Abstract
In this contribution, we report on the effect of production-induced reservoir compaction, using core material from a Permian sandstone reservoir located at the Dutch Wadden Sea coastline, at a depth of ~~3700 m TVDMSL. To assess the nature of the compaction mechanisms that operate in these reservoirs, and hence better constrain predictions of subsidence rate and magnitude, we have carried out more than 30 long-term pore pressure depletion tests on sub-samples taken from the extracted core material. Our laboratory data show that pressure depletion results in total strain of 5·10-3 -15·10-3 over the duration of the experiment of 5-12 weeks, with approximately 80% of the total strain response being rapid, and 20% time-dependent. The response is dependent on porosity and stress state, but seems rather insensitive to temperature, and pore fluid composition. Our work shows that while volumetric compaction of the sandstone reservoirs could be responsible for the magnitude of the subsidence observed in the Wadden area, it cannot directly explain the observed temporal relationship between subsidence and reservoir pressure decline, or at least not without some rescaling factor. Instead, other mechanisms such as salt flow or water-leg compaction should also be considered.
1. INTRODUCTION
Though depletion-induced reservoir compaction is a well-known phenomenon in the hydrocarbon industry, the mechanisms that operate, and the coupling between pressure decline and material response may strongly vary as a function of reservoir depth, composition, basin and production history, and material properties. Pressure solution creep, grain cracking, pore collapse, and shear failure are a number of possible operating mechanisms responsible for accommodating stress changes in the reservoir. In addition to these, upscaling of the response must be achieved in the lateral and stratigraphic directions, to arrive at a bulk description of the strain response, to serve as a means for bounding surface subsidence. Aside from the spatial component, also the temporal aspects must be addressed. In this paper, we report an experimental program that aims at improving our understanding of the magnitudes, and rates, of reservoir compaction related to gas production from Dutch Wadden Sea fields. We specifically report a series of triaxial strength, and long-duration uniaxial-strain pore pressure depletion (UPPD) tests, that enable us to assess the constitutive response of the Permian reservoir rock to production.
