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.
The drilling problems associated with the depleted reservoirs intrinsic to many of the mature fields throughout the world often make further development uneconomical. Uncontrollable drilling fluid losses frequently are unavoidable in the often large fractures characteristic of these formations. Furthermore, the typical laminated sand and shale sequences create conditions that can make drilling unduly expensive and dangerous when using conventional rig equipment. Consequently, these and a host of associated problems have led some operators to forgo continued development of these promising, yet problematic, reservoirs.
The overbalance pressure generated when using conventional drilling fluids is to blame for the majority of the loss circulation and differential sticking problems encountered when drilling these wells. The equipment required when using aerated muds or drilling underbalanced is often prohibitively expensive and meeting safety requirements can be an exhaustive effort. Furthermore, these techniques may fail to provide the hydrostatic pressure necessary to safely stabilize normally pressured formations above the reservoir.
The early wells in the Lake Maracaibo area were drilled using underbalanced drilling techniques combined with special casing designs to isolate the Miocene and Eocene formations.