This paper describes the application of a directional drilling model to wellbore spiraling and compares it to field data. In this paper, the spiraling tendency of a Bottom Hole Assembly (BHA) is determined from a stability analysis of the delay differential equations that govern the propagation of a borehole. These propagation equations are derived from a novel mathematical model, constructed by combining (1) a bit/rock interaction law, which relates the forces acting on the bit to the penetrations of the bit in the rock per revolution; (2) kinematical relationships, which link the bit motion to the local borehole geometry; and (3) a model for the BHA, which expresses the forces and moments at the bit from the external loads and the deflections imposed by the stabilizers. Spatial delays associated with the positions of the stabilizers account for the feedback of the borehole as the stabilizers interact with the wellbore. The analytical form of the propagation equations makes it possible to carry out a stability analysis and determine whether borehole spiraling is expected. The coefficients of the propagation equations embody the characteristics of a particular drilling system; these include the BHA configuration, bit properties, and the active weight, Wa, a calculated reduced weight on bit that depends on the actual Weight On Bit (WOB), the cutter wearflats, and the rock strength. If the bit trajectory is unstable, then any perturbation in the borehole geometry is amplified gradually, eventually leading to the generation of a spiraled hole. The stability of the bit trajectory essentially is controlled by the magnitude of a dimensionless group relative to a critical value that depends on the BHA configuration. This group is a function of the lateral steering resistance of the bit, the bit wear, the rock strength, and the WOB. Thus, a BHA can be either stable or unstable depending on the selected bit, its state of wear, and the WOB. Predictions of the stability analysis are compared with field data from spiral holes pertaining to eight sections from four wells drilled with different bit types and BHA configurations. The paper shows that the propensity of a BHA to spiral can be predicted by the model by assuming reasonable values for parameters such as the lateral steering resistance and the part of the WOB transmitted by the cutter wearflats. This comparison suggests that the model can be used to optimize BHA designs and critical WOB levels that will mitigate the creation of spiral holes.

This paper describes the application of a directional-drilling model to the phenomenon of wellbore spiraling and compares its predictions with field data. The spiraling tendency of a bottomhole assembly (BHA) is determined from a stability analysis of the delay differential equations (DDEs) that govern the propagation of a borehole. These propagation equations are derived from a novel mathematical model, constructed by combining a bit/rock-interaction law, which relates the force and moment acting on the bit to its penetrations per revolution through the rock; kinematic relationships, which link the bit motion to the local borehole geometry; and a model for the BHA, which expresses the force and moment at the bit as a function of the external loads and the deflection imposed by the stabilizers. Spatial delays, associated with the positions of the stabilizers, account for the feedback of the borehole geometry as the stabilizers interact with the wellbore.The analytical form of the propagation equations makes it possible to perform a stability analysis and determine whether borehole spiraling is expected. The coefficients of the propagation equations embody the characteristics of a particular drilling system; these include the BHA configuration, bit properties, and the active weight W a , a reduced downhole weight on bit (WOB) that depends on the actual downhole WOB, the state of wear of the bit, and the rock strength. If the bit trajectory is unstable, then any perturbation in the borehole geometry is amplified gradually, eventually leading to the generation of a spiraled hole.The stability of the bit trajectory essentially is controlled by the magnitude of a dimensionless group, a function of the lateralsteering resistance of the bit, the active weight, and properties of the BHA, relative to a critical value that depends only on the BHA configuration.Predictions of the stability analysis are compared with field data from spiral holes pertaining to eight sections from four wells drilled with different bit types and BHA configurations. The paper shows that the propensity of a BHA to spiral can be estimated by the model by assuming reasonable values for parameters such as the lateral-steering resistance and the part of the WOB transmitted by the cutter wear flats. This ability means that the model can be used to optimize BHA designs and determine critical WOB levels, both of which will mitigate the creation of spiraled holes.

An operator experienced sub-optimal drilling performance in a multi-rig, multi-well drilling campaign in the Sultanate of Oman. This paper describes collaboration between the operator and a bit vendor to establish key operating parameters for efficient drilling based on fundamental drilling mechanics and laboratory testing. The concept was implemented and validated in that field's 12 ¼″ section. The method makes minimum weight on bit (WOB) and torque recommendations based on the observation that a critical depth of cut (DOC) should be exceeded if a polycrystalline diamond compact (PDC) bit is to drill efficiently. Below this DOC the drilling efficiency, which is inversely related to the mechanical specific energy (MSE), can decrease significantly depending on the rock strength and the downhole environment. This behaviour has been associated with a transition between shearing and grinding as the predominant rock destruction mechanism. Critical DOCs and corresponding WOB and torque levels were determined from a wide range of laboratory drilling tests in carbonate rocks. Initially bit design details and anticipated formation properties were used before a bit run to develop minimum WOB recommendations and torque targets which were communicated to rig site personnel in a run-specific drilling roadmap. Rig crews were encouraged to maintain WOB above the minimum whenever possible while avoiding damaging stick/slip vibration and observing pre-agreed bit, bottom hole assembly (BHA) and rig imposed limits. Later the pre-drill recommendations were supplemented by computing the real time instantaneous DOC during drilling and comparing this with the critical value for efficient drilling to indicate whether the current WOB should be increased. Sub-optimal performance in early wells was frequently associated with parameters insufficient to achieve the critical DOC and torque. Penetration rate performance showed a significant and consistent improvement after adoption of the roadmap. The section average penetration rates increased by 45% and routine shoe to shoe bit runs were achieved where previously an average of 2.6 bits per well were required for this hole section. We conclude that ensuring DOC and torque exceeded the thresholds for mechanically efficient drilling provided an engineering basis for selecting WOB and was a major factor in the observed drilling performance increase. In the planning phase this approach generates an engineered parameter roadmap tailored to each specific application and bit design, and provides WOB and torque targets as input for BHA design. Many other drilling parameter optimisation schemes require significant intervals of steady drilling to calibrate underlying models. Rapid changes in formation properties over the calibration interval could make the parameter recommendations inappropriate for the rock actually drilled. In contrast the method described in this paper uses real time, instantaneous performance measurements to determine whether the parameters are delivering mechanically efficient drilling. Its recommendations are consequently more robust against fluctuations in formation drilling properties, at least for the predominantly carbonate formations so far evaluated.

This paper presents a model which aids the decision making process to determine the optimum point to trip out of hole to change a dulled drill bit. Drill bit interaction with abrasive rock and impact damage due to vibrations causes the state of wear or damage to cutters to increase. This results in a slower rate of penetration (ROP) than with a new bit with sharp cutters. The operator faces the question whether or not to take the time to pull the bit out of hole and change it for a new one. A delayed or a premature decision of when to pull the bit can add to the overall project cost. In the past the decision has been made using either the operator's experience or a simple cost model projecting a single bit run into the future. The model presented in this paper optimizes the bit strategy for an entire well with multiple bit runs to obtain the most economical choice of bits and bit trip strategy. The model can be used during planning, in real time implementation and for post well performance analysis. In particular, during real time and post run evaluation the model can be used for performance benchmarking of ROP by comparing an expected rate of progress with the actual field data. This paper further presents a case study highlighting the value of each of the features of the model including potential time and cost savings. It shows that running the model real time can reduce well costs significantly in hard rock areas. The deployment of the model is not limited to any specific application and can save costs on any well where the operator has a requirement for multiple bit runs and a good understanding of drilling performance through numerous geological intervals.