Numerous gas condensate well production data have shown that well productivity is severely affected when the bottom hole flowing pressure drops below the fluid dew point pressure. This productivity reduction is caused by liquid accumulation around the well. It is essential to take account of this ‘condensate blockage' effect when calculating well productivity since productivity losses can be significant.
Published laboratory, simulation and well-test data have shown that at bottom hole flowing pressure below the dewpoint pressure three regions are created with different liquid saturation.
Since the retrograde condensation has an important impact on gas condensate well productivity, the optimum exploitation of these reservoirs depends on the ability to diagnose condensate bank. Well test data has proven to be one of the few reliable field data of practical use to detect the existence of retrograde condensation. In Santa Barbara field, the biggest Venezuelan gas-condensate field, application of the latest practical well-test analysis methods contributed to demonstrate the presence of velocity stripping and retrograde condensation despite conventional reservoir engineering analysis did not show such phenomena.
The paper discusses how the use of two and three radial composite model helped to gain a better understanding of Santa Barbara condensate reservoir behaviour when 1999 to 2001 well tests were reinterpreted. As velocity stripping was detected, near well bore relative permeability measurements were proposed in order to take account of the phenomena which occur at high flow rates when calculating well productivity in the field-scale simulation model.
Santa Barbara field is the biggest gas condensate field in Venezuela. In this area, PDVSA concentrates most of it operational activity due to its high potential in gas and light oil. Reserves are as big as 6060 MMbls of liquid originally in place, and the accumulative production at December 2001 is approximately 530 MMbls.
During the last years, it has been observed with great concern that the reservoir pressure of Santa-Barbara field has been declining dramatically. While the initial pressure was 12000 psia at a datum elevation of 15800 feet-ss, nowadays the average reservoir pressure and temperature is around 7400 psia and 290 °F, respectively. As the bottomhole flowing pressures of some gas condensate wells started to show values below the dewpoint at the end of 1998, gas productivity impairment caused by retrograde condensation became an issue of great deal due to its negative impact on the recoverable reserves.
Numerous laboratory, theoretical and field studies have been conducted over the last forty years to try to understand condensate flow behavior. The data collected from these studies have shown that when the pressure around a well drops below the dewpoint pressure, retrograde condensation occurs and three different mobility zones with different liquid saturation are created within a radius less than 100 feet. An outer zone away from the well with initial liquid condensate saturation, a zone closer to the well with increased immobile condensate saturation and low gas mobility, and a near wellbore zone with high capillary number (velocity stripping) which increases the gas relative permeability. This increment on gas mobility at the immediate vicinity of the well compensates much of the lost caused by the condensate. It has been found that these zones are very important when calculating well deliverability1,2.
The analysis of well tests on gas condensate wells with retrograde condensation is usually based on either the two or three-zone radial composite model3. This model is based on a simplified geometry of the three regions described above. This paper explains how well test analysis was one of the few reliable field data of practical use to detect the existence of retrograde condensation and velocity stripping around gas condensate wells in Santa-Barbara field.