Predicting Subsidence in New Zealand Geothermal Fields--A Novel Approach

Abstract

A simple analytical function, based on the well known “Boltzmann Function”, is proposed as an improved method of matching historical records of observed subsidence and, by extrapolation, providing better predictions of future subsidence behavior, particularly in areas that are susceptible to creep compaction of geothermally-modified and anomalously-compressible mudstones or buried thermal clays. The function takes the form: S=S1(1+(1+exp((T-T0)/T1))-1) where S is subsidence, and T is time. S1 is a variable representing the maximum subsidence resulting from the pressure-affected part of the compressible formation, and T0 and T1 are fitted diffusion constants. A good fit to observed subsidence rates at Wairakei is achieved if S1 is represented as a linear function in time representing the slow migration of pressure change through low permeability mudstones. Within the Wairakei subsidence bowl, these mudstones, at about 100m to 250m depth, may be exhibiting creep deformation in response to pore pressure decline. At Crown Road, Tauhara, the source of a local bowl appears to be creep compaction of hydrothermal clays (kaolinite-rectorite) in a buried hydrothermal eruption deposit affected by declining pressures in a boiling groundwater zone at about 50m depth.