great potential for drilling economics when steady cutting is attainable. Deep drilling, however, typically involves long drillstring causing an array of dynamic instabilities preventing steady cutting conditions at the bit. Such behavior affects drilling performance in terms of the rate-of-penetration (ROP) and system damage and failure. This leaves a big potential for improvement of drilling performance. The first experiments with an autonomous downhole regulator constructed were completed at Ulrigg in Stavanger almost twenty years ago to tap into this potential. Several versions of similar tools have since developed using a variety of mechanical and hydraulic functions to modify and shift the forces acting on the drill-bit in order to improve drilling performance. The Norwegian operator Equinor has participated from the very start of this new automation trend. By 2020 they had deployed downhole regulators to a total of 93 well sections on the Norwegian Continental Shelf alone. In this paper, Equinor shares statistic plots from comparing these first 93 sections to well section with conventional BHA's. The data show how the continuous improvement of the regulator eventually led to gradual improvement of both ROP and footage - in addition to its initial task of reducing vibrations. By utilizing a variety of dynamic models, predictions and sensitivity analysis, it has been revealed that the downhole regulator could change the dynamic response of the bit such that the friction losses at the bit are reduced and the rock cutting efficiency is improved. In this paper, it is shown that such benefits can also be expected in real-life scenarios in which two key aspects play a role: 1) a PDC bit penetrating heterogeneous layers of rock formations, and 2) involving two frictional losses due to borehole - drillstring contact in deviated wells. This paper brings a unique insight to the fundamentals, advanced mathematical models, and statistical results from a new line of drilling technology. The autonomous regulators bring a combination of reduction in risk and time to drill that makes a significant impact on cost.
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