The availability of natural gas in the Dutch subsurface has boosted the economy of the Netherlands for decades. Nowadays the production of natural gas from conventional sources is rapidly declining while demand is ever increasing. Indeed, it is expected that within five years the Netherlands will become a net importer of gas. Many other EU countries, having far less conventional gas of their own, are already faced with reliance on imports from Russia.
However, apart from conventional resources, the Netherlands and many of its neighbouring countries also possess unconventional gas resources in very tight sandstones, shale formations and coal beds. The difference between conventional and unconventional reservoirs is that the latter have very low permeability, so that a man-made, interconnected subsurface fracture system is needed for production. These unconventional resources have been known for years but the technology to access them was not available. Recently, with the development of new drilling and subsurface fracturing techniques, unconventional gas resources have become accessible, as demonstrated by the widespread exploitation of shale gas in the U.S.A. However, the use of these technologies to exploit unconventional gas resources involves elevated economic and societal risks which need to be reduced or mitigated. Improving the location of potentially productive plays, and improving production efficiency while mitigating risks, requires substantial improvement of our understanding of natural and man-made fracture systems and of multi-scale transport processes in tight rock systems. Alternatively stated, progress in locating and producing unconventional gas in the Netherlands, Europe, and beyond, depends on achieving major improvements in our ability to predict and manipulate multi-scale fracture networks in the subsurface, matrix-to-frac transport, fluid-rock interaction and hence fluid flow.
Achieving such improvement is the overarching goal of the Basin- to pore fracture networks and fluid-rock interactions in tight sands and shales (2F2S) program. 2F2S originates from discussions between groups of the University of Utrecht, the Delft University of Technology and the Technical University of Eindhoven, and four of the industrial partners currently participating in the Dutch Top-Sector or TKI Tough-Gas initiative (EBN, GdF-Suez, Total, Wintershall), and from the shared recognition of the added value offered by integrating systematically the universities’ complementary skills and activities in the area of Tough Gas research. The five academic groups, are leaders in their field and cover a wide and complementary range of expertise resulting in working unit with unique ability to cover multiple scales and disciplines. Unlike the Dutch national (TKI) programme, 2F2S will address not only Dutch tough gas targets, as generic case studies, but also targets in other regions of Europe or even globally, depending on sponsor interests. In Part I, the various work packages proposed for 2F2S are presented. The general work-flow, the effort involved and programme coordination/governance plans are presented in Part II.
The general strategy
The overall strategy of 2F2S is built around the knowledge that efficient predictions of producibility of tough gas plays, such as gas shales, tight sands or coal seams, can be only achieved a) by adopting a multi-scale approach (cover the 105-10-5m range) to understand the behaviour and transport properties of both natural and man-made fracture systems and intervening rock matrix under in-situ P-T-chemical conditions, and b) by consciously integrating the disciplines needed to link the multiple scales involved, i.e. numerical modelling studies, laboratory experiments and field observation and verification studies.
Four overarching challenges have been defined with the aim of aligning and integrating the universities’ different expertise and activities covering both the relevant processes (Challenges 1-3) and regional tectonic evolution aspects (Challenge 4) (Fig. 1).
Fig. 1 – With 4 challenges, 2F2S tackles the multiscale character of fracture-controlled fluid- rock interactions
Challenge 1
Aims at developing tools and knowledge to predict stress and strain fields in arbitrary volumes (as small as 10mx10mx10m in size) inside a potential reservoir characterized by lithological and structural heterogeneities prior to and following fluid injection. Predictive tools are developed and tested in the P6 Natural Laboratory (P6NatLab) (see below) and subsequently applied it to other domains of interest in the NL subsurface. Other targets in and beyond Europe will also be considered according to partner priorities.
Challenge 2
Focuses on the fracture networks developing in natural rocks once stress conditions are altered. It combines innovative numerical models with rock deformation experiments. Resulting fracture networks will be compared using a range of techniques, including the latest 3D visualization tools. Associated permeabilities will derived using, for instance, Discrete Fracture Network models and compared. Boundary conditions (lithology, stress conditions at and following moment of fracturing) will be based on data derived initially from sites in the P6NatLab and subsequently from other sites of interest to the industrial partners .
Challenge 3
Investigates the interactions between fluids circulating in the fractures and the surrounding matrix, devoting special attention to processes controlling the rate at which gas can migrate from the matrix into the fractures. This will often be the key factor controlling producibility. The work will include the effects of swelling/shrinkage effects on matrix and fracture porosity/permeability. Activities in Challenge 3 will investigate methodologies and tools aimed at enhancing matrix permeabilities, so that gas migration rates to fractures can be increased. Experimental and microbeam approaches will be applied to samples from the P6NatLab and from other fields of interest to the partners.
Challenge 4
Interacting with the other components, Challenge 4 will develop regional tectonics-based approaches to predict stress (burial) and temperature trajectories for shale gas basins. These models include thermal maturity and exhumation data and provide essential information needed for a better understanding of the likely gas content, natural fracture architecture and in-situ boundary conditions pertaining to the P6NatLab. Attention will also be focused on testing the applicability of the tools developed to other potentially interesting basins in the Netherlands and elsewhere.