Improved Oil Recovery in a Tight Reservoir With Conductive Faults, ISND Shuaiba, Qatar

Abstract

Abstract The ISND Shuaiba, offshore Qatar, is a limestone formation with very low permeability and a network of conductive faults. Although the reservoir has been produced since 1964, because of the unfavorable reservoir properties only 2% of the two billion STB original oil-in-place has been recovered. A new development plan is underway that will considerably increase Shuaiba oil recovery. Most of the improved recovery will be from a waterflood using a unique well "pattern", rings of dual-lateral horizontal wells circling the flanks of the dome-shaped structure. A gas injection project is also underway to recover oil from the crest. Simulation studies to predict reservoir performance, and particularly to evaluate the effect of conductive faults, are described. The studies indicate that in the planned development, conductive faults significantly accelerate early oil production rate and moderately reduce ultimate oil recovery. Introduction The Idd El Shargi North Dome (ISND) field is 80 km offshore Qatar (Fig. 1). Since 1995, the field has been operated by Occidental Petroleum of Qatar, Ltd., under a development production sharing agreement with Qatar General Petroleum Corporation. ISND has several reservoirs; this paper concerns the largest of these, the Shuaiba limestone. (Other significant reservoirs are the deeper Arab C and D.) Shuaiba original oil-in-place is about 2 billion stock-tank barrels. Although the Shuaiba has been produced since 1964, oil production with existing technology was low because of very poor reservoir properties. Only 40 million stock-tank barrels, 2% of OOIP was produced prior to the improved recovery projects described in this paper. Factors complicating oil production include very low rock matrix permeability, a network of conductive faults, relatively high oil viscosity, and a thick water-oil transition zone. Aided by horizontal well technology, an aggressive new reservoir development is underway. The improved recovery plan consists of a "ring pattern" waterflood in the flanks of the dome-shaped structure and a pilot gas injection project in the crest. The waterflood, which affects most Shuaiba oil, uses alternating rings of horizontal production and injection wells circling the flanks. The wells have dual laterals up to 8000 ft long, completed in the upper two Shuaiba geologic zones. By early 1997 the first five flank wells had been drilled. The crestal gas injection project, intended to displace oil by drainage to wells completed in the base of the reservoir, has been under evaluation since 1994. Several reservoir simulation studies have been conducted, including a full-field model, to help design the development plan and predict future oil recovery. Topics discussed in the paper are:the Shuaiba reservoir description;production history;the improved recovery projects; andreservoir simulation, including methods of modeling conductive faults and our predicted Shuaiba oil recovery. A primary focus of this paper is the effects of Shuaiba reservoir properties - in particular, the conductive faults - on reservoir performance. Shuaiba Reservoir Description Geology. The reservoir structure is a dome with a crest at 4500 ft subsea and gently dipping flanks. Fig. 2 is a structure map showing the major faults, which are mapped based on 3D seismic data. Fig. 3 further illustrates the dome structure and fault offsets. The Lower Cretaceous (Aptian) Shuaiba is predominately very fine-grained limestone with minor amounts of shaley interbeds. The rock texture is "chalky". Porosity is typically high, but permeability is quite low. The Shuaiba averages 325 ft thick and consists of four depositional cycles, AD from top to bottom (Fig. 4). The A zone has the best reservoir properties, and, in addition, permeability in the upper 10-20 ft of the A is enhanced due to leaching and/or higher-energy deposition. P. 683^