Abstract

Removal of skin damage resulting from internal and external filter cake deposition during oil well reservoir drilling with invert emulsion drill-in fluid is desirable in order to maximize hydrocarbon recovery.Efficient filter cake cleanup is required for a number of open-hole completion operations, including the use of stand-alone and expandable sand screens, and conventional gravel pack applications for both production and water-injection wells.

Most of the currently used chemical methods to remove invert emulsion filter cake and internal formation damage are based on the use of acids, solvents and mutual solvents mixed in clear brine. Completing a well after drill-in with emulsion-based fluids is time-consuming and costly, and usually involves large volume multi-stage soak treatments.

Surfactant technology, when appropriately combined with conventional acid, allows a single-stage invert emulsion cleanup process. In this "one-step" cleanup method, the invert emulsion filter cake is incorporated into the organized surfactant in solution and the acid-soluble particles are then decomposed.

A number of injection permeability tests were performed on Berea sandstone cores and ceramic discs using various invert emulsion drilling fluids for filter cake deposition and organized surfactant in solution/acid blends for filter cake removal. This investigation demonstrated that, when using this technology to destroy the filter cake, (1) the invert emulsion is incorporated into the surfactant solution, (2) the solids become water-wet, (3) sludge formation between the acid and invert emulsion cake is prevented, and (4); and the majority of acid-soluble particles are removed.

This approach appears to be a credible method for removing formation skin damage and increasing hydrocarbon recovery and/or water injection rates.

Introduction

Many operators are interested in improving filter cake cleanup after drilling into reservoirs with invert emulsion drilling fluids.More efficient filter cake cleanup is desired for a number of open hole completions, including stand-alone and expandable sand screens, as well as for gravel pack applications for both production and water injection wells.

One-step invert emulsion filter cake cleanup technology uses a single-phase microemulsion (SPME)[1–3] and conventional acid packages, in a single blend, to solubilize the oil into the SPME, reverse the wettability of the filter cake solids, and simultaneously decompose its acid-soluble components.Reversing the wettability of the filter cake, using surface active chemistry, facilitates acidizing by preventing a sludge that could form between the acid and the emulsified cake and by making acid-soluble particles unavailable to unspent acid.

Besides the advantage of reduced skin damage, increased hydrocarbon recovery and/or increased water injection rates, a "one-step" near wellbore cleanup method will save an operator valuable rig time.

Theory of Microemulsion

In the 1950s, Schulman and co-workers added alcohol to surfactant-stabilized oil-in-water (o/w) emulsions to obtain very stable homogeneous fluids that he called microemulsions.[4] These microemulsions had an average droplet size of 10–100 nanometers (nm), much smaller than conventional emulsions.

The first studies of microemulsions in the oil industry were in the 1970's for enhanced oil recovery (EOR) applications.[5–11] Many oil operators and universities invested considerable time to research this topic. The research group led by Schechter and Wade at The University of Texas at Austin made important contributions to the understanding of the mechanism of increased production by microemulsions. However, the interest dropped due to the crude oil price decrease and because the technology was expensive, due mainly to the high concentration of surfactant required. Since then, the oil industry has conducted only a small amount of research on microemulsions.

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