Water Quality Considerations in Malaysia's First Waterflood

Abstract

Summary Malaysia's first waterflood, a seawater injection system, was placed in service in the Tapis field in 1982. This paper outlines the design basis and describes the system. Then water quality during and after startup and the formulation of a water quality guide using Millipore membrane filtration tests are discussed. The experience gained and data presented can be applied to current and future waterfloods. Introduction Geological and reservoir engineering studies of the Tapis field' located approximately 209 km 11 30 miles] northeast of Kuala Trengganu in the South China Sea (Fig. 1) have indicated that recovery can be almost doubled by gas injection and waterflooding. For the Tapis reservoirs, gas injection was initiated in late 1981 and is being used to help maintain reservoir pressure. In addition to gas injection, waterflooding was pressure. In addition to gas injection, waterflooding was initiated in Aug. 1982 to maintain pressure and to displace the oil in the reservoir. Water is lifted from the sea by seawater lift pumps (Fig. 2). Filtration removes the organic and inorganic materials from the seawater to prevent wellbore plugging. Deaeration removes oxygen from prevent wellbore plugging. Deaeration removes oxygen from seawater to minimize corrosion and wellbore plugging from corrosion products. Booster and injection pumps provide the required pressure for injection into the provide the required pressure for injection into the reservoir. The waterflood plant is designed to handle 35 772 M3 [225,000 bbl] of water a day at a maximum injection pressure of 19 300 kPa [2,800 psig]. The water is injected into the Tapis field oil zone. The injected water displaces oil toward the producing wells and maintains reservoir pressure. The type of flooding pattern chosen for the Tapis field is a "3-to-1 line drive." pattern chosen for the Tapis field is a "3-to-1 line drive." In this pattern there are three rows of oil producers to every row of water injectors (Fig. 3). The waterflood plant is installed on the Tapis B platform. Treated water is then transported from Tapis B to Tapis A, Tapis C, and Tapis D by subsea pipelines (Fig. 4). This water will be transported to Esso Production Malaysia Inc.'s (EPMI's) future Guntong field for injection. Design Basis It is of prime importance to inject water that contains carefully controlled levels of oxygen, suspended solids, and bacteria. Injection of poor-quality water, which is often the result of poor design or improper maintenance, can lead to plugging (e.g., suspended solids, calcium carbonate scale, and bacteria) and corrosion. Both of these can be minimized by careful plant design and operation and monitoring of water quality. To meet these requirements, the plant has to operate under steady-state conditions, meaning the avoidance of large changes in flow rate, which cause surging and imbalance in the treating system. To achieve high-quality water continuously, we have divided the overall water-injection plant into two separate sections. These arethe water treatment plant, comprising all equipment from the seawater lift pumps to the outlet of the vacuum tower, andthe pumping, distribution, and injection section, which we call the distribution plant (Fig. 2). The vital element in designing for a continuous steady-state operation was the provision of an overboard line from the discharge of the vacuum tower. This allows the water treatment plant to be run continuously even when problems are experienced in the distribution plant. This problems are experienced in the distribution plant. This makes startup easier and quicker. Water from the treatment plant is dumped overboard until all water-quality problems have been resolved and only injection-quality problems have been resolved and only injection-quality water is produced by the plant. The distribution plant may then be started as desired without upsetting the water treatment plant. Design Specification-Water Quality Prior to the design of the system described, a survey (including pilot plant tests) was carried out in 1978 to determine the optimal design basis. Water Sampling Water depth in the Tapis field area of the South China sea is about 64 m [210 ft]. The South China seawater was sampled at different depths to determine the optimal depth to obtain the best-quality source water. Water at 44.2 m [145 ft] below sea level was deemed most suitable as it represented the best-quality water in terms of suspended solids. A typical South China seawater analysis in the vicinity of the Tapis field is shown in Table 1. The suspended solids in the water were determined by Millipore filtration to be between 0.2 and 0.9 mg/L. This is generally considered to be good-quality water and compares very closely with North Sea water 2 containing between 0.2 and 0.8 mg/L of suspended solids. While the total weight of this material is small, it consists principally of small plant and animal organisms (marine plankton). The organism typically is a single cell filled with a gelatinous matter and surrounded by an outer skin. In the procedure of determining the suspended solids, most if procedure of determining the suspended solids, most if not all of the gelatinous matter is driven off during drying. Hence, suspended solids quoted may not be representative of the plugging tendency of the water. JPT P. 1689