Abstract

Significant improvements in well performance can be realized by using Polymer Linkage Specific Enzyme (PLSE) technology in drilling and gravel packing fluids during completion operations. This new technique allows adequate time to complete operations while still incorporating a highly effective breaker mechanism within the drilling fluid. Regardless of where fluid leakoff occurs within the wellbore, breaker is present. Subsequently eliminating pockets of polymer filter cake or whole polymer fluid damage often left behind with other breaker mechanisms.

Introduction

Completion practices for many operators in high-permeability heavy oil reservoirs follow similar procedures. In most cases, the well is drilled with a large bit to the predicted top of the pay zone. At this point, the drill bit is changed over to a smaller one (typically 6%") and the drilling fluid is changed to a 'clean' drilling fluid (usually KCl polymer). Next, the pay zone is drilled to total depth and prepared for any openhole logs. Upon analysis of the logs, the exact top of the producing formation is determined and the larger diameter hole is drilled to that depth (this is called "lowering the shoulder"). Once finished lowering the shoulder, the producing zone is sanded back or cement baskets are used to cement the casing from the top of the pay zone back to the surface. The well may be completed in a couple of different ways at this point.

In one scenario, the liner is landed with a drill-in assembly on the bottom which includes a drill bit at the end. This "tight" liner is actually drilled into place and is properly sized and slotted for the formation fines expected ahead of time. The second scenario requires that the hole be underreamed throughout the pay zone with a clean fluid. The clean fluid is normally KCl brine viscosified by hydroxyethylcellulose (HEC) polymer which may also be foamed in some cases. The slotted liner is typically run next and hung adjacent to the pay zone. In some cases, the well may be gravel packed with an HEC fluid as well. Finally, an oxidative breaker is introduced to the wellbore during the gravel packing operation to degrade the polymer viscosity and make the polymer easier to remove from the formation. A new method has been developed to enhance the break quality and minimize the residue left behind by conventional HEC polymers. Introduction of PLS Enzymes into HEC completion gels allows complete degradation of these polymers regardless of their relative position in the reservoir.

Background

The Midway Sunset Field is located in the southwestern San Joaquin Valley, about 30 miles northwest of Bakersfield, CA. Production occurs from several different horizons which vary in continuity across the field. The reservoirs range from the Pleistocene series through the Pliocene to the upper Miocene series. Examples include the Potter, Monarch, Metson, Exeter, Marvic and Moco sands. The wells detailed in this study, are located in the southern half of the northeast to southwest trending structure, and are completed in the upper Miocene Monarch sand. P. 95

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