The Impact of Fractures on Producibility and Completions in a Heavy Oil Carbonate Reservoir

The Wafra Field is located in the Partitioned Zone (PZ) between Saudi Arabia and Kuwait. Designing the right completion strategy to minimize water cut has been challenging in the Maastrichtian reservoir. One of the biggest challenges relates to variable hydrocarbon mobility and understanding of production from fractures. Also, there is uncertainty around the initial mobile water saturations in pay zones. Many wells come online with a high initial water cut, while other wells show low initial water cut before a sharp increase in produced water volume within a few months. Previous studies identified fracture production in at least one well based on core observations and production logging. In this paper, we make an integrated analysis of oil production from apparent tight zones with irregular fractures. A rich suite of borehole measurements was acquired across the field. Mud logs, wireline triple combo logs, formation pressure and fluid samples, core data, image logs, NMR logs, geochemical logs, and production logs (PLT) were collected. Initial production (IP) rates and well tests were utilized to provide the ground truth for our log interpretation. In many wells, oil production does not come from interpreted pay zones (porosity > =15%, water saturation < 50%) but rather from apparently tight rocks. The wireline logs offered limited insight into how fluids entered the wellbores. Conventional cores revealed either vugs or fractures in the rock with an aperture of several centimeters filled with oil. A PLT in one of the wells shows a significant inflow of relatively light oil and no water from an impermeable rock matrix. Wireline logs were interpreted with a multimineral model to compute rock and fluid volumes. The capture spectroscopy data provided clay concentrations, which led to a more robust porosity and matrix permeability interpretation. Image log processing revealed fracture networks with a considerable aperture in zones with higher production. Interpretation results were validated by core, fluid samples, mud logs, and production data (PLT, well tests). Ongoing assessment of fracture-related production and improved understanding of the role vugular zones and matrix permeability contribute to fluid mobility is key to future Maastrichtian development. The significance of this work is that it presents a multidisciplinary approach, combining openhole and casedhole petrophysics, to optimize producibility and completions.