Scale control has been a significant challenge for the management of well performance for several years in the Cusiana field in Colombia. Physical evidence of scale has been found in very few wells, but circumstantial evidence, including increases in gamma ray across productive zones and production declines inconsistent with reservoir properties has been widespread. Scale prediction, from water analysis, and the actual analysis of the limited scale samples suggested that carbonate scales were dominant, along with small quantities of barium sulphate scale. However, there was significant variation in the scale analysis, with both calcium and iron carbonates being identified as the major component. Furthermore, stimulation treatments to remove carbonate scale were not always successful and gamma ray readings often continued to increase after scale inhibition treatments designed for carbonate scales.
Several theories were developed based on this background information. These included the presence of mixed scales, with the sulphate scales not being removed during stimulation treatments and theories of near wellbore evaporation due to the high GOR of the produced fluids. This evaporation of connate water could lead to higher scaling potentials for conventional scales, or even the deposition of halite and other salts, and could induce the inhibition of further connate water into the evaporated zone.
In order to obtain direct evidence of the downhole scales, a multi-stage stimulation treatment was designed for a selected well. This paper describes this treatment, including operational details and data analysis. Clear evidence of both carbonate and sulphate scales downhole was found, however there was no evidence of halite deposition. It was not possible to conclude the presence of downhole iron carbonate scale, as corrosion products appeared to dominate the observed iron in flowback samples. Lessons learnt for future stimulation treatments in wells with suspected multiple scales are also discussed.
The Cusiana field has three productive horizons, the Mirador, Barco and Guadalupe reservoirs. The majority of the production is from the Mirador formation, a quartz arenite sandstone. Scale was first suspected as a damage mechanism in the field when it was observed that an otherwise unexplained decline in production coincided with an increase in gamma ray in the producing intervals. The evidence for scale deposition was not conclusive, but there was significant circumstantial evidence that suggested that it could still be the cause of decline.
Scale predictions from water analyses in each reservoir, using various models,1–2 showed that scale deposition was expected. The water analyses for each reservoir are shown in Table 1, with the associated scale predictions in Table 2. Calcium carbonate was the predominant scale predicted for all three reservoirs, with both iron carbonate and barium sulphate also being predicted in varying amounts. The quantity of scale actually deposited is clearly dependent upon both the scaling tendency and the volume of water produced. A common characteristic of the Cusiana wells was that the decline in production occurred at very low water cuts, including wells with less than 0.5% water cut.
The characteristics of the quartz arenite sandstone in the Mirador formation have previously been described 3. Due to the porosity-permeability relationship for quartz arenite sandstones, see Figure 1, any reduction in effective porosity can result in a significant reduction in permeability. Consequently, small amounts of scale deposited in the near wellbore region could cause a significant production loss.