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.

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