Improved Reservoir Characterisation and Appraisal of Tight Gas Sandstones

Tight gas sandstone (TGS) formations contain a large volume of natural gas but they are often only marginally economic to develop. Therefore, there is a need to understand the petrophysical properties in particular what controls porosity and permeability. Reliable methods need to be identified by cross comparing methods and data needs to be collected that is representative of reservoir conditions. Most importantly, there is a need to reduce reservoir characterisation timeframes by gaining data especially permeability without core analysis. It has often been suggested that the delicate nature of clays within TGS means that they need to be cleaned using critical point drying. A comparative study found that this does not seem to be the case and cleaning samples with solvents such as DCM/methanol within a Soxhlet extractor is adequate. Rock types based on the type and distribution of clays have different trends on porosity-permeability plots. This means that it is possible to estimate permeability by combining downhole measures of porosity with microstructural analysis of either side-wall cores or cuttings. Permeability can also be estimated from mercury injection analysis, however, NMR, BET or QXRD do not appear to be reliable methods to estimate permeability unless improvements are made to the models or methods. Key petrophysical properties (permeability, formation resistivity factor) are very stress sensitive due to the presence of small grain-boundary microfractures formed as the core is uplifted to the surface. However, flow properties are not likely to be as stress sensitive in the reservoir as they are in the laboratory. This means it is not beneficial to develop the reservoirs more slowly under high pore pressures (known as restricted rate practise) in the hope of maximising recovery and profitability. This thesis overall contributes to the knowledge of the properties of TGS and obtains different conclusions by studying different rock types. New methodologies were created to study the effects of core plug cleaning and the stress dependency of TGS. Alternative faster methods to obtain permeability were also found. The work could be used to increase data accuracy, identify poor vs good reservoirs, find faster experimental methods for core labs and potentially use the data to understand waste disposal and trap systems based on fluid flow and pore connectivity.