- A. Settari (U. of Calgary)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- August 2002
- Document Type
- Journal Paper
- 62 - 69
- 2002. Society of Petroleum Engineers
- 5.3.4 Integration of geomechanics in models, 5.5 Reservoir Simulation, 5.1 Reservoir Characterisation, 2.1.1 Perforating, 5.8.5 Oil Sand, Oil Shale, Bitumen, 5.1.5 Geologic Modeling, 5.4.1 Waterflooding, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 1.2.2 Geomechanics, 4.6 Natural Gas, 5.8.6 Naturally Fractured Reservoir, 5.6.4 Drillstem/Well Testing, 5.4.6 Thermal Methods, 2.1.3 Sand/Solids Control, 5.2 Reservoir Fluid Dynamics, 5.1.2 Faults and Fracture Characterisation, 5.8.7 Carbonate Reservoir, 5.3.2 Multiphase Flow, 2 Well Completion, 3.3 Well & Reservoir Surveillance and Monitoring, 5.5.8 History Matching, 1.8 Formation Damage, 3 Production and Well Operations, 1.7 Pressure Management, 5.1.10 Reservoir Geomechanics, 1.6 Drilling Operations
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Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area,these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering.
Reservoir compaction has been considered an "exotic" aspect of reservoirengineering, usually studied only when the associated surface subsidence becamea problem. In recent years, it has been recognized that reservoir compactionoccurs in many reservoirs, and it is responsible both for improved recovery anda number of field operating problems. Consequently, the understanding of theprocess and the methods for its analysis have improved dramatically. Thisarticle provides a general overview of the compaction aspects of reservoirengineering.
Compaction in Reservoirs
In the past, reservoir compaction was usually dealt with only after beingindicated by the associated surface subsidence or operational problems. Somewell-known cases include the Willmington field in California and the Ekofiskfield in the North Sea. Depletion of the Willmington field caused a subsidencebowl reaching a maximum depth of 9 m, requiring extensive remedial work in andaround the city of Long Beach.1 The sea floor under the Ekofiskplatform sank by 1984 in excess of 3.5 m, and the platform had to be extended(jacked up) at a cost of U.S. $1 billion.2 In both cases, the eventstriggered extensive reservoir/geomechanical studies and changes in fieldmanagement to arrest further subsidence. Compaction is present in many otherNorth Sea chalk reservoirs such as Eldfisk, Valhall, Dan, Tyra, and Gorm.Compaction of California Diatomite fields was the cause of numerous wellfailures, which in some cases reached 20 to 30% of the wellsdrilled.3 Better understanding of the cause of the deformations ledto a significant decrease in well failures. Compaction also has been recognizedas an important drive mechanism in Alberta and Venezuela heavy oils and oilsands. In this case, the problem is further complicated by thermal aspects andby the unconsolidated nature of the porous media. Recent exploration activitytends to discover more and more deepwater "soft" reservoirs (e.g., in the Gulfof Mexico) and high-temperature/high-pressure reservoirs, where compactionoften is an important issue.
Compaction of the reservoir itself, besides providing the additional driveenergy for production (in some cases amounting to 50 to 80% of total energy),has important consequences both inside and outside the reservoir. The mostobvious is the surface/seafloor deformation (i.e., subsidence), which createsproblems for the environment as well as for oilfield structures and seabedpipelines. Additional problems include well failure caused by casingdeformations, fault reactivation resulting in seismic activity (e.g., Groningengas field in the Netherlands), reduction in permeability leading to loss ofproductivity, and effect of deformation on overlying shales or freshwateraquifers.4 While compaction can contribute to reservoir energy andincrease recovery, its side effects are undesirable: increasing developmentcosts and creating barriers to project acceptance. Therefore, field developmentof compacting reservoirs is always more complex compared with conventionalreservoirs, and it requires a more detailed analysis. Inaccurate estimate ofthe compaction effect can lead to over- or underestimation of reserves, even ingas reservoirs.
Compaction/subsidence problems may be expected in "soft" or unconsolidatedformations and chalk reservoirs, which are overpressured or will be severelydepleted and have large thickness. Because most deformations are irreversible,it is difficult to correct the negative effect after compaction has occurred.This critical issue points to the need to screen, assess, and engineer forcompaction early in the life of a field. As deeper reservoirs are developed indeeper water with fewer, very expensive wells, there is an increased need toimprove the engineering and modeling technology to deal with these difficultissues at the planning stage.
While all reservoirs undergo deformations during exploitation, compaction isthe process in which the compressive strength of the rock is exceeded andplastic deformation occurs, resulting in irreversible reduction of porosity andpermeability. This irreversible change differentiates compaction from elasticcompression of the reservoir. The volumetric behavior of the rock under plasticdeformation determines the changes in porosity and apparent compressibility.Fig. 1a shows the customary representation of the process in whichporosity, f, is a function of pressure, p.
However, from the theory of poroelasticity5 (andthermoelasticity), the volumetric behavior of the rock is fundamentally afunction of the mean effective stresssm'=sm-ap, wheresm is the mean total stress,sx=(sy+ sz)/3; a isBiot's constant; and p is the fluid (pore) pressure. The porosity vs.effective stress (also called net pressure) during the depletion of thereservoir follows the path shown in Fig. 1b. It must be stressed thatthe fundamental physical mechanism is that of Fig. 1b, while the representationin Fig. 1a is an approximation, which must be derived from data of Fig. 1b byignoring (or making assumptions about) stress variations.
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