A new low-permeability reservoir core analysis method based on rate-transient analysis theory

Abstract

Unconventional reservoirs such as shales (mudrocks) and coals may exhibit an ultra-low matrix permeability (<0.001 md) challenging conventional laboratory-based methods for permeability measurement. Small-diameter core plug or crushed rock samples, combined with unsteady-state methods, are currently favored to reduce measurement times for ‘tight’ rocks. For core plug analysis, unsteady-state pulse-decay permeability (PDP) or steady-state methods (SS) are commonly employed in commercial laboratories, with the core plug sample subjected to confining stress. Analysis times, particularly for SS methods, may be excessive for ultra-low permeabilities in the nanodarcy range. Another limitation of both PDP and SS experiments applied to core plugs is that they do not represent the boundary conditions typically used to produce hydrocarbons from unconventional reservoirs in the subsurface through wells.Rate-transient analysis (RTA) is a technique used to quantitatively analyze production data from wells drilled into unconventional reservoirs to extract reservoir (e.g. permeability, hydrocarbons-in-place) and hydraulic fracture (conductivity, fracture length) properties. Multi-fractured horizontal wells (MFHWs) producing from low-permeability reservoirs commonly exhibit the flow-regime sequence of transient linear flow, where hydrocarbons flow through the reservoir orthogonal to hydraulic fractures or the horizontal well, followed by boundary-dominated flow caused by pressure interference between adjacent hydraulic fractures or wells. Transient linear flow may be analyzed using RTA methods to extract fracture or well-length (if permeability is known); the end of linear flow can be used to estimate permeability of the reservoir, and boundary-dominated flow to estimate hydrocarbons-in-place.In this work, a new experimental procedure and set-up, applied to core plugs under stress conditions, is developed to mimic well operating conditions encountered in the field (i.e. producing from a subsurface unconventional reservoir). After injection of methane gas into one end of the core plug, and pressure stabilization, the gas is flowed out of the same end of the core plug at constant pressure (with the aid of a backpressure regulator), and flow rates are measured with a flow meter. This procedure is applied to a core plug extracted from a low-permeability siltstone of the Montney Formation (western Canada). Repeated testing consistently demonstrated a transient linear flow period, as the gas pressure transient propagated along the core plug, followed by boundary-dominated flow after the pressure transient reached the end of the core plug; this sequence is identical to what is commonly observed in the field. Permeability was estimated using both the slope of a square-root of time plot (a common RTA method used to analyze transient linear flow), and the time at the end of linear flow combined with the linear flow distance of investigation (DOI) equation. Permeability from both techniques is in good agreement (±5% for both experiments performed), providing an important redundancy to the analysis procedure. Pore volume (and hence porosity, with bulk volume known) may be estimated from the time at the end of linear flow and the slope of the square-root of time plot – the calculated value is in excellent agreement (±2% for both experiments performed) with that obtained from pore volume/porosity estimates using a helium pycnometer (combined with calipered dimensions). Finally, test times for the ∼0.0007 md core plug sample, after the initiation of the production phase, are on the order of only a few minutes to obtain the two independent estimates of permeability and pore volume, which is faster than that achievable from a PDP test.The new innovative experimental procedure is successful in reproducing the physics of flow in unconventional reservoirs, with the results being analyzable with the same techniques applied to field data.