Selection of Completion Strategy for Sand Control and Optimal Production Rate – Field Examples from Saudi Arabia’s ‘Unayzah Sandstone Reservoir
Authors: Rahim Zillur , Al-Malki Bandar , Al-Kanaan Adnan 
Published: February 2011

In any field development, it is important to identify reservoir structure, heterogeneity, rock properties, and fluid characteristics to lay down an optimal development strategy to enhance production and increase recovery in the most cost-effective way. As such, detailed reservoir description and characterization using geophysical, geological, and engineering data are required. This paper discusses how a condensate-rich, high flow-capacity, and high-sanding deep gas reservoir was gradually developed and optimized to select the most appropriate drilling and completion technique using available geological, geomechanical, and reservoir data.

Saudi Arabia’s SA-1 field produces from the ‘Unayzah formation which is Permian in age and is subdivided into three stratigraphic units. The lower ‘Unayzah-B and C units present in the SA-1 area are dominated by generally tight, quartzose sandstones with rare siltstones intervals that are ascribed to deposition in lake fluvial-dominated environments. The ‘Unayzah-A comprises of sandstones and variable amounts of siltstones that were deposited in arid, eolian-dominated environment. ‘Unayzah-A is prolific, offers high reservoir quality, and is prone to sand production due to its highly unconsolidated nature. The first well drilled penetrating ‘Unayzah-A in 1997 showed excellent reservoir development. Cores were collected from this well and subsequently from other wells which confirmed unconsolidation of reservoir rock with low Young’s modulus and compressive strength values.

To avoid sanding during production, the wells were initially completed vertical with fracpack stimulation using premium screens. Even though some difficulties were encountered during fracpack installation, the strategy was used in the first few wells. With the advancement of technology both in drilling and completion, the development method was gradually shifted to drilling horizontal and highly slanted holes. This method elemintated deploying the fracpack system, substantially increased reservoir contact, and proved higher well performance. To protect well integrity and eliminate sand production, premium expandable sand screens (ESS) were selected for completing the wells. The overall strategy significantly improved the SA-1 field development program. Higher sustained gas rates were achieved due to reduction of non-Darcy skin, sanding was eleminated, and risk related to deployment of completion equipment (ESS) was reduced. ESS completions are attractive in open-hole completions for their easy-to-use applications, and since they have no proppant or sand-pack filter system that resulted in low or zero skin factor.

Currently, the SA-1 field is producing with wells that have high rates, on the order of 20-30 million standard cubic feet per day (MMscfd). With a high condensate level in this field (more than 400 bbl/MMscf), the wells have experienced a low to moderate decline and the reduction in reservoir pressure has been steady and within expected limits. Improved reservoir contact from horizontals has decreased pressure drop near the well, decreased the rate of condensate dropout, and has improved overall well potential and reservoir perforamance. Initially drilled vertical wells that are facing some production decline due to deterioriation of fracpack screen and proppant conductivity are now being sidetracked and completed with an ESS system.