Scale Mitigation Enhances Safety and Production

Abstract

Calcium carbonate scale represents a significant safety and operational problem in offshore carbonate fields operated by Saudi Aramco. Scale can form on any surface where the pressure drop is sufficient for the produced fluids to form the scale and unload entrained solids. These solids will deposit on wellhead equipment such as the wellhead surface valves, piping, well tubulars and may plug the perforations. Well safety is jeopardized by hindering the operation of critical safety valves such as subsurface safety valves (SSSVs) and surface safety valves (SSVs). Further, scale buildup can cause backpressure problems in the tubing as the pipe internal diameter is reduced. Identification of scale in impacted wells requires periodic inspection and repair. This paper presents Saudi Aramco's experience in eliminating calcium carbonate scale by treating existing scale using HCl acid. Scale mitigation was initially attempted using an encapsulated inhibitor placed in the rat hole of vertical wells. This method had a limited treatment effective life and Saudi Aramco has moved to using inhibitor squeeze treatments. Chemical squeezes places inhibitor (phosphonate-based) directly into the formation of wet oil producers and is now the currently employed method of long-term prevention of scale. Case studies using both methods of scale mitigation are discussed as well as future plans to improve the effectiveness of these treatments. Introduction Field "B" is located both onshore and offshore along the western edge of the Arabian Gulf. The field is a north to south trending, elongated, anticlinal structure which measures 40 Km in length and 19 Km in width. This field was discovered in June 1964 and oil production began in 1967. Field "B" is a multi-reservoir field with 11 oil-bearing reservoirs at various depths from 2,133 to 3,048 meters (7,000 to 10,000 ft). The crude grade is Arab Extra Light crude with 7.5 mol% H2S and 4.5 mol% CO2. The two main reservoirs in this field are the HN and HD. The main productive formations have a low permeability in the range of 1–50 mD. The HN and HD reservoirs are carbonates reservoirs. Bulk XRD analysis of the HN reservoir cores indicates that the zone of interest contains 97–100 wt% calcite and 0–3 wt% ankerite. On the other hand, HD reservoir cores contain 70–92 wt% calcite, 0–30 wt% dolomite and 0–5 wt% ankerite.1 For pressure support purposes, peripheral water injection began in 1973 using 14 injectors into the HN reservoir and 28 injectors into the HD reservoir. The injection water is drawn from a shallow aquifer. The produced water is injected into a separate, segregated disposal system. Water breakthrough first occurred in the HN reservoir in mid 1975 and in late 1978 in the HD reservoir. Water-handling facilities were placed in service in 1983. The water cut for both the HN and HD reservoirs has gradually increased since the commissioning of these facilities and currently the field produces crude oil with an average water-cut of 32 vol%. The first scale build-up problem was encountered in 1987. Since then, scale became a difficult problem to manage as more wells started to produce formation or injection water. Scale build-up has resulted in several operational problems and production losses. The main objectives of this study are to:give a brief summary on the scale problems encountered in Field "B" carbonate reservoirs,discuss how this problem was addressed, andsummarize field experience gained from solving this problem. Chemical Analysis of the Formation Brines The composition of the produced water varies significantly for the HN and HD reservoirs as noted in Table 1. The Total Dissolved Solids (TDS) for the HN reservoir brines varies from 27,000 to 230,000 mg/L. The calcium ion concentration is in the range of 1,904–18,876 mg/L. The TDS for the HD reservoir brines is higher than that of the HN brines and varies from 33,400 to 292,000 mg/L. Calcium ion concentration is in the range of 2,392 to 39,280 mg/L.

Research of Scale Inhibitors in Downhole Equipment

“…In the study of desorption of inhibitors, produced water was passed through the model cell [13], under the same pumping regimes. The solution at the outlet of the cell was analyzed for the content of scale inhibitors.…”