Abstract

Formation damage occurs during the lifetime of many wells. Loss of well performance due to formation damage has been the subject of several review articles. Fines migration, water, emulsion blockage, inorganic scale, asphaltene, and other organic deposition are a few mechanisms that can cause formation damage.

The present paper discusses new formation damage mechanis ms that are caused by various chemical treatments. These include: adsorption-type scale squeeze treatments (phosphonate-based inhibitor), solvent treatments (a neat mutual solvent) to remove water blockage in a tight carbonate reservoir, and regular mud acid (HCl:HF at 12:3 weight ratio) to remove drilling mud filter cake in sandstone reservoirs. These treatments were designed to remove a known form of formation damage. However, they created new forms of formation damage, which resulted in a significant decline in the performances of several wells.

Case studies of new damaging mechanisms that resulted from various chemical treatments are discussed in this paper. Details of lab and fieldwork that were performed to identify the damaging mechanisms and determine its impact on well performance were addressed. Finally, the paper highlights the remedial actions and field application that resulted in restoring the performance of various wells without affecting the integrity of the formation.

Introduction

Formation damage can occur during the lifetime of all wells starting from drilling, completion, production, and stimulation. Basically, it causes loss of well performance, and usually requires an expensive treatment to remove such damage.

Formation damage can be divided into two main categories: mechanical and chemical damage. 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.1–3 Oil droplets can also cause damage to disposal wells, especially in the presence of suspended solids.4,5 In both types of wells, suspended solids and oil can plug the formation and cause loss of well injectivity.

Another important example of mechanical formation damage is injecting or producing oil or gas wells at high rates. In this case, the fluids will exert high drag forces on clays and feldspars, which can dislodge fine particles.3 The mobilized fine particles accumulate at the pore throats, and cause formation damage. This type of damage occurs in sandstone reservoirs, which contain kaolinite and other migratory clay particles.

Formation damage due to chemical means occurs in many reservoirs. Injection of low salinity water into a sandstone reservoir with high salinity water is a typical example. In this case, the lower salinity water will trigger clay swelling and fines migration. Both can cause severe formation damage, especially in tight formations.6 Another example is the injection of incompatible water. If the injection water contains high sulfate content (seawater is a typical example) and the formation water contains high concentrations of calcium, strontium, or barium ions then the sulfate salts of these cations will precipitate in the formation and may cause severe formation damage.

An example of mixing incompatible waters occurs when disposal waters which typically contain high hydrogen sulfide content are mixed with injection water, which contains dissolved iron. Iron sulfide species will precipitate upon mixing of these two waters,7 and can cause severe loss of injectivity.5

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