Abstract

A number of maximum reservoir contact (MRC) wells have been drilled and completed in various fields (sandstone and carbonate) in Saudi Arabia. The drilling fluid, water-based mud, consists of xanthan, starch, polyanionic cellulose polymers and bridging agents like sized calcium carbonate or salt particles. Cleanup of drilling fluid filter cake in long horizontal wells completed as an open or cased hole is a challenging task. The obstacles encountered are the difficulty to ensure contact of the treating fluids with the filter cake throughout the whole interval, as well as controlling the reaction rate between the breaker and the filter cake to ensure uniform removal of the filter cake.

Today, several chemical and mechanical systems are available to remove the polymeric and bridging agents in oil and gas producers and water injectors. Chemical means (oxidizers and acids) have limitations, which can adversely affect well performance. Oxidizers and acids are highly reactive and non-specific species. Reservoir heterogeneity complicates the cleaning process. There is a need for cleanup fluids that have a delayed effect on filter cake-integrity to obtain a uniform distribution over the horizontal interval.

Specific enzymes can degrade xanthan and starch polymers in the filter cake with lower removal rates. A low reaction rate promotes a more uniform cake cleanup and should result in higher production rates. Field results indicate that utilization of enzyme treatments is very effective technique in removing drilling mud filter cake. The production rate for treated wells reached the target value without the need to acidize the well.

High pressure water jetting technique has been utilized to mechanically remove filter cake. Field results obtained from many injectors indicate that this technology has several advantages such as simplicity of use, cost effectiveness, and high success rate. This paper will discuss field application of specific enzymes and water jetting in gas, oil, and water wells. It will also highlight the advantages and disadvantages of each method based on field data.

Introduction

Drilling of maximum reservoir contact (MRC) wells is on the rise, primarily because of the enhanced productivity they provide from the reservoir.MRC wells have proved their success and economical option for field development in oil and gas fields.[1] Saudi Aramco's definition of MRC wells requires that the horizontal interval must contact at least 4,000 ft of the targeted reservoir(s).[2,3]

Water-based fluids are used in oil and gas fields. However, they create mud cake, which can cause formation damage. The filter cake comprises solids, such as calcium carbonate particles, formation fines, and polymers such as xanthan and starch.Proper removal of filter cake is required to ensure good well performance.

Xanthan gum and starch polymers are commonly used in drilling fluids. They work in a synergistics manner to provide enhanced static suspension and dynamic transport of suspended solids. Combining these two polymers improves the suspension properties of the drilling fluid. These polymers, however can cause formation damage through several mechanisms. First, they constitute a portion of the drilling mud filter cake.Second, they can leakoff into the formation and cause damage.[4] Third, XC-polymer and starch are biopolymers, which are very good nutrients to sulfate reducing bacteria (SRB). These bacteria produce biomass and iron sulfide, which can cause formation damage. More details on methods used to remove biomass and iron sulfide are given by Nasr-El-Din et al.[5–7]

Fluid invasion by polymer-based drilling fluids can also cause operational problems and reduce wellbore stability. A high fluid loss results in a thicker filter cake, which can cause problems with drill-pipe sticking.[8,9]Fluid loss reducers include starch and polyanioinic cellulose (PAC). These polymers can limit the loss of drilling fluid to under-pressurized or high-permeability formations. Starches have thermal stability up to nearly 220°F.Polyanionic cellulose is used at temperatures up to 280°F.In many of the drilling fluid formulations, the weight ratio of starch to XC-polymer is greater than unity, and thus removal of starch alone from the filter cake can significantly reduce the flow initiation pressure and permeability impairment.[10]

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