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EXPERIMENTAL INVESTIGATION OF MATRIX PERMEABILITY OF SHALE

(2015) Mathur, Ashish; Sondergeld, Carl H.; Rai, Chandra S.; Devegowda, Deepak

Matrix permeability of shale is an important parameter needed to evaluate the economic viability of unconventional reservoirs. Laboratory measurement of shale permeability has therefore become important. Due to the inconsistencies in values reported for the pressure decay permeability measurements on crushed rock samples, measurements on core plugs have become the recent focus. An experimental apparatus has been developed in our laboratory which has the capability of performing steady state permeability measurement and transient techniques such as pressure build up and pulse decay. Various researchers have pointed out that due to the nanometer sized pores in shale, transition regime is dominant, especially at low pore pressures. In the transition flow regime, gas permeability measurements cannot be corrected with a simple Klinkenberg correction and require second order permeability corrections. An experimental investigation to understand the flow regime at different pore pressures has been carried out on two Wolfcamp shale samples. Steady state permeability of these samples was measured using helium and nitrogen at different pore pressure ranges (100 psi to 3000 psi) at a constant effective pressure of 3000 psi. It was assumed that whichever correction provided a similar value of permeability i.e. Klinkenberg and double slip, for both the gases for a pore pressure range, would be the appropriate correction for that range. It was found that when pore pressures were less than 250 psi, permeabilities required second order corrections which suggest transition flow. At pore pressures close to 1000 psi, permeabilities required the Klinkenberg correction. Interestingly, permeability measurements at pore pressures greater than 2000 psi required negligible corrections. Various plug permeability methods have been developed for shales which yield inconsistent result. It has therefore become important to compare them in order to standardize the permeability measurement. A round robin study for permeability measurement was conducted. Permeability of nine pyrophyllite samples was measured by four different laboratories using nitrogen (99.999% pure) as pore fluid. The laboratories used different measurement methods such as steady state, step decay and pressure build up. The confining pressure applied was 1000 psi while pore pressures were maintained below 250 psi. Second order corrections resulted in good agreement of permeability values while the Klinkenberg correction resulted in large discrepancies. A test was performed to compare the permeability values measured on shales using the steady state method with transient methods like pulse decay and pressure build up. Permeability of five horizontal W olfcamp shale samples as measured at an effective pressure of 3000 psi using nitrogen as pore fluid. The pore pressure ranged between 100 to 250 psi for the measurement and hence second order corrections were made. The three methods yielded similar permeabilities with differences being within 30%. Due to growing interest in production from oil shales, it has become imperative to know whether corrected gas permeability values are comparable to liquid permeability. Comparison of permeability measurements using dodecane (liquid) and nitrogen (gas) as pore fluids was conducted. Steady state permeability measurements were made on seven horizontal Wolfcamp shales at 6000 psi confining pressure and a mean pore pressure of 3000 psi with both fluids. Corrected gas permeability values were observed to be higher_than liquid permeabilities by a factor of two. NMR measurements were made before and after permeability measurements. Water saturation remained constant at 22% after permeability measurements with either fluid. This means that the sample is not completely saturated with the flowing pore fluid during regular permeability measurements and hence relative permeability is essentially measured. Permeability creep studies were also conducted on Wolfcamp, Eagle Ford, and Vaca Muerta shales. The steady state method was used for Vaca Muerta and Wolfcamp, while pressure build up was employed for Eagle Ford samples. Both gas, i.e. nitogen, and liquid, i.e. dodecane, were used as pore fluids in different tests. The Vaca Muerta shale creep test was done at three different effective stresses ranging 2500 to 4500 psi while on the other samples the value of effective stress was held constant at 3000 psi. None of the shales showed any significant change in permeability with time over 30 days. This is in contrast with some published studies. The values of effective pressure coefficient were measured for Wolfcamp and Eagle Ford shales. The steady state method was employed for Wolfcamp while pressure build up was used for Eagle Ford. Nitrogen was used as the pore fluid in both cases. The coefficient was found to be less than or equal to one which is low compared to conventional rocks. This has a positive impact during production, as effective pressure is less sensitive to changes in pore pressure.