The characteristics of gas and condensate flow in porous media are significantly different to those of conventional gas-oil systems. The flow of gas and condensate in reservoirs has been studied in this laboratory since 1986, investigating several recovery process including pressure depletion, gravity drainage and water encroachment. The above studies have resulted in development of a number of facilities and expertise. Three core test rigs, including in-situ saturation monitoring using x-ray, which allow both steady and unsteady state relative permeability tests, have been designed, commissioned and evaluated successfully. Appropriate experimental procedures for testing gas and condensate fluids have been developed. Theoretical and numerical models, based on mechanistic approaches, have been developed to interpret and generalise the results.

The effect of interfacial tension (IFT) on relative permeability at low IFT values has been known for very long time (Bardon and Longeron, 1980). We demonstrated experimentally for the first time in 1994 (Danesh et al.) that the relative permeabilities of gas-condensate in the near wellbore region may improve significantly by increasing the rate (Figure 1). Figure 2 shows that at lower condensate saturation and higher velocities the relative permeability values decreases, due to dominance of the inertial effect but the positive coupling effect surpasses the inertial effect at higher condensate saturation resulting in a net increase of relative permeability with velocity. Our recent experimental results have also shown that for tight core and high permeability propped fractures inertia could be more dominant especially at lower IFT values.

We were also first to study the improvement in relative permeability of low IFT systems as velocity increases and/or IFT decreases, known as coupling effect mechanistically using a combined experimental and theoretical approach (Jamiolahmady et. al, 2000). A unique flow pattern consisting of simultaneous flow of the two phases with intermittent opening and closure of gas passage by condensate was observed in the conducted flow visulatiasion experiments using high pressure micromodels (Figure 3). A single-pore model was developed based on the basic constitutive equations capturing the competition of viscous and capillary forces at the pore level in this cyclic flow pattern. The model, consistent with the experiments, showed that for these low IFT systems, there is a highly conductive film of condensate flowing with the gas. The condensate evolves at the pore throat and blocks the gas passage, after which the flow of gas should continue till it overcome the capillary barrier (Figure 2). As IFT increases the number of pores promoting this flow pattern gradually diminishes switching to conventional Darcy type of flow mechanism used in the channel flow concept. Hence, when the effect of multiple pore interaction was included in a network of pore (Jamiolahmady et. al, 2003) kr values comparable with experimentally measured values were reported (Figure 4).

There are saturation based relative permeability correlations currently available in the two leading commercial reservoir simulators (i.e., ECLIPSE and VIP) referred to as VDRP (Velocity-Dependent-Relative-Permeability) formulations, which were developed at Heriot-Watt University in the previous phases of the project. These formulations express the positive coupling and negative inertial effects separately with inertia considered for the gas phase only. They also require core specific constants to predict near-wellbore relative permeability more efficiently. This has resulted in lack of generality limiting their use. Therefore, in our new generation of relative permeability correlation the combined effects of positive coupling and negative inertia are expressed simultaneously in one formulation. The generalised correlation has either universal parameters or those parameters that can be estimated from readily available petrophysical data. The input rock data to use this correlation, for predicting gas condensate relative permeability at different IFT and velocity, are absolute permeability, porosity, single-phase inertial factor, mercury porosimetry Pc curve and a base kr curve measured at high IFT and low velocities conditions (conventional kr measurements).

Based on our extensive experience in simulating gas condensate flow, a number of in-house simulators describing gas-condensate flow around perforated/hydraulically fractured/deviated wells have been developed. Based on the results of these in-house simulators, we also proposed reliable methodologies for calculation of gas condensate well productivity, under various completion strategies, using an equivalent 1-D radial open hole system with a modified well bore radius, which is calculated based on our in-house skin formulations. These correlations eliminate the need for cumbersome and costly 3-D fine gridding exercise, such as those implemented in our in-house simulators, to capture accurately the variations of pressure and velocities for such complex flow geometries. A new formulation for the optimization of a hydraulic fracture design has also been introduced.

Our NeW-COIN (Near Wellbore Coupling INertial) software, is a desktop compositional simulator suitable for researchers and field reservoir engineers. It includes all our formulations and proposed methodologies to reliably calculate gas-condensate relative permeability and estimate well productivity of open hole, perforated/hydraulically fractured/highly deviated gas-condensate wells.

For more information about the project please contact Prof. M Jamiolahmady.

Project Publications available here: