Comprehensive Cuttings-Transport Model Optimizes Drilling Operations for Hole Cleaning
This paper discusses a new, comprehensive cuttings-transport model designed to enable safe and improved hole-cleaning operations.
This paper discusses a new, comprehensive cuttings-transport model designed to enable safe and improved hole-cleaning operations. Local velocity profiles are calculated for a given fluid and compared against the local critical velocity for cuttings transport. Then, the location and the magnitude of annulus blockage are numerically assessed. After an annulus becomes partially blocked with cuttings deposited in a cuttings bed, continuity and momentum equations are solved for the blocked annulus to estimate the new local velocities. The annulus is divided into small sections, and the pressure profile is calculated with the progression of time. The results from this study agree with large-scale flow-loop experiments and field observations, showing that axial flow alone is not enough for effective hole cleaning in high-deviation and horizontal wells. The results also provide a way to predict packoff events during which the string can become stuck, the equivalent circulating density can spike, and lost circulation can occur.
Currently, no sensor exists that measures hole-cleaning effectiveness, or the level of cuttings deposition in a wellbore, directly. Instead, sensor data collected during drilling usually are analyzed to evaluate the effectiveness of hole-cleaning practices. Decision-making for effective hole cleaning currently is dominated by rules of thumb. This may be explained by the lack of computational capabilities to run real-time hole-cleaning simulations at the rigsite or at remote operations centers. Therefore, years of research on cuttings transport and drillers’ repeated experiences have been translated into rules of thumb and sometimes exercised as best practices.
Despite the operational success of such guidelines, they are not universally valid—particularly in more-challenging drilling operations—and can impose severe and costly restrictions. According to the authors, the industry’s aim should, therefore, be to move away from such generic rules of thumb and use today’s advanced computational and modeling capabilities to provide customized, case-based cleaning recommendations.
Numerical and Pseudotransient Modeling of Cuttings Transport
The complete paper presents a detailed discussion of a new, steady-state cuttings-transport model based on the present numerical-modeling approach, then improved subsequently to include the effect of time to become a pseudotransient model. Numerically solving the time-dependent mass and momentum equations for the flow in annuli, including the effects of eccentricity and rotation for this research area, would not be practical for engineering purposes because of the high computational cost and implementation complexity. A practical approach is taken in the complete paper, and the steady-state model is extended to be a pseudotransient cuttings transport model. This is accomplished by recalculating the entire wellbore-cuttings profile at each timestep, taking the results from the previous state into account. By doing so, an easy-to-implement and computationally less-resource-consuming method is achieved. The method can be executed in near-real-time, which is practical for field-application purposes.
Using the proposed steady-state model, the cuttings deposition at a wellbore can be estimated at three spatial dimensions including along the trajectory of the wellbore. This is achieved by dividing the wellbore into sections and showing the estimation of cuttings accumulation at various positions along the wellbore (Fig. 1).
Cuttings at each cell can be tracked through time (integrated with well-construction data) while also taking into account the rate of penetration (ROP). The result is the ability to simulate whether more or fewer cuttings will accumulate or be transported for various drilling parameters with respect to time. An example of this algorithm in use reflects the cuttings accumulation over time for the various states seen while sliding and rotating (Fig. 2).
Several scenarios are shown at a deviated well at arbitrary times to illustrate the capabilities of the model. At the time of t1, the drill bit reaches the bottom and the cuttings start to accumulate at the estimated locations. At t15, as drilling progresses, the cuttings accumulate in more cells. At t50, the inner pipe rotation is increased and a significant reduction in cuttings accumulation is estimated. At the instance of t100, both the flow rate and pipe rotation are elevated.
The complete paper explains how wellbore cuttings volume was reconciled with penetration, then presents a detailed discussion of an analysis using the proposed model with actual drilling data from a stuck-pipe incident to simulate hole cleaning while drilling.
- A time-dependent cuttings-transport model that incorporates the effects of pipe rotation, eccentricity, and partial annulus blockage is presented. A new approach that differs from those reported in the literature previously is accomplished by introducing a local-critical-velocity concept and comparing the results to local resulting velocities at the cross section in annuli. By use of this method, cuttings transport along the wellbore can be analyzed numerically.
- Without pipe rotation, even at elevated axial flow rates in a fully eccentric annulus, cuttings will deposit on the low side of the hole and around the drillstring in a cuttings bed. For example, during slide drilling with a directional motor (when the drillstring is not rotated from the surface), local velocities are significantly lower compared with rotary drilling because of the reduced local velocities on the low side when the drillstring is eccentric. These low local velocities together with gravity allow for increased levels of cuttings accumulation. This is further validated by the field data used in the case study, after the curve is built by sliding with a directional motor. Because of inefficient hole cleaning during drilling operations, the bottomhole assembly packed off while tripping, resulting in a stuck-pipe event.
- Using the time-dependent model presented in this work, drillers can now estimate the required duration of a cleanup cycle before tripping out of the hole. The simulations can be run in near-real-time, and they are significantly more practical to implement, compared with numerically solving the transient continuity and momentum equations.
- Using the proposed numerical modeling, drilling operations can be optimized for hole cleaning. During the planning phase, the modeling can flag any potential design issues that could negatively affect hole cleaning and allow for optimization. During well construction, the model can be used in conjunction with along-string pressure measurements to generate an accurate representation of the state of cuttings evacuation in the well, which, in turn, will aid in better real-time hole-cleaning management and problem avoidance.
This article, written by JPT Technology Editor Judy Feder, contains highlights of paper IADC/SPE 199587, “Modeling Cuttings Transport and Annular Packoff Using Local Fluid Velocities With the Effects of Drillstring Rotation and Eccentricity,” by Oney Erge, SPE, and Eric van Oort, SPE, The University of Texas at Austin, prepared for the 2020 IADC/SPE International Drilling Conference and Exhibition, Galveston, Texas, 3–5 March. The paper has not been peer reviewed