Abstract

A vertical well has a history of problems which has contributed to its disappointing performance. A 15 wt% HCl acid treatment was performed in an attempt to remove drilling mud filter cake and improve wellbore to formation connectivity, however the solubility of the formation's cementation material (calcite) was overlooked and sand production resulted after the treatment. A screen liner running operation was then performed, with a need for sand control, and massive mud losses and polymer pills were lost to the formation during the process.

The workover fluids were displaced deep into the formation while loading the hole to maintain well control. The presence of biopolymers in the completion fluids enhanced the activities of sulfate reducing bacteria (SRB). These bacteria further compounded the problem, and the damaged environment was ideal to initiate the production of plugging FeS and biomass in the near wellbore region. Ultimately, the formation was not able to deliver the necessary flow rate to supply the ESP pump when the well was returned to production.

The damage mechanism is a combination of polymer residue, biomass and FeS plugging the screen liner. A three-stage treatment was designed to remove the damaging material. The first stage was based on tetrakishydroxy methyl phosphonium sulfate (THPS) and was designed to remove iron sulfide and hydrogen sulfide, and control the growth of bacteria.The second stage was based on formic acid and was intended to remove FeS and some of the polymer residue in the near wellbore area. The third stage was based on a two-part oxidative treatment and was designed to remove polymer damage deep in the formation.Analysis of well flowback samples before and after the treatments was used to confirm the damaging mechanisms and to evaluate the treatment.

This paper reviews the procedure used to identify the problem, and the unique treatment which successfully restored well productivity.

Introduction

Formation damage can occur during drilling, completion, or workover operations.[1–3]It can also occur during normal daily injection or production. Finally, it can occur during well stimulation treatments, e.g., matrix acidizing of sandstone and carbonate reservoirs.[4] Basically, formation damage causes loss of well performance, and usually requires an expensive chemical treatment to restore well performance.

Formation damage can be divided into two main categories: mechanical and chemical.[5] Mechanical damage occurs when particulate solids, emulsion, asphaltene, or inorganic scales physically plug the pore spaces. A typical example of formation damage due to suspended solids occurs in water injectors.[6,7] Suspended oil droplets can also cause damage to water disposal wells, especially in the presence of suspended solids.[8,9] In both types of wells, suspended solids and/or oil can plug the formation in the critical near wellbore area, and cause loss of well injectivity.Details of various formation damage mechanisms were discussed by Civan3 and Nasr-El-Din.[10]

Polymers are frequently employed in drilling, completion and stimulation operations.The polymers used are selected based upon their ability to provide viscosification, proppant transport and/or suspension, fluid loss control and zonal isolation.Yet the very properties for which they are chosen also make them difficult to break down following their application. Unbroken filter cake and insoluble high molecular weight polymer fragments are just two forms of damage produced by polymers.It is these residual effects of polymers that are responsible for reducing well productivity through damage to the formation.

Some treatments require gel systems utilizing high polymer loadings in order to solve a present problem such as sand production, but the polymer becomes concentrated on the formation faces and causes some other productivity problems. At times the concentration of this polymer becomes so high that breaker additives are no longer able to thoroughly degrade it.The goal then becomes the reduction or removal of the polymer damage in order to obtain optimum productivity in a cost-effective manner.

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