Solids-cleanouts using Coiled Tubing (CT) remain a major part of total activity in the CT industry. Due to the multitude of parameters that influence solids-transport, it can be very challenging to design and successfully execute solids- cleanouts with CT in highly deviated, larger wellbores with e.g. 7″ 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 instead focused 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, which 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-bottom hole assemblies (BHA's) 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 of using 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, as well as estimates of solids- volumes during the cleanout to be established, based on feedback from the cleanout BHA before any solids have actually reached surface.
It is a known fact 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 are changing during an operation. Most of the previous solids- transport studies in the oil industry mainly focused 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's 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 volumes circulation to clean the well, annular fluid velocity two times of the solids-settling velocity or performing cleanout stages of a certain length.
In our previous studies1–6, a comprehensive experimental test of solids-transport for both the stationary circulation and the wiper trip was conducted. The effect of multi-phase 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.