Fast Appraisal And Development Of A Stratigraphic Carbonate Play
Authors: Raghunathan Murali , Al-Zaabi Yousuf Salim , Creusen Anouk , Carrera Mariano , Singh Maniesh , Spiteri Richard Raymond , Soek Harry , Roeterdink Roeland Christiaan , Wigley Paula 
Published: March 2015

This paper discusses the hydrocarbon maturation strategy of a stratigraphic carbonate play in North Oman It illustrates the appraisal and development cycle from play identification and concept definition to project execution, pilot design, execution and early production.

A sequence of carbonate clinoform reservoirs was deposited as an infill to the Bab Basin during the Cretaceous Era. These Upper Shuaiba clinoform trends are around 800–1,000 m wide and over 10 km long and can be imaged on seismic amplitude extractions due to tuning effects at the up-dip pinch-out. The clinoform shoal facies, in the proximal part of the clinoform, forms producible reservoir geo-bodies embedded within argillaceous facies.

The fields in this carbonate play area were discovered over different times and are at different stages of maturation ranging from 'discovered non-commercial' to 'full-field water-flood'. These fields also vary significantly in size and geometry and, together with their heterogeneity, pose key challenges to field appraisal decisions related to pipeline tie-up and facilities, remote operations, and the quantity and quality of water available for injection.

The appraisal strategy was based on drilling off-structure vertical pilot holes followed by horizontal side tracks targeting the prolific shoal facies. Sophisticated geosteering tools were used to place the horizontal wells within the reservoir at desired 'distance to bed boundary'. In order to capture the wide range of uncertainties in the reservoir geometry and reservoir heterogeneity, an integrated approach to data gathering and analysis was adopted. Seismic amplitude extractions were used to establish trend data and to constrain the broad-scale shape of the geo-bodies. Directional density and resistivity data were used for well placement and to help underpin reservoir thickness. Well-tests, production and pressure data were all used to constrain the extent of the producible facies. Logs and core-data were used to identify facies. In addition to conventional logging tools, other tools like Nuclear Magnetic Resonance and Borehole Imaging were also used for reservoir characterization.

Recent water-flood pilots have been used to help ensure commercial feasibility. Analytical concepts and analogue data from areas that are further along in their development stage were used to constrain the reservoir models. In addition to updating 3D geological models for dynamic studies, simple simulation 'box-models' were used to plan wells and to support the adopted 'drill and learn' philosophy to optimize well spacing while allowing sufficient flexibility for corrective measures to mitigate all risks. A balance and synergy between an aggressive drilling schedule and early water injection was implemented in the development strategy for this play.

Various surveillance activities like frequent well testing, temperature monitoring along well bores, rate measurements, surface/subsurface pressures and water salinity sampling are used to monitor the water flooding process and to adjust injection and production rates as required. A proactive multi-disciplinary team is in place to deal with all subsurface activities establishing robust workflows and providing continuous feedback to management.

Introduction

The Upper Shuaiba clinoforms were deposited in the Late Aptian, during a second order low stand on either side of the 70 km wide strait between the Shuaiba platforms (Figure 1). These clinoform geometries are recognizable on 3D seismic data and prograde inwards from the northern and southern margins of the Bab Basin (Figure 2).