Implementing Convection in a Reservoir Simulator: A Key Feature in Adequately Modeling the Exploitation of the Cantarell Complex
- E. Manceau (IFP) | E. Delamaide (IFP) | J.C. Sabathier (IFP) | S. Jullian (IFP) | F. Kalaydjian (IFP) | J.E. Ladron De Guevara (Pemex) | J.L. Sanchez Bujanos (Pemex) | F.D. Rodriguez (Pemex)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- April 2001
- Document Type
- Journal Paper
- 128 - 134
- 2001. Society of Petroleum Engineers
- 4.1.4 Gas Processing, 5.8.6 Naturally Fractured Reservoir, 5.5.8 History Matching, 2 Well Completion, 4.3.3 Aspaltenes, 5.2.1 Phase Behavior and PVT Measurements, 5.6.9 Production Forecasting, 1.6 Drilling Operations, 5.4.2 Gas Injection Methods, 5.6.4 Drillstem/Well Testing, 5.5 Reservoir Simulation, 5.2 Reservoir Fluid Dynamics, 5.1.1 Exploration, Development, Structural Geology, 4.1.5 Processing Equipment, 2.2.2 Perforating, 5.2.2 Fluid Modeling, Equations of State
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As with some thick and highly fractured Iranian fields, the Cantarell complex located offshore Mexico presents features [decreases in the production gas/oil ratio (GOR) and bubblepoint pressure with time] that reveal the effect of convection.
This effect on the past homogenization of fluid properties is discussed and supported by a thorough characterization of the thermodynamic properties of actual reservoir fluids.
To model convection, the reservoir simulator used for this study was purpose adapted. Sensitivity runs were performed to demonstrate the necessity of accounting for convection when matching the past history of the Akal field, which is part of the Cantarell complex.
Presentation of the Cantarell Complex.
The Cantarell complex is the most important oil field in Mexico, and the sixth-largest in the world. To economically optimize its value, it has been decided to initiate a major recovery process by injecting nitrogen for pressure-maintenance purposes. Cantarell field is a thick, highly fractured reservoir; therefore, it is the kind of reservoir where convection phenomena may occur. Convection is a complex process that is characterized by a vertical homogenization of fluid properties in the fractures. This may have an essential impact on production and injection profiles, in particular on the quantity of nitrogen in the effluents as well as nitrogen breakthrough times, and therefore on the overall nitrogen-injection efficiency.
The Cantarell complex is located offshore approximately 85 km from Ciudad del Carmen. It includes four adjacent oil fields known as Akal, Chac, Kutz, and Nohoch. Akal is the largest oil accumulation, with more than 90% of the 35 billion barrels of oil in place. The reservoir is an anticline producing from the fractured carbonates of the Cretaceous and upper Jurassic formations, which also contain many vugs and caves. The Upper Cretaceous is the most fractured and brecciated. Fracturing decreases with depth in the Middle and Lower Cretaceous. The average thickness of the whole reservoir is about 775 m, and the depth of the top Cretaceous ranges between 1100 and 3600 m true vertical depth subsea (SS). Below the Cretaceous sequence, the Upper Jurassic (Oxfordian, Kimmeridjian, Tithonian) is a stratigraphic reservoir with poor reservoir characteristics.
Field production started in June 1979, reaching a peak of 1.157 MMBOPD in April 1981, with 40 producing wells. A total of 184 wells were drilled in Cantarell, among them 173 wells in Akal alone. Cantarell crude is a 19 to 22°API Maya type, with an initial bubblepoint pressure close to 150 bar. Initially, the reservoir pressure was above the bubblepoint pressure and was equal to 266 bar at 2300 mSS; therefore, there was no initial gas cap. The reservoir pressure rapidly reached the bubblepoint pressure, and a secondary gas cap appeared in 1981. The gas/oil contact (GOC) was located at 1800 mSS in 1997. The corresponding cumulative oil production was around 5.5 billion STB.
Accounting for Convection in Cantarell Complex.
Cantarell field appears to have all the characteristics of a reservoir where convection may occur. As observed, for instance, in a major Iranian field,1 convection is a complex phenomenon that occurs in thick and highly fractured reservoirs. As explained in detail by Saidi,2 it results from a combination of thermal gradients, gas liberation at the GOC, and gravity segregation, and it is made possible by high vertical permeabilities. When the oil initially reaches the bubblepoint pressure, it liberates gas in solution, thus becoming heavier. Because of the high vertical permeability, this heavier oil can move downward while lighter oil heated from below expands and rises. A convection flux is then established, finally leading to a fast homogenization of the oil properties along the vertical depth. This leads to a reduction of the bubblepoint pressure in a vertical oil column. Indications that convection is taking place include more homogeneous oil properties and temperature on the vertical, change of the oil composition with time, and decline of bubblepoint pressure and production GOR with time.
For Akal, producing GOR's were plotted vs. time for each well. The initial mean GOR value is approximately 90 vol/vol. The wells were organized into four classes: wells with decreasing then increasing GOR, wells with increasing GOR, wells with a constant GOR, and wells with a decreasing GOR. Fig. 1 shows a typical well with a decreasing behavior. Such a well generally begins producing with an initial GOR value of 90 vol/vol, then its GOR slowly decreases down to around 60 vol/vol. This means that the oil produced becomes heavier with time. Typically, this can be explained by convection. Fig. 2 shows the location of all the wells on the Cantarell field with their classification as of 1993. The corresponding GOC is also drawn. One can observe that the major part of the wells areally allocated close to the top of the structure, despite the vertical position of their completion, shows a GOR behavior other than constant as the reservoir pressure goes down, while the wells allocated through the flanks of the structure show a constant GOR behavior. This means that a complex phenomenon affecting the original fluid properties is taking place. Even though there is no evidence of convection, it is assumed that convection also takes place in the gas cap, leading to a faster homogenization in this area. However, this is not the main focus of this paper.
To confirm this statement, three oil samples were taken in 1997 from three different zones of the reservoir: sample 1.07 was found close to the GOC, sample 1.11 was at an intermediate location, and sample 1.16 was in a deep zone, close to the water/oil contact (WOC). The bubblepoint pressures, as well as the flash GOR measured for each oil sample, are presented in Table 1. It can be observed that, for each crude sample, the oil is heavier than the initial oil in place and that the deeper the oil, the lighter it is.
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