Multiphase Pump Recovers More Oil in a Mature Carbonate Reservoir

Abstract

Technology Today Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering. Abstract A field test for a multiphase pump (MPP) was conducted in Abqaiq field, a mature carbonate reservoir in Saudi Arabia. Test results showed that large quantities of oil, water, and gas mixtures could be pumped over long distances without separating gas from the liquids. The helico-axial MPP successfully lowered backpressure, revived dead wells, and was able to transport as much as 75,000 B/D total fluid to a separation plant 10 km away. From test startup (8 June 2001) until 1 June 2002, the pump logged 5,160 operational hours (215 days). Economic payout was achieved rapidly, with up to 12,000 B/D of incremental oil production. The technology offers an attractive alternative to constructing expensive gas/oil separation plants in remote areas. This paper presents a summary of the field test. Introduction Multiphase pumps1,2 are modified liquid pumps that are capable of pumping various combinations of oil, water, gas, and minor amounts of sand in the same pipeline without separation. MPPs are most commonly used to add energy to unprocessed fluids for transportation to central processing facilities a long distance downstream. A reduction, consolidation, or elimination of the production infrastructure, such as separation equipment and offshore platforms, also can be achieved. In this way, marginal fields in hostile environments can be developed more economically. In mature fields, MPPs have potential to reduce the backpressure on producing wells, leading to an increase in production rate and recovery efficiency. There also is an environmental advantage of MPP application: the possibility to reduce gas emission and flaring. Field and Reservoir The Abqaiq field, shown in Fig. 1, is one of the oldest in Saudi Arabia. It is approximately 37 miles long and 6 miles wide. It has been on production since 1946. The light crude is produced from the Arab D carbonate formation at an average depth of 6,500 ft and an average pay thickness of 240 ft. More than 50% of the initial oil in place has been produced with the support of peripheral water injection, its natural aquifer, and, between 1954 and 1974, partial gas injection. The average reservoir permeability is approximately 400 md with tighter zones at the top and the bottom. The average reservoir porosity is approximately 20%. The reservoir crude is 36°API gravity, with an average solution gas/oil ratio of 860 scf/STB at the saturation pressure of 2,560 psi. The original reservoir pressure was 3,400 psi at 6,500 ft subsea. The reservoir is maintained above the bubblepoint pressure. Original oil formation volume factor was 1.53, and the average oil viscosity is approximately 0.4 cp. Sulfur content of the mixture is 1.4 wt%, and the gas contains 3.7 mol% of H2S. No solids production is reported. Average well productivity index is more than 100 B/(D-psi). Producing-well water cut varies from 0 to 80% throughout the field. Test Location The test location was chosen in the North Nose of the field (Fig. 1). There is a production manifold in this part of the field 18.6 miles from the engineering offices. The North Nose remote manifold (Figs. 1 and 2) is 6.2 miles from the nearest gas/oil separation plant (GOSP). Fourteen oil-production wells are connected to the manifold, 12 of which are not capable of flowing into the production line because of high trunkline pressure and high producing water cuts, greater than 70%. The main objective of the MPP is to reduce the backpressure on the dead wells and restore their production. The manifold was originally constructed to divert wells from the production line to a well-test line. The pump was installed in a manner to allow a combination of wells to be placed on pump suction header selectively. The pump discharge header is connected to a 16-in. production trunkline and to a 10-in. test line, which flows to a conventional test separator at the GOSP. A multiphase flowmeter was installed at the manifold upstream of the MPP to allow testing of individual wells, combined wells, or total pump discharge. Reservoir Conditions and Well Performance at the Test Location Fig. 3 shows an east/west vertical cross section showing the water saturation in the test location. On average, 30 to 40 ft of oil column stays at the top of the water. The thickness of the oil column varies with location. Most of the wells are completed vertically except one multilateral discussed later in the paper. The current pressure distribution within the same area is shown in Fig. 4. A relatively lower area exists in the crest where most of the manifold wells are connected. Similarly, rock permeability distribution is given in Fig. 5. A high-permeability pocket is in the crestal area where a horizontal well, later converted to a dual-lateral well, was completed. A typical well pressure and water cut starting from 1985 are shown in Fig. 6 for demonstration purposes. Although there is enough reservoir pressure to sustain flow, high water cut, which increases the weight of the production column, and the long distances (6.2 to 9.3 miles) that the wells would have to flow, end up killing the wells. Therefore, 12 wells, which were producing at rates with 70 to 75% water cut, were unable to sustain production against the high trunkline pressure of 750 psi. These wells were shut in and classified as dead wells. Multiphase Pump The Sulzer helico-axial pump3 is a rotodynamic turbine machine capable of handling a mixture of oil, water, and gas containing H2S, CO2, and a certain amount of solids. The MPP package is mounted on a horizontal skid (Fig. 7).