oa Understanding and Managing Water Advance in a Heterogeneous Carbonate Reservoir: from Integrated Subsurface Approach

Rapid and uneven water advance is observed in heterogeneous platform-interior carbonate reservoirs<br>in the south of a super-giant Middle East Field, producing since the early 1960s. Crestal oil production<br>is supported through peripheral water injectors typically 5-7km from the producing area. Historical<br>production imbalance has accentuated water movement in the south of the field, with wells ceasing to<br>flow naturally even with relatively low water cut.<br>A multidisciplinary approach has been used to understand controls on water encroachment. This<br>involved integration of geology (core facies, correlation, reservoir properties), geophysics (seismic<br>attributes, fault / fracture characterization), petrophysics (openhole & time-lapse cased hole logs) and<br>reservoir engineering data (simulation models, production / injection).<br>Water advance is focused in certain areas of the field, and within limited stratigraphic layers. In<br>particular, two prominent ‘water fingers’ are present within most reservoir units at the limit between<br>the platform interior and thick prograding platform margin belt.<br>Stratigraphic water fingering is controlled mainly by vertical variations in reservoir quality, particularly<br>permeability within the 5 main Reservoir Units (named 1 - 5 from bottom to top). Water advance is<br>less rapid in the lower two and the uppermost Reservoir Units (1, 2 and 5) which show interbedded<br>stylolitic dense layers and lower matrix permeability (typically 1-10md). Within Units 3 and 4, water<br>movement is more advanced and initially concentrated along super-high (darcy scale) permeability<br>layers a few feet thick. Sequence stratigraphic analysis calibrated with core confirms that such layers<br>are associated with 3rd & 4th Order sequence boundaries, with partial cementation but preserving<br>vuggy porosity. Within Units 3 and 4, poorer reservoir quality facies (more cemented, coral-rich in Unit<br>3) and non-reservoir carbonate mudstones (within Unit 4) act as isolated local baffles to horizontal<br>water movement.<br>Controls on the location of the two prominent areal water fingers include faulting / fracturing,<br>stratigraphic contrast between the platform margin and interior, and production / injection imbalance.<br>An improved understanding of water movement is being used to predict future water breakthrough,<br>refine infill drilling locations, plan future Artificial Lift requirements, design selective well completions<br>and optimize field development.