59100schD07R1.qxp:59100schD07R1 11/20/06 10:01 PM Page 50 Compaction and Subsidence Pressure drawdown in a producing field can lead to reservoir compaction, movement of the overburden and subsidence of the surface above the reservoir. This compaction and subsidence can prove costly, both for production and surface facilities. Dirk Doornhof Venice, the Italian city known for the romance of 1969 had increased from its low historic rate to Nederlandse Aardolie its canals, is slowly sinking into its surrounding 1.7 cm/yr [0.7 in./yr] in the industrial area and Maatschappij (NAM) B.V. lagoon, mostly due to natural causes. This is an 1.4 cm/yr [0.6 in./yr] in the city center.2 Assen, The Netherlands improvement over the recent past: for several This dramatically higher rate of subsidence decades of the 20th century, Venice was rapidly was caused by compaction, which is a decrease in Tron Golder Kristiansen sinking into the lagoon. From the 1940s until volume of a reservoir resulting from pressure BP Norway the 1970s, extraction of water and natural reduction and production of fluids, in this case, Stavanger, Norway gas from below the city radically increased the water and gas. The terms compaction and subsi - 1 Neal B. Nagel subsidence rate. dence describe two distinct processes. ConocoPhillips Subsidence is a sinking of a surface, such as Compaction is a volumetric change in a reservoir, Houston, Texas, USA ground level, relative to a stable reference point. while subsidence is a change of level of a surface. It occurs naturally as a result of plate-tectonic That surface could be a formation top, the Phillip D. Pattillo activity, above active faults and in places where mudline in a submarine area or a section of the BP America fluid is expelled from underlying sediments. Earth’s surface above the compacting formation, Houston, Texas Fluid expulsion is common at river deltas, such as is the case with Venice. as the Po River delta around Venice. This effect, A record 2-m [6.6-ft] flood submerged Venice Colin Sayers which amounts to a subsidence rate of a few in November 1966.3 After the flood, extraction of Houston, Texas centimeters per century in Venice, is distinct both natural gas and water was essentially halted For help in preparation of this article, thanks to Olav from eustasy, or a change in sea level, which around the city to control subsidence. Aquifer Barkved, BP, Stavanger; Tom Bratton, Denver; Rob Marsden, accounts for a rise of about 13 cm [5 in.] per levels rose again, and the ground rebounded a Gatwick, England; Frank Mitchum, BP, Houston; and Marc Thiercelin, Moscow. One author acknowledges permission century in Venice. few centimeters. However, the rebound was to publish this article from ConocoPhillips Norge and After World War II, two practices increased only a fraction of the ground-level change coventurers, including TOTAL E&P Norge AS, Eni Norge AS, Norsk Hydro Produksjon a.s., Statoil ASA and Petoro AS. the subsidence rate of Venice. First, the amount that had occurred over the period of water and One author acknowledges permission to publish this article of water withdrawn from aquifers underlying the gas extraction. Today, the slow natural from BP Norge AS and coventurers Hess Norge AS, Enterprise Oil Norge AS and TOTAL E&P Norge AS. city increased dramatically to accommodate a subsidence continues. ECLIPSE and Sonic Scanner are marks of Schlumberger. growing population. As a result, the water levels In the oil and gas industry, some cases of CMI Compaction Monitoring Instrument is a mark of in these aquifers dropped significantly. Secondly, subsidence have become well-known. Goose Baker Atlas. FCMT Formation Compaction Monitoring Tool is a mark of Halliburton. VISAGE is a mark of V.I.P.S. natural gas was extracted from an industrial zone Creek field south of Houston was one of the first on the mainland just across the lagoon. The that received intense study. Subsidence over that subsidence rate measured between 1968 and field was first noticed in 1918, eventually reaching more than 3 ft [0.9 m] and submerging 50 Oilfield Review 59100schD07R1.qxp:59100schD07R1 11/20/06 10:01 PM Page 51 the Gaillard Peninsula, which lay over the center of the field.4 The Wilmington field in California, USA, several fields at Lake Maracaibo in Venezuela, and the Groningen field in The Netherlands all had noticeable subsidence that required remediation because the surface above the reservoirs was at or near sea level.5 The chalk fields in the Norwegian North Sea, notably Ekofisk, Eldfisk and Valhall fields, have compacted, and the resulting subsidence at the mudline generated concern for platform safety. Low-strength carbonate reservoirs in Northwest Java field, Indonesia, and fields offshore Sarawak, Malaysia, have also experienced significant subsidence.6 The Belridge field in California and neighboring diatomite fields subsided and had numerous well failures.7 The