Chemical Descaling to Restore Full Wellbore Access and Hydrocarbon Production: Use of 10% Hydrochloric Acid System and Customized Bottom Hole Assembly to Remove Deposited Scales on the Tubing Walls
Authors: Bougha Arthur , Iyengumwena Ricky , Alalade Temitope , Okeke Chinyere , Ndukauba Godian , Abudu Rahman 
Published: July 2023

Solids deposits is commonly experienced in oil field well tubulars mostly in mature fields where the wells have been produced over time. Full wellbore access is required to execute most thru-tubing intervention activities. It is also imperative that well production is restored to sustain the asset value of the operator. Restoring and sustaining production from brown field reservoirs has often proven to be critical to the success of oil and gas companies across the world. These increases in oil production are often achieved through various well intervention techniques. The relative instability in global oil prices makes such activities hugely attractive in the oil and gas sector as they are typically a lot cheaper than new well developments which also come with the inherent risks associated with drilling and completions operations. The key objective here was to descale a well with deposited scales in the tubing which prevented downhole intervention and also led to production deferment.

The well intervention targeted at improving the production from Well AA-052 in the Niger Delta region of Nigeria was initially planned as a slickline gas lift valve change out due to the observed issues with the vertical lift performance of the string. However, difficulties were experienced during the execution of the gas lift valve change out. Debris and scales present in the wellbore hindered access of the slickline tools deployed to achieve the gas lift valve change out. As a result, the activity was temporarily aborted

In this paper, the detailed planning and execution of AA-052 is addressed. With the debris recovered from the well, an extensive laboratory testing to ascertain the nature of solid deposit was conducted. These scale deposits were confirmed from the laboratory tests to be mainly calcium carbonate, hence a tailored fluid system consisting of 10% Hydrochloric acid was identified to effectively dissolve and remove the solid deposits. The treatment was deployed via Coiled Tubing utilizing a rotating downhole tool which provided additional mechanical means with 360 degrees coverage. The combination of the chemical treatment and mechanical means (rotating downhole tool) provided an effective and efficient descaling of the wellbore.

The key project deliverable was achieved as the full wellbore access was regained. Production was also restored as the well which originally had a shut-in tubing head pressure of 0psi has now increased to 1,360psi post the chemical descaling. The earlier attempt to carry out a planned slickline gas-lift valve change was suspended due to the wellbore access challenge as a result of the scale build-up. However, post the chemical descaling activity, the planned slickline gas-lift valve change out activity was successfully carried out. Initial production post chemical descaling was 1,000bopd. With the gas-lift valve change out executed, an additional gain of 200bopd in production was achieved. The well is currently producing with ca.1,200bopd with a recoverable volume of 0.93 MMstb reserves.

This paper represents a typical case of flawless diagnostic, design and delivery process. Getting a scale sample and performing an extensive laboratory testing and fluid selection was key to the successful descaling operation. Utilizing Coiled tubing for pinpoint treatment by reciprocating across the solid build up sections while pumping 10% HCL acid through an incorporated high jetting and rotating tool made it most effective