Abstract

Reservoir heterogeneity understanding is critical to predict gas-water contact (GWC) movement in order to take early preventative measures to delay water breakthrough in wells and hence improve Well, Reservoir and Facilities Management (WRFM). This maximizes ultimate recovery of a gas brownfield.

The carbonate gas field investigated in this paper (called Delta Field) is located in Central Luconia, with 20 years of production history. It is a platform type carbonate build-up, penetrated by eight production wells and a water disposal well. The contact movement of the reservoir was monitored using well surveillance logs. After 5 years of production, a horizontal well (Delta-110S1) was drilled in the southern part of the field. The well targeted attic gas at the top of the reservoir and was designed with a horizontal drainage bore to avoid early water breakthrough. However, it encountered water breakthrough in Feb 2004 after 1.5 years of production. By 2007, all Delta Field wells were producing water.

This indicated that the gas-water contact (GWC) rose non-uniformly with water encroachment faster in the southern area of the field compared to the north, resulting in a tilted GWC. Hence, a ‘back-to-basics' reservoir characterization was required to understand the reservoir architecture and heterogeneities.

Reservoir characterization was carried out to identify and map out the distribution of low porosity zones within the carbonate reservoir, based on seismic and well-log data as well as regional understanding. In the Central Luconia carbonate fields two types of tight layers with very different dynamic behaviours are encountered: (1) Tight layers that formed by floodings events with a large lateral extent and continuity, acting as baffles to gas and water flow, and (2) tight layers related to exposure-events with a more patchy distribution, which can act as minor, local baffles but can also function as conduits for water when they are related to karsts and/or fractures. To distinguish the nature of the tight layers and predict their dynamic behaviour, they were mapped out within and between platforms and calibrated with core and seismic data, to understand their depositional origins that control their properties impacting flow and their continuity.

Subsurface reservoir models focusing on the key reservoir heterogeneities that matter for flow were built by incorporating core, well log, drilling and seismic data. Integrating this static understanding with completion data (perforation intervals, water shut offs) and dynamic data (GWC-rise history, production and water-cut rates) provided valuable insights into the evolution of the GWC movement and allowed more accurate prediction of production rates and identification of suitable infill well locations.

Through this study, an infill well opportunity was identified to target remaining attic gas in the northern part of the field. In 2013, Delta-111S1 was abandoned and side-tracked to target remaining attic gas in the north of the field. Delta-111S2 achieved the first gas in December 2013 and has produced very successfully.

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