Student Seminar October 21, 2016 3:00 pm
8-207 Donadeo Innovation Centre for Engineering
Speaker: Dr. Alireza A Moghadam
Supervisor: Prof. Rick Chalaturnyk
Title: Analytical and Experimental Study of Gas Flow Regime in the Matrix and Fractures of Tight Porous Media
One of the most significant differences between conventional and unconventional gas resources is the ultra-low matrix permeability of unconventional gas reservoirs such as shale, coal and tight gas. Matrix permeability of shales is an elusive but important parameter in characterizing shale gas reservoirs. It is believed that the long-term performance of shale wells is controlled by matrix permeability. Permeability is typically measured using steady-state flow tests or the more timely transient methods such as pulse-decay. Due to the low permeability nature of shale rocks, a slip or transition flow regime is observed to be dominant at the pore scale and as a result, permeability changes with pore pressure. Traditionally, permeability is measured at various mean pore pressures and the data is used to extract the Klinkenberg or absolute permeability. However, the assumptions behind the Klinkenberg’s equation do not apply to the nanoscale pore networks in shales and other tight rocks. Additionally, various methods of permeability measurement can lead to significantly different results at similar test conditions.
In this work, a fundamental analytical study was conducted to understand the dominant flow regimes under laboratory and reservoir conditions, which is important for modelling gas flow and predicting gas production. Based on the findings, an analytical model is developed capable of modelling gas flow in low permeability reservoirs while retaining the simplicity of Klinkenberg’s original formulation. The theory suggests, that the gas permeability is in essence a function of the pore radius open to gas flow, but further enhanced by the slippage of gas molecules at the pore wall. The slippage factor is also a function of available pore radius to flow, increasing as the pores diminish in size. These findings imply that any parameter that changes the effective pore radius in the porous media (water saturation, adsorption, effective stress, etc.) alters the absolute permeability and the slip factor. This alteration in gas permeability can be quantified using the approach proposed and verified in this work. Extensive laboratory testing has been conducted to measure gas permeability in tight rocks at various conditions. The lab tests are uniquely designed to capture the influence of pressure, effective stress, temperature, gas type, and flow rate. The influence of gas flow rate on permeability has never been investigated before in this class of materials. This phenomenon casts a shadow of doubt around the common non-steady-state permeability measurement methods. A more complete picture of the gas flow regime as a function of pore pressure and velocity is introduced.
The outcomes of this research focus on designing more representative gas permeability tests, proposing accurate methods of analyzing lab results, and finally to be able to convert the lab results to the in-situ values to model gas flow in the reservoir. This work lays the foundation to revisit the basic definition of gas permeability, in order to set up new standards (concerning testing pressure, rate, and stress state) to obtain meaningful and comparable permeability measurements.