Cylindrical core plug samples or drill cuttings can be used to evaluate the sealing capacity of overlying caprock. Threshold pressures obtained from laboratory tests are converted to a maximum hydrocarbon column height that could potentially be retained by the seal. At PanTerra we often use this laboratory test in combination with petrography in order to classify the seals and to relate seal capacity to mineralogical composition and/or depositional environment.
Two methods are available: |
| For this method a vertical sample is drilled from the caprock. This sample is mounted into a core holder, stress applied and the initial vertical permeability to brine is measured. Subsequently gas pressures are applied to the plug at incremental steps to determine at which pressure level the brine is being displaced from the core plug. Although these are time consuming and costly tests, they have the benefit that reservoir conditions can be simulated. |
| During this test cuttings or core plugs are subjected to incremental injection pressures of mercury. It provides the air-mercury threshold pressure, which needs to be converted to a subsurface system. Associated uncertainty in IFT and contact angle in the conversion may imply that the seal capacity values from these test are fair estimates only. The advantage is that they can be carried out fast and are relatively cheap. |
