Abstract

Hydraulic fracturing in the sandstone and carbonate reservoirs has been adopted as the primary means to enhance production from the gas wells in the Ghawar field of Saudi Arabia. The initial incremental production from acid treatments is readily noticed from all stimulated wells in the Khuff carbonates. For the Pre-Khuff sandstones, because of the sanding tendency of the unconsolidated formation, selective perforations and indirect fracture treatments are performed to stimulate the wells. Thus the incremental production is not ascertained through actual production data, rather the post-fracture production is compared to the simulated pre-fracture potential. In either case, as the production continues and the reservoirs are depleted, production impairment occurs due to potential water encroachment, condensate build up around the wellbore, and changes in non-Darcy flow effects as gas enters production string.

This paper presents a comprehensive study of the impact of pressure depletion on production for Ghawar gas-condensate reservoirs. The paper quantifies the effect of condensate dropout on the well productivity with and without induced hydraulic fractures.The effects of non-Darcy flow have also been investigated and presented. A compositional model has been used to forecast and optimize long-term production under various reservoir conditions and operational scenarios.

Introduction

Saudi Aramco's development plans for its non-associated gas found in deep carbonate and sandstone reservoirs includes stimulation treatments of the gas wells. Underlying the giant oil bearing Ghawar Arab-D reservoirs are gas bearing Khuff and Pre-Khuff formations. Khuff is carbonate, whereas Pre-Khuff (Jauf and 'Unayzah) is sandstone. There are also recently discovered fields south of Ghawar containing Pre-Khuff sweet gas.

As the gas bearing formations are found across the greater Ghawar area, the gas quality varies considerably from North to South. The condensate yield varies from 30 bbl/mmscf for lean gas to 300 bbl/mmscf for very rich gas. Lean gas has been in production for more than a decade. Gas from condensate rich reservoirs has entered the production stream recently. The rich gas from gas-condensate areas are being developed and produced to feed to the newly built giant gas plants.

Varying reservoir characteristics and fluid characterisitics influence the gas development program. One of the concerns in developing a gas-condensate reservoir is the possibility of loss of gas productivity due to the buildup of condensate near the wellbore. The other concern is the drop of condensate inside the reservoir leaving a substantial amount at the end of exploitation period. A comprehensive understanding of the magnitude of liquid dropout near the wellbore and its effect on the well's gas productivity would be essential for the optimal development program. In the past, many authors have investigated the impact of liquid dropout on gas productivity. O' Dell and Miller1 has studied a very rich gas condensate field with 45% liquid dropout via analytical method. He proposed a method of calculating near wellbore condensation. Others such as Bourblaux2 has used 2-D radial compositional model to investigate liquid dropout in gas-condensate reservoir.

The objective of this study is to assess the decline of gas productivity due to near wellbore condensation for a range of gas qualities found in Ghawar and its surrounding areas. A state-of-art single well compositional simulation model was used to investigate the problem.

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