economic consequences of compaction and subsidence can be huge, but not all of them are negative. Compaction may be beneficial, as it provides a potentially strong production-drive mechanism. In this article, we examine the issues relating to compaction and subsidence, illustrated by reservoir-management approaches in the North Sea, The Netherlands and the Gulf of Mexico. Compaction Physics A porous medium, such as a hydrocarbon- producing formation, contains fluid within its solid structure. This simple observation has profound implications when the material is placed under stress. For example, sediment deposited under water may have high porosity immediately after deposition, and it may behave more like a liquid with solid material in suspension, rather than a solid material con - taining liquid. As more sediment accumu lates, the original layer must support the weight of the new material. As long as fluid pathways exist, some of the liquid will be expelled, and porosity will decline. 1. For more on subsidence in Venice: Brighenti G, Borgia GC 5. For more on subsidence in Wilmington field and Lake For more on subsidence in fields offshore Sarawak: and Mesini E: “Subsidence Studies in Italy,” in Maracaibo fields: Poland JF and Davis GH: “Land Mah K-G and Draup A: “Managing Subsidence Risk in Chilingarian GV, Donaldson EC and Yen TF (eds): Subsidence Due to Withdrawal of Fluids,” in Varnes DJ: Gas Carbonate Fields Offshore Sarawak,” paper SPE Subsidence Due to Fluid Withdrawal, Developments in Reviews in Engineering Geology II. Boulder, Colorado, 88573, presented at the SPE Asia Pacific Oil and Gas Petroleum Science 41. Amsterdam: Elsevier Science USA: Geological Society of America (1969): 187–268. Conference, Perth, Australia, October 18–20, 2004. (1995): 248–253. 6. For more on subsidence in Northwest Java field: 7. Fredrich JT, Arguello JG, Thorne BJ, Wawersik WR, 2. Brighenti et al, reference 1. Susilo Y, Rahamanda Z, Wibowo W, Tjahyadi R and Deitrick GL, de Rouffignac EP, Myer LR and Bruno MS: 3. See http://www-geology.ucdavis.edu/~cowen/~GEL115/ Silitonga FJ: “Stimulation Efforts in Carbonate Gas “Three-Dimensional Geomechanical Simulation of 115CHXXsubsidence.html (accessed October 17, 2006). Reservoir Experiencing Subsidence in Offshore North Reservoir Compaction and Implications for Well Failures West Java Field—Indonesia,” paper SPE 82264, in the Belridge Diatomite,” paper SPE 36698, presented 4. Pratt WE and Johnson DW: “Local Subsidence of the presented at the SPE European Formation Damage at the SPE Annual Technical Conference and Exhibition, Goose Creek Oil Field,” Journal of Geology 34, no. 7– Conference, The Hague, May 13–14, 2003. Denver, October 6–9, 1996. part 1 (October–November 1926): 577–590. Autumn 2006 51 59100schD07R1.qxp:59100schD07R1 11/20/06 10:01 PM Page 52 change in pore volume caused by a change in applied stress. Bulk compressibility, Cbv, is a measure of the change in bulk volume due to a Overburden change in applied stress; it is the inverse of the stress bulk modulus. Under an assumption that the grains are incompressible, Cbv is the product of porosity and Cpv. The compressibility value depends on the rock composition and Depth depositional history, and can vary with changing Pore pore-fluid composition. pressure A competent, granular sandstone typically has -4 -6 a Cpv value of about 5 x 10 /MPa [3 x 10 /psi], but it can exceed 15 x 10-4/MPa [100 x 10-6/psi] for highly compressible North Sea chalks (below). Pressure Grain-bonding cement tends to increase rock > Overburden stress and pore pressure. The overburden stress (yellow arrows, stiffness, decreasing its compressibility. right) on a formation increases with depth because of the added weight of over- Depositional history is important because burden. The overburden stress (yellow line, left) is determined by integrating the compaction tends to cause irreversible changes density of the overburden. The pore pressure (blue) also increases with depth, in the rock fabric. Grains shift; clay particles with a gradient determined by the brine density. Beneath an impermeable stratum (red), the pore fluid becomes overpressured as the formation compacts deform; cemented bonds break; and even grains under additional weight without being able to release pore fluid. can break under loading. Since these changes are irreversible, the rock displays hysteresis. When stress on the rock decreases, which would occur if some of the overburden weight were As burial depth of a sediment layer increases, concept is the effective-stress principle, which eroded away or if formation pressure increased the weight of the overlying sediments increases, states that the stress affecting the behavior of a without additional deposition, the unloading tending to squeeze fluid out of the layer and solid material is the applied stress minus the material is less compressible than when it was reduce its porosity (above).