Successful Field Application of Organophilic Clay-Free Invert Emulsion Fluid to Protect the Reservoir Core from Drilling Fluid Damage: Case Study from a Kuwait Field

Abstract

To optimize production from a key reservoir, obtaining a core sample with minimum fluid invasion and damage was necessary. In addition, operational nonproductive time (NPT) related to drilling challenges, such as interbedded formations of varying formation pressures, wellbore instability in the reactive, stressed shale sections, and hole cleaning concerns, needed to be mitigated. This paper describes the design of the drilling fluid and its performance in the field. After completion of the first dump flood water injection well drilled using an 80/20 conventional nonaqueous fluid (NAF) weighted with barite, low injectivity was observed, which led to acquiring cores to analyze permeability and porosity along with the change in mineralogy resulting from long exposure of the reservoir in the water zone. A 70/30 organophilic clay-free (OCF) NAF was selected to mitigate equivalent circulating density (ECD) risks and minimize damage. Proprietary software was used to customize the bridging design, which was verified during laboratory testing, and to help ensure adequate hole cleaning with the customized low-ECD fluid. The engineered OCF NAF contained no damaging materials, such as barite, asphaltic material, or organophilic clay. OCF NAFs are well suited to low-ECD drilling operations because they are more resistant to weighting material sag than conventional NAF systems of similar rheology. This is a product of the high gel strengths developed, even in low-rheology (low-ECD) fluids. Downhole pressure fluctuations are low because these gels are fragile and break easily. For the well in which this OCF NAF was used, drilling, coring, and logging operations were successfully completed without incident. Four cores were acquired with minimal damage compared to the previous wells resulting from the engineered design of the bridging material and fluid-loss control polymers. In addition, there was minimal erosion to these four cores, which was a result of the low-ECD fragile gel fluid used. The fluid-loss control properties of the fluid were also effective in strengthening the wellbore and eliminating differential stuck pipe tendencies that had been observed in previous wells. The fluid properties resulted in minimal ECD, and the OCF NAF displayed excellent suspension along with improved pressure management; no pressure spikes occurred while breaking circulation. There was no NPT related to wellbore instability or any of the drilling challenges previously identified. This unique organophilic clay-free and organolignite-free drilling and coring fluid relies on a specialized technology involving an interaction between the emulsifier package and the polymer additives in the fluid. This provides the behaviors needed for reliable weight material suspension and suitable hole cleaning properties in a low-ECD drilling fluid. Together with the appropriately designed bridging package, the OCF NAF provided a better understanding of the reservoir characteristics by delivering the core with minimal damage.