Bias in Engineering Estimation
Authors: Brush Randal M. , Marsden Sullivan S. 
Published: December 2007

This paper presents 5-year-operation results of successful pressure maintenance by formation water dumping into a partially depleted oil reservoir of limestone rock. Water is being dumped into light 33 deg. API(0.86-g/c3)oil in the reservoir periphery. The light-oil deposit is underlain by a heavy-oil mat [from 28 to 90API (0.887 to 1.007 g/c3)]. Such heavy oil could not be recovered by natural water dumping. Light-oil-recovery performance is compared with natural-depletion oil recovery by means of a simulation model. Some operational problems encountered during our field practices are discussed.

Introduction

The Ratawi limestone oil reservoir of the Khafji field, 25 miles (40 km) east offshore Ras Al-Khafji, Saudi Arabia was discovered in 1963 (Fig. 1). The oil is light, sweet, and undersaturated with dissolving sour gas. It is in the upper part of the Hout formation of the Lower Cretaceous Thamama Group, which forms asymmetrical anticline trending northeast to southwest, having its crest at about 6.900 ft (2100 m) subsea. Below 7,300 ft (2225 m) subsea, the oil rapidly increases ingravity up to 9 deg. API(1.007 g/c3) and is completelyunderlain by the bottom aquifer at 7.385 ft (2250 m)subsea.

Regular oil production started with 30,000 B/D (4770m3/d) in Aug. 1969 and increased to 66,000 B/D (10485 m3/d) in late 1972 by completing 15 producers. In late 1975, the producing GOR had increased rapidly in the lower pressure area. Five producers were closed, resulting in production of 33,000 B/D (5200 m3/d) oil. Nevertheless the reservoir pressure was depleted substantially to 2,650 psig (18 300 kPa), about 1000 psi(6900 kPa) below the original, and the producing GOR was leveling continuously to 1,550 scf/STB(280 stdm3/stock-tank m3) like a typical solution-gas drive reservoir.

To maintain oil production by controlling GOR and to improve ultimate oil recovery, the pressure-maintenance operation was started in Nov. 1975 by dumping the shallow aquifer waters into the peripheral oil zone. After 5 years of operation, the reservoir pressure has been maintained at around 2,600 psig (18 000 kPa) and the oil production has been maintained at 40,000 to 45,000 B/D(6360 to 7150 m3/d) by controlling the GOR below1,000 scf/STB(180 std m3/stock-tank m3). Currently eight dump flood wells are dumping water at 29,000BWPD (4600 m3/d water).

This paper presents reservoir descriptions and the primary oil recovery performance and describes how we achieved successful pressure maintenance by means of an economical dumpflood scheme. Several problems encountered during our field practices also are discussed.

Geology

Geological structure of the Ratawi limestone reservoir isa symmetrical anticline trending northeast-southwest(Fig. 2). The flanks of anticline dip at 5.8 deg. on the north-western side and at 2.9 deg. on the southeastern side. The reservoir structure extends to the Safaniya field of the adjacent oil company, where no regular oil production operation has begun. The details of the stratigraphicsequence in offshore Khafji are shown in Fig. 3. The reservoir is defined as porous limestone in the Houtformation of Lower Cretaceous Thamama Group, which issubdivided into Zone B and Zone C according to porosity distribution.

JPT

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