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Integrated Understanding of Enhanced Reservoir Properties through Fracturing and Dissolution: Case Study from the Late Jurassic Arab Formation in a Large Gas Field, Onshore UAE

Al Darmaki, Fatima *1; Lawrence, David 1; Singleton, Richard 1; Reddy, Budideti A.1; Popa, Georgeta 1; Cortes, Pascal 2
(1) Subsurface, Al Hosn Gas, Abu Dhabi, United Arab Emirates. (2) Badley Ashton & Associates, Abu Dhabi, United Arab Emirates.

A major gas accumulation located onshore UAE in the Late Jurassic Arab Formation is currently being developed from eight drilling pads. The field is a large NE-SW trending four-way dip closed anticline. The main Arab Formation reservoirs underly the regionally extensive Hith Anyhdrite topseal and four reservoir units are identified (Arab A, B, C & D). The upper Arab A & B reservoirs comprise complex alternations of dolomitic limestone and anhydrite layers. The lower Arab Formation contains the main Arab C reservoir dominated by oolitic grainstone shoal deposits. This is underlain by the Arab D consisting of mud-rich limestones / wackestones with very low matrix porosity and permeability.

The initial development plan was based on drilling horizontal gas producers in the principal Arab C oolitic limestone reservoir. During early development drilling in a previously poorly understood part of the field, the first deviated pilot hole encountered significantly enhanced reservoir properties in both the Arab D and Arab C reservoirs. An extensive program of data acquisition included coring and pressure measurements in the pilot hole, borehole image logging in the pilot hole / horizontal wells and seismic re-processing. Initial results indicate that enhancement of reservoir properties is related to both dissolution along deep-rooted faults / fracture corridors and bedding-parallel leaching along initially high permeability pathways (stylolitic bed boundaries, stylofractures, grainy & bioclast-rich facies).

Detailed information from a variety of well configurations on one drilling pad (deviated pilot hole, single laterals in Arab C, dual laterals in Arab C & D) has enabled inferences to be made on the orientation and extent of high-permeability pathways related to fracturing and bedding-parallel dissolution. This has a significant impact on the productivity potential of the Arab D in a large part of the field and future development drilling of the main reservoir targets. Reservoir connectivity, well productivity and drainage areas especially in the Arab D still carry high uncertainties which are being reduced by focused continued data acquisition, integration and modeling efforts.


AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain