Reservoir compaction is a positive contributor to reservoir energy and hence hydrocarbon recovery. Reservoir compaction above a certain magnitude, however, will result in casing deformation and, in some cases, well collapse in standard oil well constructions. Large amounts of reservoir compaction will also induce seafloor subsidence and potential risk to platform constructions and pipelines. The Valhall field is a chalk reservoir in the southern most part of the Norwegian sector of the North Sea. The highly porous chalk (in excess of 50% in many cases) is weak and loses a significant part of its original porosity when the effective stress increases during hydrocarbon production (pore pressure depletion). This pore collapse causes reservoir compaction, and currently the associated seafloor subsidence has reached 4 meters at the platform complex.
Since Valhall production was initiated in 1982, the recoverable reserves have been upgraded from approximately 250 MMSTBO to 700 MMSTBO. At the same time 28 out of 102 production wells have been sidetracked due to severe tubular deformations. Most of these deformations (60%) have occurred in the overburden. The first casing deformations in the reservoir were experienced almost instantaneously as a result of chalk production and near wellbore compaction. The first casing deformation experienced in the overburden which resulted in the need for a sidetrack was in 1986, when the seafloor had subsided less than a meter. The work performed through the years to analyze the available data indicates mostly shear loading of the casing in the overburden. Some of the well failures in the reservoir are due to shear loading as well; others are due to cross-sectional collapse, while still others are buckling failures. With the amount of reservoir compaction experienced so far, and the amount expected with continued hydrocarbon production from Valhall, one expects casing deformations to be part of the operational cost. Experience from other fields indicates that one can not stop the compaction and associated kinematics, so the best strategy will be to extend well life as well as possible with minimal cost additions to standard oil field well constructions. Even if the additions are low cost, there is a potential for optimization and risk reduction if better prediction capability could be developed for the casing deformations.
Passive seismic monitoring is a relatively new technology in the oil and gas industry. Seismic geophones are installed in wells in order to record small micro-seismic events induced by hydrocarbon production of fluid or gas. The technology has not been explored in great detail in the industry so far. It has the potential to provide useful engineering information for reservoir management in the areas of earth stress determination, waterflood monitoring and hydraulic fracturing monitoring (both stimulation and waste injection). This paper discusses a field test of passive seismic monitoring performed at Valhall and how the data collected can be used in well and casing design. An array of six geophones installed in a well above the reservoir recorded 572 micro-seismic events over a period of 57 days. The events ranged from 0 to 10 per day in the vicinity of the observation well. The data recorded can be processed to yield useful information for a risk based well and casing design at Valhall in order to optimize well life.