Calcium carbonate scale was detected in several vertical wells in a sandstone reservoir in Saudi Arabia. The scale was found downhole, plugging gravel-pack screens and the intake of submersible pumps. Scale build-up caused decline in oil production from this field. The sandstone reservoir is watersensitive and has a bottom-hole temperature of 160ºF.
An emulsified-type scale inhibitor (phosphonate-type) treatment was designed to mitigate scale formation in this field. The treatment was successfully applied in more than forty-five wells. Some of these wells were de-scaled before the treatment, while others were treated before scale detection. Moreover, some of the wells were subjected to two scale mitigation treatments.
The main objective of this paper is to assess the long term outcome of the scale squeeze treatment based on field data. A second objective is to determine the actual minimum inhibitor concentration (MIC) and treatment lifetime. In addition, the impact of the second squeeze will be investigated. Samples of produced water following the scale mitigation treatment were collected and analyzed over the last five years.
The emulsified scale squeeze treatment was successfully conducted in wells with water-cut that ranges from 3–80 vol%. No operational problems were encountered during the first and second scale inhibitor squeeze treatments. The inhibitor return following the second squeeze treatment was similar to that noted following the first treatment. Lower scale inhibitor return in term of phosphorous was noted after re-squeezed treatments than that of the first squeeze. The volume of the scale inhibitor was affected the levels of the inhibitor in well flow back samples. Most of the treated wells have lifetime more than two years and the actual MIC in Field H was found to be nearly 0.9 mg/l.
Introduction and History of Field H
Oil production from Field H started in August of 1994. Water injection started in this field in late 1994 and early 1995, using a peripheral water injection pattern to maintain reservoir pressure. The injection water is obtained from a shallow aquifer and is produced from 17 supply wells with electrical submersible pumps (ESPs) and sent to four water injection plants (WIPS). The water pressure is increased to approximately 1,200 psig and pumped into the injection wells. Table 1 gives the chemical analysis of the water produced from selected supply wells. Note that the supply water contains high levels of sulfate ion, which contributed in the growth of sulfate-reducing bacteria and subsequent well plugging problems encountered in water supply wells and injectors in this field.1–3
Many of the oil wells started producing wet-crude shortly after oil production started. Oil production continued from this field without encountering any major problems until 1997, where a decline in oil production was noted in some wet wells. This decline was attributed to calcium carbonate scale formation, which was found covering the gravel pack screens, pump intake and on the impellers of the ESPs. In addition, down-hole video camera verified the presence of scale on the gravel packed screens and on the perforation.4
An extensive sampling campaign started in this field since its commissioning. Chemical and isotopic finger printing were conducted on the injection and produced waters to determine the time of injection water breakthrough.5 Typical chemical analysis of the produced water from some of the treated wells is shown in Table 2. Note that the produced water has a much lower TDS than the injection water. It has significant sulfate content, albeit it is less than that of the injection water.
To avoid any future decline in oil production due to scale formation in Field H, scale mitigation campaigns have been commissioned since mid 2000. Different types of scale mitigation treatment were tried in this field. These varied between encapsulated treatment, adsorption/desorption squeeze and forced precipitation squeeze.6–20 However, the latter, emulsified scale inhibitor squeeze was found to be the effective type of treatment for this sandstone field.21–24
The objectives of this study are to:Investigate the outcome of the emulsified treatment based on long term monitoring,Determine the actual minimum inhibitor concentration (MIC) and treatment lifetime, andAssess the impact of the re-squeezed treatment in the field based on analysis of the flow back samples.
