The New Geophysics: implications for hydrocarbon recovery and possible contamination of time-lapse seismics
Recent theory and observations suggest that the fluid-saturated microcracks in hydrocarbon reservoirs (and most other in situ rocks) are so closely spaced that they are critical systems verging on failure by fracturing. As a result reservoirs are highly compliant and respond to small changes with ‘butterfly wings’ sensitivity. However, these phenomena cannot be imaged with conventional technology and the largest effects may be possible complications when recovering hydrocarbons. This critical behaviour leads to a New Geophysics, where the response to changes in fluid-saturated rock (during hydrocarbon production, for example) necessarily varies both spatially and temporally so that detailed measurements degrade with time from the moment they are made. This means that behaviour cannot be averaged. Consequently, many (perhaps most) standard oil-field procedures may not be wholly or strictly valid. The typical (and extraordinarily low) 30% recovery from most oil reservoirs is at least partly explained by the sensitivity of reservoirs and behaviour inexplicable in terms of conventional geophysics. The New Geophysics is the cause of at least some of the difficulties in standard oil-field procedures, but does offer enormous potential advantages that may be exploitable. This article discusses new ways of monitoring production, where the response of the reservoir to production procedures can be calculated, possibly predicted, and even potentially controlled by feed back.