Improved Near-Well Reservoir Characterisation of a Complex Gas Condensate Reservoir in Saudi Arabia by Integrating Core, Log, Seismic and Extended Well Test Data

Abstract

Abstract The Jauf reservoir is a deep sequence of thin sand/shale beds with a large degree of variation in permeability and porosity saturated with rich gas. 3-D seismic interpretation indicates high degree of faulting and reservoir fluid variations indicate compartmentalization. This paper presents the results of three pressure drawdown and buildup tests from a Jauf well, in which PLT survey as part of the second test indicated non-uniform flow profile reflecting the contrast in permeability between the various sand layers. A number of interpretation models were investigated using data from the first and second tests. The tests exhibit boundary effects that were interpreted to represent limited extension of the high permeability layer but not conclusive for the lower permeability layers. The third test was designed with long flow and shut-in periods, based on the second test interpretation, to confirm reservoir layering and compartmentalization. The analysis integrates pressure transient, PLT, seismic and core data and demonstrates the importance of knowing the flow profile and designing test duration in order to get a more realistic interpretation. The core data confirmed that the non-uniform profile was not the result of damage but in fact reflects variation in permeability, which was not clear from openhole logs. The interpretation of the third test indicates two main reservoir systems, the first is high permeability closed system of finite extent and the second is very tight infinite acting system. The interpretation also shows that significant portion of the gas in place in this reservoir is in rock of lower permeability than initially thought. The implications of this finding on well completion and productivity enhancement options are also presented. Introduction As part of the effort to satisfy the Kingdom's increasing demand, of gas, fuel and feedstock, Saudi Aramco has accelerated the development of many recently discovered gas reservoirs. At present, the primary gas production comes from the prolific Khuff carbonate formation, which underlies the major oil production zone - Ghawar Arab-D. The deeper Jauf (Pre-Khuff) formation was found also containing significant amount of gas reserves, and more importantly, the gas in this formation is sweet. The Jauf reservoir was found gas bearing in several areas throughout the eastern region of Saudi Arabia. However, this paper is concentrated on the Jauf reservoir in Hawiyah area of the Ghawar field. A massive program has been launched to bring the Hawiyah Juaf reservoir to production by September 2001. An extensive reservoir appraisal program was one of the major tasks that were implemented early on to address many issues related to the field development and production optimization. The appraisal strategy included extensive coring, logging and testing programs. In this paper we will focus on one of the initial appraisal wells that exhibited performance anomaly during a short flow test while the rig was on location. Reservoir Geology The Juaf formation is an early Devonian sandstone. It consists of shallow marine sands, deposited over a broad shelf. The formation includes fine-to-medium grained sandstone and the hydrocarbon traps are found in simple four-way closures as well as more complex structural-stratigraphic traps. The primary source rock for the Jauf reservoir is the basal hot shale of the Lower Siliurian Qalibah Formation.