UNCONVENTIONAL TIGHT SAND/SHALE GAS PRODUCTION BASED ON DIFFUSION MASS FLOW (CASE STUDY, PANNONIA BASIN, EU)

The unconventional gas producing shales are all different in their own ways, but they do present an analogy indispensable for exploration and production in uncharted areas that can spare us hundreds of wells. For a meaningful comparison, besides the mineralogical and fluid product characteristics, it is vital to know rock strength and diffusion parameters, which form the basis of the production forecast. It is essential to measure these parameters directly (whether by destructive or non-destructive methods) in conjunction with log interpretation. From the hysteresis under increasing or decreasing load, inferences can be drawn with regard to the optimal process to be applied (slick water vs. crosslinked fluid and/or combined frac). Here, the scanning or AFM analysis of the surfaces can be very instructive.

The Darcy permeability does not come into play in the unperturbed, non-fractured nano range of pore size, simply because of the absence of the pressure gradient that could enable gas flow. This role is performed instead by the concentration-driven diffusion as a function of the surface of the fracked rock mass.

Conventional asset calculations are out of the question with shale gas due to the uncertainty of recovery. In order for a dynamic forecast of recoverable gas quantity, it is therefore crucial to introduce the diffusion coefficient, ideally with a linear (horizontal and vertical) and flooding mass flow under simplified lab conditions, but real formation pressure and temperature conditions, possibly for multi-component (CH, water, CO2, H2S) flow. A purpose-designed laser-based photoacoustic apparatus (PA Hilase) enables us to track changes of the diffusion characteristics. Even though the boundary of the Darcy flow mechanism can be determined by the pressure build-up well test method, this will not lend itself to deriving direct data for the above uconventional gas flow.