Maximizing Yibal's Remaining Value
- F.C.J. Mijnssen (Petroleum Development Oman) | D.G. Rayes (Petroleum Development Oman) | I. Ferguson (Consultant) | S.M. Al Abri (Petroleum Development Oman) | G.F. Mueller (Petroleum Development Oman) | P.H.M.A. Razali (Petroleum Development Oman) | R. Nieuwenhuijs (Petroleum Development Oman) | G.H. Henderson (Petroleum Development Oman)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- August 2003
- Document Type
- Journal Paper
- 255 - 263
- 2003. Society of Petroleum Engineers
- 5.5 Reservoir Simulation, 3 Production and Well Operations, 3.3.1 Production Logging, 3.1.6 Gas Lift, 5.6.5 Tracers, 5.2.1 Phase Behavior and PVT Measurements, 5.7.2 Recovery Factors, 5.5.8 History Matching, 6.5.2 Water use, produced water discharge and disposal, 5.5.2 Core Analysis, 3.3.6 Integrated Modeling, 5.6.2 Core Analysis, 1.14.1 Casing Design, 5.1.2 Faults and Fracture Characterisation, 5.8.7 Carbonate Reservoir, 2.2.2 Perforating, 4.3.4 Scale, 1.6 Drilling Operations, 7.4.5 Future of energy/oil and gas, 1.6.6 Directional Drilling, 5.3.4 Reduction of Residual Oil Saturation, 5.4.1 Waterflooding
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Reserves in Yibal traditionally have been targeted by infill drilling, initially with vertical wells and, since 1994, with horizontal wells. Since 1999, new well results have been disappointing, and drilling was suspended in early 2001. An integrated field review was kicked off to design the next era of Yibal development to maximize the remaining value.
The review began with an integrated well and production review. This review concluded that horizontal wells drilled in 1994 and 1995 produced significantly more oil than the original vertical wells. However, later horizontal wells produced similar amounts of oil as the vertical wells, but at much higher water cut.
Based on the well and production review, 13 development opportunities have been identified. A focus area was identified within the field to test these development opportunities. A comprehensive data-gathering campaign of production and saturation logging has been completed in addition to detailed integrated modeling. Subsequently, two infill wells were drilled targeting bypassed oil pockets. These wells confirmed the data-gathering and modeling results and highlighted the further development potential in the Yibal field.
The Yibal field was discovered in 1962 and is situated in North Oman approximately 450 km west/southwest of Muscat. Oil production is mainly from the Lower Cretaceous Shuaiba formation, while gas is produced from the overlying Natih formation. This paper will concentrate on production from the Shuaiba formation.
In the Yibal field, the Shuaiba formation is heavily faulted and consists of intrashelf basin floor carbonate muds.1 Porosities range from 30 to 35%. Permeabilities range from 1 md at the base of the Shuaiba to 200 md in thief zones. Average matrix permeability is approximately 10 md. Permeabilities in fault and fracture zones are believed to be orders of magnitude higher than the average matrix permeability. At the moment, there are approximately 850 penetrations into the Shuaiba formation. Oil is relatively light (40° API).
Production commenced in 1969 and gradually built up to a peak oil production of approximately 225,000 B/D in 1997. Since 1997, oil production has decreased rapidly to current levels of approximately 95,000 B/D. Water production steadily increased until 1997; since then, water production has accelerated, resulting in a current gross rate of approximately 850,000 B/D. Current recovery factor in the field is 40%. Key uncertainties with respect to ultimate recovery are expected to be waterflood sweep efficiency and residual oil saturation.
Historic Production Review
Five development phases are recognized in the Yibal field. These are primary depletion (1969-72), water injection in the oil leg (1972-81),2,3 aquifer injection (1981-93),4 horizontal infill drilling (1993-2001),5-8 and production optimization (2001-present). Fig. 1 shows the well density in the Yibal field at these various stages in field life.
Fig. 2 shows liquid potential for the Yibal field. To compare production behavior through time, production is split according to wells that came on stream in a particular year. The plot shows both oil (solid colors) and water production (pastel colors).
Fig. 2 shows that Yibal saw a big increase in production in 1972 as a result of the large number of wells drilled in that year. Production declined from 1972 to 1980; this suggests that the 22 injectors drilled between 1972 and 1980 could not adequately maintain reservoir pressure. Producers drilled between 1980 and 1990 generally show minor or no decline. This suggests good pressure support from the 75 injectors that were drilled in the 1980s. From 1980 until 1990, water production slowly increased to approximately 50% of total production. Yibal again experienced a rapid increase in production potential in 1990, when 71 new vertical producers were drilled. However, the impact of this large number of wells resulted in an increased decline of the wells drilled in earlier years. This decline seemed to decrease gradually until 1994, when Yibal experienced the next rapid increase in production. This is related to the onset of drilling horizontal wells. From 1994 onward, wells drilled in earlier years kept on declining, while overall oil production slowly increased until early 1997, when Yibal reached peak oil production of approximately 225,000 B/D. Since then, the field has experienced a rapid decline, which started to flatten off when drilling ceased (2001). It is interesting to note that the water production in Yibal dramatically increased in 1996. Fig. 2 shows that this increase in water production is caused only by new wells.
Fig. 3 shows a plot of recovery factor (RF) and time vs. water/oil ratio (WOR). This figure shows a rapid acceleration of the WOR increase after the Yibal RF reached 30% in 1996. This coincides with the large increase in water production from the newly drilled horizontal wells (Fig. 2). The WOR development suggests that significant pockets of oil in the Yibal matrix are poorly drained as a result of the bypassing of water through the highly conductive fault and fracture network.
Average Well Performance
Fig. 4 shows the average first-year production of the producers that came on stream in a particular year. Vertical wells (1969-93) generally produced at gross rates of approximately 1,250 to 1,900 B/D. Average first-year water cut of the vertical wells remained at approximately 10% until 1980, after which it increased to approximately 20 to 30% in the early 1990s. No wells were drilled in 1977.
In 1994 and 1995, both gross and net rates increased dramatically, coinciding with the onset of horizontal drilling in the Yibal field (Fig. 4). However, by 1997 both gross and net rates had dropped significantly. The reason for this sudden drop is not clear. After 1997, gross rates sharply increased again, but net rates further declined to approximately 950 to 1,250 B/D. This is similar to the net rates of the vertical wells drilled in the mid-1980s.
To investigate the effectiveness of the perforated intervals in the horizontal well stock in the Yibal field, perforation length has been plotted vs. first-year average oil potential, first-year average gross potential, and first-year average water cut (BS&W) (Figs. 5 through 7).
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