Learning from Mining: Applications to Unconventional Reservoirs
Production from unconventional reservoirs is a relatively recent phenomenon. In contrast, mining dates back thousands of years, and as a result has gone through many of the teething problems that are now facing the oil industry. In addition, the hands on nature of mining has provided unique insights into the behaviour of rocks under stress, and the architecture of depositional systems.
Examples that will be presented include a Portland Stone mine in the United Kingdom, which is mining building stone using a room and pillar system, the only mine of this type in the country. The mine has accessed natural fractures in the subsurface, as well as complex depositional patterns high-lighted by chert replacement, that have necessitated novel mining approaches that can readily be applied to unconventional production.
Another mine under the spotlight will be St. Helena Gold Mine in the Free State of South Africa. A unique data set has been collected from the Leader Reef Zone of the Witwatersrand Supergroup, exposed close to one kilometre underground. A total of 156 sections, ranging from less than 1 metre to around 3 metres in thickness, were measured through this stratigraphic horizon. The dimensions of the entire study area were around 120 metres by 150 metres. These sections were used to construct a complex 3-dimensional depositional model, which was then populated by a proxy for reservoir porosity. The resulting model of porosity distribution allows wells to be placed with maximum efficiency in a braided river depositional system. The methodology can be directly applied to other depositional settings.
Even mines focusing on igneous rocks and their associated minerals can yield important data on fracture distribution and fault behaviour. Examples from chromitite and vanadium mines in Mpumalunga, mining within the Bushveld Complex in South Africa, demonstrate how even relatively small scale faulting can prove disastrous to horizontal well paths, while slumping and the development of pot holes can actually enhance potential production. The encroachment of gabbro in such subsurface deposits bears many similarities to secondary cementation of reservoirs, with consequent potential as an analogue to silica and calcite cements.
In addition to structural and sedimentological interpretation, economic aspects of mining can also provide guidance when assessing new unconventional targets for exploration potential. Models will be presented that demonstrate how mining models can be applied in the subsurface. Current economic analyses focus on spreadsheets, but kriging and other statistical tactics can enhance results and the understanding of production potential. Overall it is clear that we have barely scratched the surface of a valuable and detailed database with the potential for huge synergies with the energy industry.
AAPG Search and Discovery Article #90207 © AAPG Geoscience Technology Workshop, Unconventionals Update, November 4-5, 2014, Austin, Texas