Water Flooding Efficiency in a Low-Permeability Mixed-Wet to Oil-Wet Carbonate Reservoir; A Case Study from the Kharaib B Reservoir in the Al Shaheen Field, Offshore Qatar

Abstract

Abstract This paper describes the studies and measurements performed to assess the sweep efficiency and remaining oil saturation after water flooding in the mixed-wet to oil-wet, low permeability Kharaib B carbonate reservoir, which is further characterized by pronounced homogeneneity of the reservoir matrix and substantial lateral variation in fluid properties. The Kharaib B reservoir is developed with extended reach horizontal wells and has been successfully water flooded since the field development started in the mid-1990s. The remaining oil saturation and three dimensional sweep efficiency after water flooding has been studied by both vertical and horizontal penetrations of the flood front. Furthermore, core has been cut behind the front to improve the log evaluation in the flooded intervals and with that the accuracy of the measured remaining oil saturations. The core and log data results from the evaluated wells show that water flooding is an efficient method for recovering oil in the oil-wet and mixed-wet parts of the Kharaib B reservoir, an observation which is widely supported by the special core analysis experiments performed and the extensive production history. Introduction The Al Shaheen field is one of the giant oil fields of the Middle East situated in Block 5, offshore Qatar (Figure 1). Maersk Oil has been developing the field with Qatar Petroleum since 1992 with start-up of production in 1994 and implementation of water injection in 1996 (Thomasen et al., 2005). The Kharaib B reservoir is one of the three main producing reservoirs. It consists of a thin oil column (< 80 ft) covering a large area (25 km x 45 km) in low permeability carbonate rock of Cretaceous age making the economic development of this reservoir very challenging. Furthermore, pressure measurements have delineated a considerable lateral pressure gradient in the aquifer resulting in non-horizontal fluid contacts (Engstrøm and Toft, 2005) complicating the estimation of the oil volume in place and description of the petrophysical properties. The Kharaib B reservoir shows large lateral variations in oil gravity, solution gas-oil ratio (GOR) and saturation pressures (Lindeloff et al., 2008). A hypothesis for the origin of the observed complex fluid variations is that the reservoir has been charged by separate oil pulses followed by gas influx and biodegradation. The Kharaib B reservoir is being developed with long horizontal wells under appliance of extensive water injection to accommodate the large areal extent of the hydrocarbon accumulation and relatively poor vertical well productivity. The length of the horizontal development wells has been gradually increased and in 2008 the limit was further extended when a producer was drilled to a world-record depth of 40,320 ft MDRT. As the result of the application of extended reach horizontal wells, Al Shaheen is at present developed from only nine platform locations; for comparison in excess of 40 locations would have been necessary for a development scheme comprising wells with a horizontal reservoir section of 5,000 ft.