Establishing Inflow Performance Relationship (IPR) for Gas Condensate Wells

Abstract

Abstract A new simple method of establishing Inflow Performance Relationship for gas condensate wells is proposed. The proposed method uses transient pressure test data to estimate effective permeability as function of pressure and then uses it to convert production BHFP data into pseudopressure to establish well performance. Requirement of relative permeability as function of saturation thus has been completely eliminated. Effective permeability of either phase can be used to predict the production of second phase. A scheme has also been devised to estimate the effective permeability using well testing mathematical models available in literature. Also mathematical models of well deliverability loss due to condensate deposition when dew point pressure is reached, and deliverability gain due to condensate mobility when P* is reached have been developed. Pseudopressure curves for both oil and gas phase have been developed for quick conversion of pressure data into pseudopressure. Relative permeability curves if available can also be used, however, the knowledge of saturation has to be known at all the stages of the depletion to be able to use them. Gas condensate reservoirs are primarily gas reservoirs. As the pressure declines with depletion, reservoir conditions of pressure may go below dew point and liquid begins to buildup. Such reservoirs may go under liquid buildup without showing any trace of liquid production. Sudden well deliverability loss and very high skin factor estimates from pressure tests are strong indicators of liquid buildup. PVT characteristics like phase diagram help identify the problem too. As the critical conditions are reached such reservoirs become two phase in nature. Finally, a field example is analyzed to show the use of new method developed and a step-by-step procedure is used to establish the well performance. Small operators, Independents, will benefit from this method at the most, since data acquisition like relative permeability curves require the laboratory experiments on cores, an expensive procedure. Introduction Retrograde Gas-condensate systems have not been treated so intensively as solution gas reservoirs have been. Main reason is the phase behavior of light (C1-C10) hydrocarbons in the reservoirs. Retrograde gas-condensate reservoirs are primarily gas reservoirs. A zone of liquid begins to form as the dew point pressure is reached. The liquid keeps accumulating and does not flow until the critical liquid saturation is reached. Pressure at this point in the reservoir is termed P*. Interestingly, this liquid may re-vaporize as the pressure further crosses the lower line on two-phase envelope of phase diagram. This behavior of re-vaporization of the oil phase is called the "Retrograde behavior." Fig.2 through Fig.4 show the schematics of such phenomenon in vertical and horizontal well. Deliverability loss in such conditions is mainly due to two reasons:Gas undergoing liquid phase andpermeability impairment by the liquid. Thus both have to be handled mathematically to predict the well performance with reasonable accuracy.