Gas Turbulence Factor in a Microvugular Carbonate

A laboratory study of the turbulence factor in low-permeability carbonateswas undertaken, because turbulence is an important factor in gas wells in suchsystems. This study shows that, as has been observed with other rock types, the turbulence factor correlates with permeability. For the microvugular carbonatesystems, however, the correlation yields a turbulence factor that, for any given permeability, is an order of magnitude higher than found in earlier work.The presence of a static second phase, such as connate water or condensate, alters the correlation between the turbulence factor and permeability. As thesaturation of the static second phase increases, the turbulence factor first decreases and then rapidly increases, whereas the permeability decreases in thenormal manner. INTRODUCTION GAS FLOW IN THE VICINITY OF THE WELL BORE is generally in the so-calledturbulent-flow regime. To be able to predict gas-well performance accurately, it is necessary to evaluate the additional pressure drop resulting fromturbulence. The term "turbulence," as applied to the flow through consolidated porous solids with relatively small flow passages, describes an excessive pressure drop which is associated with the inertia effects of the gas travelling through narrow passages. The question is whether this excessive pressure drop is caused by eddies or by a turbulent-flow regime as encounteredin pipes or beds of pebbles, glass beads, etc. – although the pressure-dropflow-rate relations are the same or similar for both estimates of the"turbulence" effect can be obtained by gas-well tests, but it is also helpfulto measure this parameter in the laboratory; such measurements in the past haveresulted in turbulence factor – permeability correlations. The first objective of the work described in this paper was to determine the turbulence factor - permeability correlation for microvugular carbonates.Interest in microvugular-carbonate gas reservoirs is high in Western Canada andwe wanted to compare the correlation for this rock type with published data. Toachieve this objective, a suite of cores was selected from the Turner Valleymember of the Rundle formation, a rock of Mississippian age. The samples were chosen to cover the formation's permeability range. The second objective was to determine how the turbulence factor -permeability correlation is affected by the presence of a static second phase. In the reservoir, a second phase is present in the form of connate water;furthermore, the liquid saturation around the wellbore can be increased bymechanisms such as mud-filtrate invasion, coning, acidizing or retrogradecondensation. In the laboratory, on the other hand, most measurements have beenmade on dry cores. The resulting dry-core turbulence factor – permeabilitycorrelation, together with a relative-permeability curve, offers a procedurefor estimating the effect of the second phase on the turbulence factor, provided that the turbulence factor – permeability correlation is unaffected bythe presence of the liquid. Therefore, we wanted to find out, experimentally, whether this is the case. This paper presents the results of the investigation by first revealing the theoretical considerations involved. It then explains the experimental procedure and the results obtained. Finally, it discusses the results.