Abstract

Early return on investment, high productivity indexes, late water/gas breakthrough and tight reservoir oil recovery are among the benefits of horizontal well over vertical well. Many operators have perfected and standardized the drilling and completion procedures of horizontal wells, however, the results of the different drain hole cleanup approaches have been inconsistent (especially where oil or pseudo-oil base mud were used during drain hole drilling).

Conventionally, the removal of the damage caused by drilling mud filter cakes and filtrate is achieved with the aid of acid cleanup, Nitrogen lifting or brine wash. While the results of enzymes or acid cleanup in water based mud filter cakes removals are encouraging, the results in oil or pseudo-oil base filter cakes have been considerably inconsistent.

This paper details the Pop-Off approach to the removal of drilling fluid damage and filter cake in horizontal wells with high heterogeneous permeability profiles. The paper described how the application of filter cake flow initiation pressure combined with treated drill-in fluid base fluid and reservoir's fluid solvent and dispersion power have delivered superior and consistent value on investment. The results from the application of this method showed remarkably low drawdowns and high productivity indexes. Field data resulting from this method demonstrates a history of 100% technical and economical success with more than 150% improvement in recovery and projected productivity index. In addition, there has been consistent improvement in sustenance of well productivity with wells treated with Pop-Off method over wells treated with previously applied conventional methods of filter cake removal.

Introduction

About twelve years ago, drilling horizontal wells was a new technology. Nowadays, horizontally placed wells are as popular as vertically placed wells. However, each well placement style has its benefits and areas of application. Horizontal well drilling and completion are continuously increasing because of its high flow area, high productivity index, longer water/gas breakthrough time, and suitability for tight reservoirs oil recovery and early return on investment.

In the Niger-Delta most of the reservoirs are highly heterogeneous with high permeability. Consequently, the pay zone drilling mud is designed with wide range of particle sizes that filter and bridge rapidly with minimum filtrate penetration. The type of drilling fluids used in Niger-Delta has continuously been changing from Water base or polymer base to Oil and Pseudo-Oil base mud. Whilst water based mud is cheap, economical and has good rheological properties, its inability to effectively control highly reactive shale in Niger-Delta reservoirs has lead to increasing usage of Pseudo-Oil based mud. Despite the high cost of sales, high environmental impact, non compatibility with cements and low to medium impairment disadvantages of oil or pseudo oil based drill mud, it is preferred to water based drilling mud due to its high shale inhibition, high lubricity, controllable rheology and most of all their low fluid loss, thin filter cake and generally better return permeability. Calcium Carbonates and Barite are commonly used as weighting agent or as particulate for filter cake formation.

Conventionally, the removal of the damage caused by drilling mud filter cakes and filtrate are achieved with the aid of acid cleanup, Nitrogen lifting or brine wash. While the results of enzymes or acid cleanup in water based mud filter cakes removals are encouraging, the results in oil or pseudo-oil base filter cakes have been considerably inconsistent.

The poor performance of these various clean-up approaches in Oil phase is mainly attributed to downhole emulsion formation between oil and water phases, incompatibility between treatment fluids, drilling mud and in some cases formation of damaging byproducts. Wettability change and pronounced water block problems may also arise in low permeability zones.

In order to eliminate the above listed problems, Pop-Off, an approach to clean horizontal hole was introduced and successfully applied. The approach has given consistent and rewarding results.

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