The Wafra First Eocene Reservoir Partitioned Neutral Zone (PNZ), Saudi Arabia and Kuwait: Geology, Stratigraphy, and Static Modeling
- William Scott Meddaugh (Chevron ETC) | Dennis Dull | Raymond garber (Chevron Energy Technology Company) | Stewart Griest (Chevron ETC) | David Lee Barge (Chevron Corp.)
- Document ID
- Society of Petroleum Engineers
- SPE Middle East Oil and Gas Show and Conference, 11-14 March, Manama, Bahrain
- Publication Date
- Document Type
- Conference Paper
- 2007. Society of Petroleum Engineers
- 5.6.1 Open hole/cased hole log analysis, 5.8.7 Carbonate Reservoir, 5.4.6 Thermal Methods, 4.3.4 Scale, 5.1.5 Geologic Modeling, 1.6.9 Coring, Fishing, 5.5 Reservoir Simulation, 4.6 Natural Gas, 1.6 Drilling Operations
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The First Eocene reservoir at Wafra Field was discovered in 1954 and has produced about 290 million barrels of 17-19° API, high sulfur oil. The dolomite reservoir is Eocene/Paleocene age. The average porosity is 35% and the average permeability is 250 md over the gross interval. Well log and core plug porosity values over 45% are common and measured permeability values range up to 5000 md. The First Eocene reservoir has an average depth of about 1300 feet and a gross thickness of about 750 feet. The OOIP is about 9 billion barrels.
The reservoir largely consists of dolomitized packstones and grainstones deposited under arid to semi-arid conditions on a shallow, very gently dipping, low to moderate energy inner shelf or ramp setting in a gently dipping, restricted ramp environment. The presence of some interbedded evaporites suggests restriction was occasionally sufficient for the development of hyper-saline lagoons and sabkhas. The shallowing-upward cycles are capped by mud-dominated rocks, hardgrounds and exposure surfaces that are correlative with gamma ray highs that can be easily correlated across the 20 km length of the field and are the basis for the current sequence stratigraphic interpretation.
Several full field and small, highly detailed reservoir models have been generated using stochastic workflows to assist management of on-going primary development as well as two steamflood test/pilot projects. The reservoir models utilize the new stratigraphic framework and are constrained by over 285 wells with high quality porosity logs and six wells with high quality core descriptions and core plug data. Model properties were distributed by stratigraphic layer and constrained by layer-appropriate data (or transforms) and layer-appropriate semivariograms. The range parameter for the semivariograms models used in full field static models varied between 1100-2200 m with moderate anisotropy in the direction N120°E.
Fig. 1 shows the location of the major fields in the PNZ. The First Eocene is the shallowest reservoir at Wafra field.1,2 Average depth to the top of the reservoir is about 1000 feet (400' subsea). A generalized stratigraphic column for the PNZ is given in Fig. 2. The First Eocene stratigraphic interval (Rus or 1st Anhydrite to 2nd Anhydrite) averages 750 feet thick with a gross average porosity of 35% based on well log and core data and a gross average permeability of 250 md based on core plug measurements. Based on current field practice a porosity cutoff of 35% is used to define net reservoir. The average porosity in the net reservoir is 43% and the net average permeability is about 280 md. Fig. 3 provides a type log for the First Eocene reservoir interval.
Oil was first discovered in the First Eocene in 1954 but full scale development and production did not commence until 1956. The First Eocene has a cumulative production of over 300 MMBO. Current production exceeds 25,000 BOPD of 14-20 ºAPI high-sulfur oil. The First Eocene is a depletion drive reservoir with partial solution gas drive and limited aquifer support. The aquifer support is not sufficient to maintain reservoir pressure at current production rate. A small, single 5-spot, steamflood pilot began steam injection in early 2006.
Fig. 4 provides a structure map for the First Eocene reservoir at Wafra field. Note that for a probable OOWC of 700-750 feet subsea, the structure is not closed within the known production limit of the field. Wells drilled outside the producing limits have reasonable well log porosity (30-45%) and Sw values. However, these wells have little or no fluid production (oil and/or water) from the First Eocene reservoir interval strongly suggesting that the field limits correspond to a facies or lithologic change. The estimated OOIP within the defined field limit is about 9 billion bbls (reservoir).
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