Non-Darcy flow increases the pressure drop required to establish a desired gas well production rate, thus decreasing productivity. This increased pressure drop is exacerbated by the liquid drop-out and build-up which occurs in gas condensate wells. The additional pressure drop caused by the two phase non-Darcy flow can have a dramatic effect on the flowing bottom hole pressure required to maintain producing rates, especially in high rate gas condensate systems. Neglecting or underestimating this effect will cause optimistic predictions of the maintenance of gas rate plateau.
Literature values for measured inertial coefficients, (which quantify non-Darcy flow pressure drops) show a scatter of a one to two orders magnitude for a given permeability. Measurements of inertial coefficients (betas) may or may not agree with literature reported values. Use of literature reported permeability - beta relationships to calculate beta may not be appropriate and may lead to significant errors. These errors increase with increasing flow rate and decreasing permeability.
This paper presents transient flow beta measurements obtained in dry core samples and in samples containing different saturations of immobile liquid. Results from core samples from three different reservoirs and one outcrop are included. A method has been developed to estimate the effective beta at different liquid saturations using core samples containing various saturations of solidified paraffin wax that mimic an immobile condensate phase. Continued validation of this technique will allow simplified and inexpensive beta measurements as functions of saturation and permeability
A relationship has been derived which allows the inertial coefficient to be estimated as a function of effective permeability and effective porosity. This relationship appears to hold for almost all of the core samples tested and includes both dry and saturated samples. The relationship, which includes both permeability and porosity, provides significantly reduced scatter compared with previous permeability-beta relationships and allows better understanding and estimation of beta.