The Obaiyed Gas Recovery Project - A Fully Integrated Study
- M. Bouman (Shell International Exploration and Production) | A. Koopman (Shell International Exploration and Production) | N.S. Ibrahim (Badr Petroleum Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Middle East Oil Show, 17-20 March, Manama, Bahrain
- Publication Date
- Document Type
- Conference Paper
- 2001. Society of Petroleum Engineers
- 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.1.2 Faults and Fracture Characterisation, 4.1.2 Separation and Treating, 5.5 Reservoir Simulation, 3.3.2 Borehole Imaging and Wellbore Seismic, 5.1.3 Sedimentology, 1.14 Casing and Cementing, 1.1 Well Planning, 4.1.5 Processing Equipment, 5.1.5 Geologic Modeling, 5.1.1 Exploration, Development, Structural Geology, 5.5.8 History Matching, 4.3.4 Scale
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A fully integrated study was required to solve the puzzle of the rapidly declining reservoir pressures in the Obaiyed Field. A more regional sedimentological and structural approach formed the basis for a new reservoir model. Consequently the field is now subdivided in a number of discrete blocks, each with its own static and dynamic parameters to predict reservoir properties.
The Obaiyed Gas Field is situated in the Western Desert of Egypt, approximately 450 km WNW of Cairo (Figure 1). It is operated by BAPETCO (a Government-Shell joint venture) and is the largest onshore gas field in Egypt. It commenced production in late 1999 and produces from tight Middle Jurassic sandstones at a depth of some 4000 m.
The field was contracted to supply a DCQ (Daily Contractual Quantity) of 300 MMscf/d for seven years. Very early into production wellhead pressures began dropping sharply. Static surveys confirmed reservoir pressures were declining rapidly suggesting the field would fail to meet DCQ by mid 2001.
To safeguard the DCQ, a study team was set-up with Shell Technology E & P in Rijswijk, The Netherlands to identify the reasons for the decline and recommend remedial action. The resulting Obaiyed recovery study was successfully completed in November and the main results are discussed in this paper. The appraisal and development phases of the field encountered so many unexpected well results that they may be best described as ‘EWD': ‘Exploring While Developing'.
The main reservoir succession is part of the Middle Jurassic Khatatba Formation (Figure 2) overlying the Base Mesozoic Unconformity and Palaeozoic sands of the Shifah Formation below. The reservoir section of the field is interpreted as the proximal sandy facies of the Lower Safa Member of the Khatatba Formation, representing fluvial/alluvial estuarine -incised valley- sediments. Deposition of the Lower Safa Member was affected by palaeo-topography and possibly also by syn-sedimentary tectonics, with onlap / non-deposition / erosion on palaeo-highs.
The field is situated in a large structure, charged with gas but reservoir quality is lacking in a number of wells and for a variety of reasons.
Find the Sweetspots
Production appears to be mainly from relatively few permeable units (tidal channels of some 1-20 m thickness) in the otherwise massive tight Lower Safa sandstones. The permeable zones have permeabilities in the order of 10-100 mD, in contrast to a tight background with permeabilities of 0.01-1 mD.
Predicting the presence of well connected permeable reservoir has been difficult. Complexities in primary deposition, diagenesis and structural overprint interplay and the poor seismic resolution at reservoir level hampered the mapping of non-depositional areas and reservoir thickness. Reservoir diagenesis has been studied in detail, based on a good set of core data but the results can not be used in a predictive way. Quantitative seismic studies failed to predict reservoir development because of poor seismic resolution and small impedance differences at the main objective level. Well correlation at reservoir level is difficult mainly because of poor biostratigraphical control in the barren sandstones.
Exploration and development in Obaiyed are therefore very much a matter of finding the sweetspots in terms of reservoir development against a complex geological background.
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