This case study attempts to explain a fully integrated approach, to develop a comprehensive understanding of variable fluid contacts in a giant carbonate field (with stacked reservoirs) of Middle Eastern region. With very little to no water handling facility at surface, this study helps the operating unit to sustain dry hydrocarbon production by identification of potential water producing zones. The outcome of this study also has a direct impact to optimize future well locations/numbers, and to come up with a perforation strategy integrating surface and subsurface disciplines.
A detailed fluid fill analysis is done with the available data. Oil, water, and transition zones are picked in individual wells (~50) and correlated with Drill Stem Test (DST) data, mud log data, special log data (viz. NMR), and electro-facies data (rock typing from logs) to arrive at an understanding about lateral and vertical fluid distribution in the field. Reservoir surveillance data, in few wells, is also used to calibrate the fluid distribution model.
Fluid-fill distribution mapping has shown that fluid fill varies both vertically (between zones) and laterally. Vertical variation can be attributed to capillarity of different rock types – rocks with good porosity and permeability having the deepest oil fills with little to no transition zones, while low porosity rocks have long transition zones and shallow oil fills. A consistent inverse relationship is observed between the porosity and thickness of the transition zone for all reservoirs. Also, lateral variation in the fluid fill is observed in ENE-SSW direction where aquifer is present. Open hole log interpretation shows residual oil signature in wells in W-NW direction which is further confirmed by available mudlog data. A structural-tilting theory helps us to explain these lateral variations across the field. Observations infer that a tilting episode has sunk the NW part of the structure, post oil migration. Consequently, the oil-filled rock that submerged below Free Water Level (FWL) underwent imbibition. The evidence of imbibition is now seen in the form of a residual oil signature. Understanding and mapping of the residual oil zones is critical to sustain the production of dry oil and delay/minimize the water production in the field.
The novelty of this study is to reiterate cross discipline integration, where different pieces of data when put together helps to solve the most complex of the problems (viz. variable fluid contact in carbonate reservoirs, rock typing scheme), thereby having a direct impact on the field development plan and consequently on the ultimate hydrocarbon recovery.