Steamflood Strategies for a Steeply Dipping Reservoir
- K.C. Hong (Chevron Oil Field Research Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- May 1988
- Document Type
- Journal Paper
- 431 - 439
- 1988. Society of Petroleum Engineers
- 5.3.4 Reduction of Residual Oil Saturation, 5.4.6 Thermal Methods, 1.10 Drilling Equipment, 2.4.3 Sand/Solids Control, 4.1.5 Processing Equipment, 5.4.2 Gas Injection Methods, 5.1.5 Geologic Modeling, 3 Production and Well Operations, 4.1.2 Separation and Treating, 5.2.1 Phase Behavior and PVT Measurements, 5.2 Reservoir Fluid Dynamics
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A computer simulation study was conducted to determine the best steamflood strategy for a steeply dipping (20 deg. [0.35-rad]) reservoir. A compositional steamflood simulator was used with both two-dimensional (2D cross section) and three-dimensional (3D) models of the reservoir.
The study showed that gravity drainage of the heated oil is the main production mechanism in steamflooding steeply dipping reservoirs. production mechanism in steamflooding steeply dipping reservoirs. Consequently, oil production is strongly affected by injector and producer locations. The best steamflood strategy for this type of reservoir, therefore, involves judicious selection of injector and producer locations, reducing steam injection rate and shutting in wells selectively at appropriate times, and injecting a noncondensable gas to prevent steam cycling in the depleted updip portion of the reservoir.
This paper compares oil production performances and steamflood efficiencies for nine different operating strategies and recommends the best strategy for a steeply dipping reservoir.
Steam injection is the principal enhanced recovery method in use today, accounting for 78 % of all oil produced in the world by enhanced recovery methods. Of the two steam methods, cyclic and continuous steam injection, the latter has been more popular and accounts for an increasing share of oil production by steam methods. The continuous steam injection, more commonly called steamflooding or steamdrive, however, has been applied almost exclusively to reservoirs of negligible dip, unconsolidated sand, and massive thickness. There are only a few reported cases of steamdrive application in reservoirs with significant dip.
A number of investigations dealing with steamdrive in dipping reservoirs have been reported in the literature. All these investigations point to the importance of the gravity-drainage mechanism in steeply dipping reservoirs. Different steamflood strategies were proposed to exploit this gravity-drainage mechanism, and some of them were actually applied in the field successfully. Among the more common steps included in the proposed strategies are (1) development of the flood as a line drive in the direction of dip; (2) use of strategically located injectors and producers; (3) increase or decrease of injection rates during the flood; and (4) conversion of injectors to producers or vice versa. These strategies are quite different from those more commonly applied to reservoirs with little or no dip. Nondipping reservoirs are usually developed on repeated patterns, such as five-spot or inverted nine-spot. In the absence of patterns, such as five-spot or inverted nine-spot. In the absence of reservoir dip, flow across the pattern boundaries may be ignored and each pattern may be assumed to behave independently of others. When a reservoir dips significantly, however, the assumption of no flow across pattern boundaries cannot be made. In such reservoirs, the oil displacement is primarily by gravity drainage of the heated oil from the updip sections and upper horizons of the reservoir to the downstructure and lower horizons. To capture the displaced oil requires placing the producers primarily in the downdip section on lines perpendicular to the direction of the dip.
While most investigations attempted to address how best to take advantage of the gravity-drainage mechanism, no study has elucidated what changes are taking place in different parts of the reservoir during steamflooding and how each step in the injection strategy is directly tied with the reservoir behavior. It appeared that an optimum strategy should address not only the gravity-drainage mechanism but other complex changes that are taking place in dipping reservoirs. For example, steam cycling in the undersaturated, updip portions of the reservoir may waste injected heat and therefore be detrimental to oil recovery. The term "steam cycling" is used here to represent the phenomenon of injected steam rising to the top of a reservoir where it condenses because of cooling, and the condensed water falls toward the bottom as a result of gravity.
This study was conducted to determine how the various mechanisms work together in steamflooding of a steeply dipping reservoir to displace the heated oil and how each step in a steamflooding strategy relates to the recovery of the heated oil. The study also attempted to evaluate the potential use of unheated produced water or a noncondensable gas together with steam to improve the steamflood efficiency. Water or gas injection into the depleted updip portions of the reservoir is thought to reduce steam cycling there and portions of the reservoir is thought to reduce steam cycling there and divert the steam to unheated areas to further the recovery of oil.
This paper also discusses the numerical simulation of the interaction between recovery mechanisms and operating procedures, and the effect of this interaction on oil recovery from a steeply dipping reservoir.
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