Extreme Challenges in FeS Scale Cleanout Operation Overcome using Temporary Isolation, High Pressure Coiled Tubing, and Tailored Fluid Systems

Abstract

The carbonate gas producing zones of the Ghawar field have been impacted by extensive FeS scale deposition, reducing overall gas production and significantly increasing risks of well interventions. Previous remediation included the use of workover rigs, which can be costly because of the time necessary for workovers and lost production. H 2 S levels (2 to 5%) found in the reservoir also contribute to higher costs and risks when using workover rigs.A chemical solution was also considered, but the FeS could not be 100% dissolved with HCl and the chemical reaction generated large amounts of H 2 S in addition to existing high levels of H 2 S in the reservoir. This poses a safety concern with the returns at surface along with potential corrosion of the coiled tubing (CT) and completion. Therefore, the safest and most economical method was deemed to be mechanical descaling with CT.This paper discusses two wells where mechanical descaling was applied using CT. Each well involved four major challenges that included low reservoir pressure, increased reservoir temperature, horizontal openhole completion, and scale with high specific gravity (3.7 to 4.3). The low reservoir pressure required pay zone isolation to allow for returns to circulate out the heavy scale and to minimize fluid losses to the formation. The fact that the wells had long, openhole sections created another challenge for isolation and cleanout. With a bottomhole temperature (BHT) as high as to 310°F, the operational envelope of temporary chemical packers in combination with loss circulation materials (LCMs) to isolate the openhole section had to be expanded. Following mechanical descaling with CT, the final challenge discussed in this paper is the process to clean out the LCM in the horizontal openhole and bring the well back to maximum gas production using pinpoint stimulation techniques.

Corrosion and Scale Formation in High Temperature Sour Gas Wells: Chemistry and Field Practice

“…It has been found that a fluidic oscillation technology cleaning device (Webb et al 2006, Leal et al 2007, Buali et al 2014) and a high pressure jetting tool with a down-hole motor or turbine with mills (Bolarinwa et al 2012, Mukhliss et al 2014) are effective in removing iron sulfide scales. Current mechanical procedures involve temporary reservoir isolation without formation damage (Buali et al 2014, Mukhliss et al 2014.…”

“…Current mechanical procedures involve temporary reservoir isolation without formation damage (Buali et al 2014, Mukhliss et al 2014. In a severely scaled well, the differential of pressure between the pressure upstream of the choke and downstream of the choke is not changed even though one has decreased the choke of the well.…”

“…In a severely scaled well, the differential of pressure between the pressure upstream of the choke and downstream of the choke is not changed even though one has decreased the choke of the well. Mechanical methods of descaling are currently used in the Southern area (Leal et al 2007;Buali et al 2014). In one instance 316 kg of solid materials were recovered at the surface.…”

“…In one instance 316 kg of solid materials were recovered at the surface. Well descaling efforts have allowed increased well gas production and overall obtainable gas reserves (Buali et al 2014, Mukhliss et al 2014.…”