On Reservoir Fluid-Flow Control with Smart Completions
- T.S. Ramakrishnan (Schlumberger-Doll Research)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Operations
- Publication Date
- February 2007
- Document Type
- Journal Paper
- 4 - 12
- 2007. Society of Petroleum Engineers
- 5.6.4 Drillstem/Well Testing, 5.7.2 Recovery Factors, 2.3 Completion Monitoring Systems/Intelligent Wells, 5.2 Reservoir Fluid Dynamics, 1.14 Casing and Cementing, 4.1.2 Separation and Treating, 5.4.1 Waterflooding, 7.6.6 Artificial Intelligence, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 3.3.4 Downhole Monitoring and Control, 4.3.4 Scale, 1.8 Formation Damage, 4.1.5 Processing Equipment, 5.3.1 Flow in Porous Media, 2.2.2 Perforating, 1.7.5 Well Control, 5.2.1 Phase Behavior and PVT Measurements
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Poor displacement efficiency in hydrocarbon formations is often caused by the natural variation in the mobility of fluids across the reservoir strata. Historically, completions with cemented casing, packers, conformance controlling fluids/gels, and selective perforations have been used to mitigate the disparities in water encroachment over the reservoir interval. Recently, completion technologies using downhole valves, which allow production and injection control over multiple zones, have become available. The central idea is that downhole control may be used to adjust flow distributions along the wellbore to correct undesired fluid-front movement.
In this paper, we address several technical issues related to downhole controls. We consider a single system comprising the reservoir, the completion, the measurement, and the feedback algorithm that adjusts flow-control devices, with quantitative models for each of the components. Both pressure and flow-rate control systems are discussed. Downhole control is modeled for electrical, reversible hydraulic, and unidirectional hydraulic valves. The design methodology for different valve systems is described and the disadvantages of hydraulic systems are discussed. In particular, it is shown that in conjunction with an automated feedback control, hydraulic valves will oscillate. Computations also show that all other factors remaining equal, these oscillations occur most easily in low-permeability zones. For unidirectional hydraulic valves, we also illustrate novel anticipatory control algorithms that prevent overshooting.
For communicating layered systems, a front movement equation is derived using perturbation techniques. This technique provides the zone of influence of wellbore flow-control devices, and illustrates the maximal benefit that may be obtained through downhole control, thus providing a ready comparison with the cost of completion.
Two of the common problems that plague waterfloods are poor sweep efficiency and low contact factor. By our convention, sweep refers to areal displacement. The contact factor is determined by improper displacement in a direction orthogonal to reservoir strata. These are discussed by Herbeck et al. (1976). Here, we use the terms sweep and contact factor to simply distinguish efficiencies of recovery in the two orthogonal directions.
In a laterally heterogeneous system, or when bedding planes exhibit anisotropy, poor areal displacement efficiency is expected. That is, for a given economically acceptable water cut in the production wells, large areas of the reservoir are left with high oil saturation. This may also be termed as poor "load balancing.?? The literature on the concept of locating missed pockets of oil goes back to Hurst (1979) and the concept of controlling production was suggested by Rinaldi (1987). Recent papers [for an example see Graf et al (2006)] address optimization of intelligent completions. To some extent, load-balancing problems may be corrected by proper placement of wells, if the reservoir is characterized perfectly a priori. And incorrect well placement may be partially compensated by adjusting wellbores' flow rates with respect to each other. This is best done by establishing flow-rate control through variable completions. Thus, for areal nonuniformity and heterogeneity, we expect a reservoir optimization scheme to prescribe qi (t) for each well i, given all of the reservoir information estimated until that point. The smart completion may then be programmed to follow this qi (t) as closely as possible.
In contrast to poor sweep problems among vertical wells, to improve contact, one may think of having a well with segmented intervals as shown in Fig. 1. Each of the strata commingles only at the wellbore. Within the segmented intervals, a smart completion may be put in place. Through observations or other modeling means, a feedback-control system to adjust the flow-rates in and out of each of the independent interval may be implemented. In contrast, one may also design a feedback system to maintain sandface pressure. Normally, when control is established in an injection well, the desire is to produce each layer as fast as possible, limited by the maximum allowable injection pressure. Uniform frontal displacement may be secondary. Thus, one expects injection at the maximum allowable pressure pj (t), where jis the completion interval.
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