Tight sandstones are becoming an increasingly important source of natural gas due to a combination of increased demand and decline in production from conventional reservoirs. Tight gas sandstones have vast quantities of gas which can extend for thousands of square miles, yet there are many problems that encounter the production from these reservoirs. For example, the production rate is not only low relative to that in conventional reservoirs, but drops dramatically with increasing the total effective stress. In addition, tight reservoirs have very low permeability (poor reservoir quality) and the identification of sweet spots (regions of high fracture density) is of great importance. This research, firstly, explores the stress dependence of tight sandstones properties mainly the stress dependence of gas permeability (absolute, effective and relative) and the stress dependence of ultrasonic velocity (dry and water partially saturated), in addition to microstructural analysis. Secondly, a finite difference wave propagation code, WAVE, is used to highlight the seismic attributes which can be used to detect sweet spots. Microstructural analysis indicated that tight sandstones have variable microstructure where the microstructure of some tight samples is characterized by isolated pore spaces and narrow pore throats (may exhibit crack like shapes). Low porosity « 10%), low permeability « 0.1 mD) samples are found to be more permeability stress sensitive than high porosity, high permeability samples. It should be emphasized, however, that laboratory measurements conducted at low stress conditions (~ < 3000-4000 psi) might overestimate in situ stress changes. This is because microcracks density develops during coring (stress release) and stress changes at low stress conditions are likely to be related primarily to changes in microcracks aspect ratio. In an attempt to correlate stress changes of permeability to that of ultrasonic velocity, the results showed quite broad scatter and there is no quantitative measure between the stress sensitivity of permeability to that of ultrasonic velocity. Using finite difference wave propagation modelling, scattered waves (e.g. P-S converted waves and/or Rayleigh waves) might be a potential seismic attribute in the detection of sweet spots.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:581873 |
| Date | January 2012 |
| Creators | Al-Rajhi, Mohamed Salim Mohamed |
| Publisher | University of Leeds |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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