Nano-Technology Based Bridging System Helps Drilling Success in Highly Depleted Mature Fields
Authors: Addagalla Ajay , Maley Iain , Moroni Luigi , Khafagy Mohamed 
Published: November 2018

As operators reduce spending on exploration, more wells with increasing complexity are being drilled in mature fields. As the well complexity increases, a dilemma often occurs over how to address borehole stability requirements while mitigating the risks of shale instability, differential sticking or mud losses due to high differential pressures. These wells may have little or no operating margin between the pore pressure and formation breakdown. High mud weight may be required to prevent shale collapse. Options tend to be limited to accepting the drilling risks or setting a contingency casing string—both very expensive choices in terms of equipment and lost time. Downhole losses and differential sticking occur because the drilling fluid hydrostatic pressure is higher than the formation pressure, a situation referred to as ‘overbalanced pressure’ while drilling through formations that are fractured, under pressured, cavernous or highly permeable. Traditionally, the wells in these types of fields experience some or all of these drilling problems.

Wells drilled through the interbedded shale and sandstone Barik formation usually encounter severe wellbore stability issues that result in increased drilling time and the possibility of side tracks. Wellbore instability occurs because the Barik formation requires a high fluid density to stabilize the shale sections due to the depleted, low-pressure sandstone sections. Wells drilled in the Barik formation have been drilled with an advanced invert emulsion fluid. These wells experienced wellbore instability and differential sticking, resulting in significant non-productive time (NPT). One of the limitations of bridging wells with traditional materials is the impact these materials may have on rheological properties and the generally poor performance they demonstrate with respect to wellbore stability at overbalance pressures greater than 2,000 psi.

A water-based fluid with an optimized bridging and sealing system was custom-designed using proprietary software and based on deformable nanotechnology material with synthetic carbon-based additives. This combination of additives provides a wide particle size distribution range to cover the micro-fractures (&lt; 200 nanometers) and the macro-fractures (&gt;10 microns). This bridging system comes in a single sack, which helps reduce transportation costs, logistics, rigsite footprint and minimises manual handling.

This was the first application of water-based mud to drill in the field, and the customized bridging and sealing package was successful in sealing the Barik sandstone, allowing the horizontal well to be drilled to planned depth without NPT, despite the formation being subjected to high overbalance pressures greater than 5,600psi. The bridging and wellbore strengthening properties of the custom-designed water-based fluid prevented instability of the shale by sealing the formation, delivering excellent wellbore stability while simultaneously sealing the sandstone and preventing differential sticking. This approach resulted in a significant reduction in NPT compared to the previous attempts.