A guide to petrophysical models for shale gas reservoirs based on sensitivity analysis of key variables

Shale gas is seen as an attractive proposition to UK government and industry based on success in the USA. A key uncertainty in evaluating the resource is the gas in place and a plethora of petrophysical approaches or models exist that are at best confusing and at worst misleading. This catalyst proposal aims to address this and provide industry, government and society with a guide through the petrophysical maze. It addresses the NERC call in enabling the development and extension of our industry collaboration on shale gas through the development of a substantial JIP, and falls directly into the theme: Unconventional Hydrocarbons.

Shale gas refers to fine-grained formations where organic matter has matured to produce predominantly gas, but that gas has not migrated any significant distance. The source rock is effectively the reservoir, although the geological, and petrophysical, nature of the reservoir can be incredibly variable. Shale gas is frequently characterized by two distinct gas components: free gas is able to move and occupies the pores, while adsorbed gas is fixed onto organic surfaces and held in place by pressure; these two occupy adjacent volumes and separating out the two to avoid double counting is an important step. Our understanding of shale gas petrophysics has changed significantly in the last 5 years, partly as a result of our own research and partly because of significant developments in the published literature. Calculating the quantity of gas in place is done through the development and application of a petrophysical model and for shale gas there are a number of different approaches. We propose to build on our work on shale gas petrophysics and develop a guide to these petrophysical models based on our own analysis of the Haynesville shale, published data, peer-reviewed papers, and industry experience. We will identify key variables within the models and explore the sensitivity of the models to imposed changes in geological and petrophysical parameters. We will also carry out a preliminary investigation of whether there are underlying geological characteristics that control or determine the petrophysical characteristics of the shale gas play.

This is at a critical time for the UK, and providing clarity of the differences between the models, and the ability to identify an appropriate choice of model is an urgent requirement. In the UK, recent estimates by the British Geological Survey for the Carboniferous Bowland Shale suggest very significant amounts of gas exist and quantify the gas at 1329 tcf or 37.6 tcm, as a central (P50) figure. The BGS, however, acknowledge that these estimates necessitated making considerable assumptions concerning the petrophysical modeling.

The proposed catalyst funding builds on these developments at a critical time for the UK in improving our understanding but moreover in providing knowledge exchange between academia and industry, and society generally. This proposal will provide clear guidelines for calculating the gas in place and will communicate these to industry, government and public.

The main deliverables will be a clear explanation of how petrophysical models for shale gas vary, and guidelines for their use. This will improve the general understanding of shale gas, but moreover provides knowledge exchange between academia and industry, and society generally, through communicating these guidelines to industry, government and public alike. Our outcomes will include a peer-reviewed publication of the guidelines, an industry KE workshop on shale gas petrophysics models, a briefing paper for government, an outreach article to inform society. This initial study will then be developed and strengthened through the development of a JIP to further evaluate shale gas models with emphasis on a UK context. A focus of the JIP will be to bring substantial new geological research to bear on the petrophysical study.