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Technical Paper: Successful Drilling and Deployment of an Open Hole Multistage Fracturing System in a Deep and Hostile Sandstone Gas Reservoir

Society: SPE
Paper Number: 149062
Presentation Date: 2011
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Abstract

The pre-Khuff Unayzah gas reservoir in Saudi Arabia is a competent sandstone formation with Unayzah-A and B/C sand units separated by a variable thickness siltstone unit, which is regarded as a sealing barrier between the Unayzah-A and Unayzah-B reservoirs. They are known to contain significant reserves of natural gas, however they are at low permeability, which has made it difficult to optimize production. Drilling through highly abrasive sandstone at high bottom-hole temperatures (BHT) has also been challenging. To meet Saudi Arabia’s future gas requirements, it was essential to establish a means to exploit these reserves.  Multiple methods have been used to drill and complete these wells, including long horizontals and multilaterals. Well trajectories were planned with the horizontal sections drilled along the direction of maximum in-situ stress (parallel to the fracture plane) for enhanced wellbore stability and fast penetration rates. Fracture treatments were pumped generating mostly longitudinal fractures propagating in the same direction as the maximum horizontal stress (parallel to the wellbore axis) resulting in a poor fracture network and suboptimal production results.  In April 2010, a horizontal well was successfully drilled into the Unayzah reservoir towards the minimum in-situ stress direction and completed with an open hole, multistage (OHMS) fracturing system. A four-stage proppant fracturing job was successfully conducted in November 2010 without issues. Unique fracture signatures were observed in each zone and production from the well has exceeded expectations.  This paper analyzes Saudi Aramco’s successful initiative to drill the lateral section along the direction of minimum in-situ stress and the challenges associated with wellbore stability, a highly abrasive formation and high static BHT. Also highlighted are Saudi Aramco’s successful techniques for deploying the OHMS fracturing system with special emphases on borehole preparation. Lessons from the successful drilling/completion of this well could have a significant impact on how future Unayzah wells are drilled and completed to maximize productivity from tight gas reservoirs in Saudi Arabia.

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