Aphron Drilling Fluid: Field Lessons From Successful Application in Drilling Depleted Reservoirs in Lake Maracaibo

Abstract

Depleted reservoirs pose numerous technical challenges in both the construction and completion phases for wells in dozens of producing fields, often putting into question the economical viability of these fields. Wellbore instability, severe lost circulation, and stuck pipe are just a few of the problems encountered when drilling into these low-pressured reservoir formations.No area better illustrates the problems with depleted reservoirs than the Lake Maracaibo region. Water-wet sands that frequently triggered costly seepage losses and differential sticking typify many of these zones. Some contain microfractured sandstone formations where uncontrollable losses of whole drilling fluid previously were the norm rather than the exception. Others are characterized by laminated sand and shale sequences, which create the conditions for slow, dangerous, and unduly expensive drilling. Attempts were made with underbalanced drilling, but in addition to the extra time and equipment required, wellbore instability lead to failed well construction and thus seriously degrading project economics.Over the past two years, a specialized drilling fluid has being utilized to drill these depleted reservoirs in Lake Maracaibo. This fluid combines certain surfactants and polymers to create a system of "micro-bubbles" known as aphrons encapsulated in a uniquely viscosified system. These aphrons are non-coalescing, therefore creating a micro-bubble network for stopping or slowing the entry of fluids into the formation.The aphrons allow conventional drilling equipment to be used to successfully complete many reservoirs that previously would have been candidates for underbalanced drilling only. This paper describes the development and application of the specialized "micro-bubbles" or aphron-based drilling fluid for drilling depleted reservoirs by controlling downhole mud loss and formation damage. The authors will detail the operational procedures and the field applications of this drilling fluid, with particular emphasis on the lessons learned in the Lake Maracaibo implementation of the system.

Modeling of micro-bubble surfactant multi-layer drilling fluid stability based on single bubble behavior under pressure and temperature in a deviated gas well

“…It is not necessary simultaneous gas injection during this operation (Ramalho and Davidson, 2006). MBSML fluid (or Aphron fluids) has been recommended as a drilling fluid for low pressure reservoir drilling instead of aerated fluids, which simultaneously maintains well-bore stability and controls mud losses (Montilva et al, 2002). Micro-bubble drilling fluids are different from foam and aerated drilling fluids in structural, rheological and stability properties (Spinelli et al, 2010).…”

How Aphron Drilling Fluids Work

“…While the formations above and below these producing zones typically have much higher pore pressures and require high fluid density to stabilize them, exposure of a depleted zone to this high-density fluid can result in significant loss of whole drilling fluid and differential sticking. [1][2][3][4] Furthermore, pressured shales are often found interbedded with depleted sands, thus requiring simultaneous stabilization of multiple pressure sequences. Drilling such zones safely and economically is very difficult with conventional rig equipment.…”

Enhanced Wellbore Stabilization and Reservoir Productivity with Aphron Drilling Fluid Technology

“…1 Aphron drilling fluids are thought to seal permeable zones by virtue of their very high low-shear rheology and the unique bridging capability of specially designed microbubbles dubbed "aphrons". 2 These aphrons are composed of two fundamental elements: 3 a core of air and a protective shell composed of multiple layers of surfactants and polymers.…”