Well bore cleanouts have been performed for many years with coiled tubing and it may be said that the cleanout operation is one of the most routine services performed in the coiled tubing industry; but until recently design was no more than a rule-of-thumb approach. Sufficient information is now available to facilitate informed decisions and achieve reliable engineering solutions, utilizing a sophisticated particle transport model.
A methodical thought process, based on extensive research and mathematical modeling, will be presented for designing specific well bore cleanouts in a challenging offshore field in the Middle East. Case histories from a number of wells are documented detailing the conclusions from each. Laboratory analysis was performed on samples collected from the cleanout operations to further optimize and validate the design process. It is the intent of this paper to demonstrate the importance of sound engineering design, aided by comprehensive computer modeling technology.
Particle transport is a process to remove material such as formation fines, drill cuttings, sand, etc. out of wells. The use of coiled tubing to convey circulation fluids and tools to facilitate a thru-tubing cleanout is considered standard industry practice in the oilfield. Typical fluids used to facilitate these operations are water, brine, and water based gellant systems. Hydrocarbon fluids may also be pumped, however inherent HSE issues must be addressed when pumping hydrocarbon fluids. It may be safe to say that "cleanouts with coiled tubing is the number one application". Having said that, one may also say that the industry has really lacked a good understanding of how to effectively transport particles. The fact that regular operations are conducted on a daily basis globally does not necessarily mean that everyone conducting cleanout operations has a full understanding of how to effectively design and execute a particular cleanout.
Based on a comprehensive experimental test of solids transport in the previous studies1–7, a patented fill cleanout process technology8 with CT has been developed and proved by field operations9–13. The process includes running CT into the well while circulating fluid using a nozzle with a "high energy" jetting action pointing forwards down the well to stir up the particulate solids and allow the CT to reach a target depth or bottom of the well (penetration stage). When the bottom or desired depth is reached, the hole can then be cleaned either by circulating a fluid while keeping the CT stationary (circulation stage) or by pulling the CT out of the wellbore with continuous circulation (wiper trip stage), or by a combination of these stages. In the wiper trip mode, a reversing jetting nozzle with low energy is used to circulate the fluids and to create a particle re-entrainment action to enhance agitation of the solids and then entrain the particulates in suspension for transport out of the wellbore while pulling the CT out of the hole. The reverse jetting action along with a controlled pump rate and POOH speed can produce a solids transport action which cleans the hole completely by keeping the solids in front (upward) of the end of the CT in continuous agitation. The low energy nozzles have a low pressure drop which allows for higher flow rates and results in improved cleanout efficiency. This method and tool is more efficient than existing methods since the process may be limited to one pass or sweep with the option of resetting the tool for repeated cycles if required.
Factors Affecting Clean Out Operations
There are several factors which affect the clean out operation and hence the efficiency of the clean out job. When designing a clean out job, those factors should be taken into consideration to achieve a successful clean out job
Fill type: Fill type plays an important role in sand clean out design. Fill type can be fines (small grain size) or coarse (large grain size). The fill type affects fluid selection, penetration and wiper rate determination. The larger grain size is more challenging to transport in a cleanout job. The density is related to the circulation time (how long to circulate at TD to move heavy solids out of well). The fundamental reason for circulation and turbulent flow is to prevent solids falling back into the well when performing a wiper trip.