Repeatedly Increased Efficiency and Success Rate From a New Solids-Cleanout Process Using Coiled Tubing: A Review of Recent Achievements From More Than 100 Operations

Abstract

Summary Solids cleanouts using coiled tubing (CT) remain a major part of total activity in the CT industry. Because of the multitude of parameters that influence solids transport, it can be very challenging to design and execute solids cleanouts successfully with CT in highly deviated, larger wellbores with 7-in. production tubing, or even larger tubulars, installed. Numerous papers have been written about the development of wiper-trip cleanout technology and associated engineering design tools, but this paper is focused instead on important practical issues that directly impact the effective implementation of wiper-trip technology in the field. This paper presents the results and lessons learned based on a database that was compiled from more than 100 solids cleanout operations using wiper-trip methodologies. Results will be presented showing how the wiper trip cleanout methodology has improved cleanout efficiency and success rate. Examples are presented showing how the effectiveness of cleanout bottomhole assemblies (BHAs) involving positive-displacement motors (PDMs) and mills has been improved, while simultaneously reducing stress on surface equipment during the operation. Circulation rates higher than specified maximum rates for the PDM are being used without danger of damaging the PDM, while reducing the total volume pumped through the PDM during the cleanout by 80-90%. Larger outer diameter (OD) items in the BHA are kept clean of solids while wiper tripping, reducing the risk of stuck CT and protecting sensitive completion components from undesirable interactions with the PDM/mill BHA. Multiple wiper trips can be performed in one run without the use of drop balls, while having the ability to use selected functions of critical BHA components and full-size drifting of the wellbore for subsequent operations. Field-proven procedures are explained, allowing solids loading in the annulus to be controlled and reduced when necessary, and allowing estimates of solids volumes during the cleanout to be established, on the basis of feedback from the cleanout BHA before any solids have actually reached surface. Background It is well known that many CT operations start with a cleanout before being able to conduct other work in the wellbore. A review of all CT operations performed in the Norwegian sector of the North Sea since 2001 shows that 74% of the CT operations involve cleanouts of solids from the wellbore. This is reason enough to concentrate on performing repeatedly successful cleanouts, so that the process is economical and allows subsequent operations in the wellbore to continue as planned. Solids transport is affected by many variables, and the complexity of the phenomena presents challenges to the field engineer who is trying to determine how the parameters affect solids transport even as one, or more than one, of the variables is changing during an operation. Most of the previous solids-transport studies in the oil industry focused mainly on finding the minimum critical velocity in the wellbore annulus for conventional rotary drilling with mud fluids. The studies lack information related to the prediction of the equilibrium solids-bed height during tripping in, the wiper-trip speed during tripping out, and the prediction of the hole-cleaning time. In field operations, people often use outdated "rules of thumb" (i.e., 2-hole-volume circulation to clean the well, annular fluid velocity two times the solids-settling velocity, or performing cleanout stages of a certain length). In our previous studies (Li and Walker 2001; Walker and Li 2001, 1991; Li et al. 2002; Li et al. 2005; Li and Wilde 2005), a comprehensive experimental test of solids transport for both the stationary circulation and the wiper trip was conducted. The effect of multiphase flow, rate of penetration (ROP), deviation angle, circulation fluid properties, particle density and size, fluid rheology, pipe eccentricity, wiper-trip speed and nozzle type on solids transport was investigated. On the basis of comprehensive research (Li and Walker 2001; Walker and Li 2000, 2001; Li et al. 2002; Li et al. 2005; Li and Wilde 2005), an effective solids-cleanout methodology/process using CT (see Fig. 1) has been developed, patented (Walker et al. 2005), and proved by field operations (Engel and Rae 2002; Ovesen et al. 2003; Hobbs and Liles 2002; Gilmore et al. 2005; Nasr-El-Din et al. 2006; Li et al. 2006). The developed solids-cleanout methodology/process includes a specialized downhole cleanout tool and a solids-transport simulator for CT in vertical, deviated, and horizontal well conditions. Empirical formulas are applied to predict surface and downhole pressures, fluid velocities, and solids transport effectiveness. The simulator is a powerful analytical tool that can characterize wellbore hydraulics and solids transport considering downhole conditions, especially when applying the concept of removing solids from wellbores by use of wiper tripping. Its use has resulted in better-designed and -performed cleanouts (Engel and Rae 2002; Ovesen et al. 2003; Hobbs and Liles 2002; Gilmore et al. 2005; Nasr-El-Din et al. 2006; Li et al. 2006). The specialized downhole cleanout tool offers the option of using downhole-facing, high-energy jetting nozzles or a PDM, in order to ensure sufficient penetration energy required for harder solids depositions. Having penetrated the targeted solids, the specialized downhole cleanout tool allows the fluid pumped during the cleanout to be redirected to uphole-facing, low-energy nozzles, simultaneously stopping the fluid stream through the jetting nozzle or PDM. A surface indication of the specialized-downhole-cleanout tool position is provided for the CT operator.

The Effects of Fluid Hammer Tools on the Efficiencies of Coiled Tubing Plug Milling - A Comparative Best Practices Study

“…Use of specialized washing tools with multi-cycle, backward-facing jets, combined with computer modeling, considerably improves solids cleanout efficiencies (Sach et al, 2007). The technology has led to a step change in how the industry uses coiled tubing to clean wells.…”

“…Higher viscosities typical of gels hinder the creation of turbulent flow, which is ideal to entrain solids in the flow stream. For this reason it is believed that gels are not significantly advantageous over water in horizontal sections (Sach et al, 2007). The best practice in this case is to limit the number of gel sweeps to two as indicated above.…”

Optimization of Operation Parameters for Helical Flow Cleanout with Supercritical CO2 in Horizontal Wells Using Back-Propagation Artificial Neural Network

“…Water jets are used extensively in conventional horizontal wellbore cleanouts to break up consolidated sand deposits and sweep away solids. In horizontal wells, the flow direction is perpendicular to the gravity, so the sands have a tendency to settle down, resulting in low cleanout efficiency or even worse troubles such as stuck-pipe [ 1 ]. Over the past years, several improved cleanout techniques, such as wiper tripping and sand vacuuming [ 2 ], have been developed.…”

Mechanism and Characteristics of Horizontal-Wellbore Cleanout by Annular Helical Flow

“…Wellbore cleanout with coiled tubing (CT) has been applied for decades (Courville et al 2000;Hobbs and Liles 2002;Sach and Li 2008). Several cleanout techniques have been developed throughout the years, including stationary circulation, wiper tripping, reverse circulation, sand vacuuming, and Venturi sand/junk bailers.…”