Drilling depleted reservoirs is fraught with a host of technical and economic problems that often make it unprofitable to further develop some mature fields. Most of the problems center around uncontrollable losses in the large fractures that commonly characterize these reservoirs. Frequently, less expensive drilling fluids will be used in a particular interval, even though it may have the propensity to damage the formation. The reasoning holds that such fluids will offset the high costs of losing more expensive muds to the formation.

A specialized invasion control drilling fluid has been developed to drill reservoirs prone to lost circulation. This fluid combines certain surfactants and polymers to create a system of "micro-bubbles" known as aphrons that are encapsulated in a uniquely viscosified system. These aphrons are non-coalescing, thereby creating a micro-bubble network for stopping or slowing the entry of fluids into the formation. The unique viscosity builds to create a resistance to movement into and through the zone, thus generating a true non-invasive and at-balance fluid. Test data confirms its enhanced hole cleaning and suspension properties.

This paper describes the development and application of the specialized "micro-bubbles"-base drilling fluid for controlling downhole mud loss and formation damage. The authors will detail the laboratory methods used to generate appropriate formulations, the operational procedures, and field applications.

As detailed in the paper, this novel drilling fluid relies entirely on "micro-bubbles" network bridging and does not contain any conventional particulate agent for sealing the loss zone. Therefore, the fluid can be pumped through narrow drill pipe, coil tubing and downhole tools. Case histories show that drilling problems are reduced, while drilling fluid losses are prevented or minimized dramatically.


The drilling problems associated with the depleted reservoirs intrinsic to many of the mature fields throughout the world often make further development uneconomical. The water-wet sands that typify many of these zones propagate seepage losses and differential sticking, both of which are extremely expensive to correct. 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.

Recently, aphron (micro-bubble) fluid technology was employed to successfully drill depleted reservoirs in a major drilling campaign in California. Aphron technology entails the creation of a micro-bubble environment to control fluid invasion into the rock. The density of the fluid is not reduced substantially and no special equipment is required. Besides California, the technology has been used extensively in North and South America to drill fractured and depleted zones. The use of aphron-base drilling fluids has proved to be a successful and cost-effective alternative to drilling underbalanced.

Aphron Structure Description

An aphron comprises two fundamental elements:1

  • A core that is commonly, but not always, spherical. Typically, the core is liquid or gaseous.

  • A thin aqueous protective shell

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