Introduction This chapter is organized to help perform acidizing on a well candidate in a logical step-by-step process and then select and execute an appropriate chemical treatment for the oil/gas well. The guidelines are practical in intent and avoid the more complicated acid reaction chemistries, although such investigations and the use of geochemical models are recommended for more complicated formations or reservoir conditions. Effective acidizing is guided by practical limits in volumes and types of acid and procedures so as to achieve an optimum removal of the formation damage around the wellbore. Most of this chapter is an outgrowth of field case studies and of concepts derived from experimental testing and research. Justification for the practices and recommendations proposed herein are contained in the referenced documents. The reader is referred to the author's previous papers on matrix acidizing for references published before 1990. Concepts and techniques presented have ...

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Abstract

Pretreatment analysis, job planning, and well preparation lead to acidizing success in sandstones with permeabilities greater than 50 md. Formation mineral analysis improves success in sandstones with lower permeabilities. Injection pressure responses to acid injection provide data for onsite decisions.

Introduction

Many papers have been written about specific pro ducts for acidizing sandstone formations. Most often these products are designed to correct specific problems and are primarily based on laboratory research in sandstone cores. Very little has been written about evaluating problems that exist in oil and gas wells completed in sandstone formations. Identifying problems in real wells which can be solved with specific products is an important part of the overall acidizing program. This paper will attempt to provide insights into identifying problems and selecting the best product or process to remove the specific damage.

Also, many papers have been written about specific acidizing design models. Most of these papers predict the spending of HF as it penetrates the formation and sometimes the permeability increase. These papers show that regular 3 percent HF only penetrates formations about 6-12 inches before the HF completely spends. More recent retarded HF acids have achieved deeper penetration. It has been the authors experience that the volume of acid predict by these models is not the real key to successful acidizing, but rather it is the control of injection into all the perforations in the formation that determines success. Moreover, some formations respond well to HF acids and others can be damaged by the application of either HF acid or HF acid. This paper will discuss the reasons for this with recommendation on which wells are potential HF acid successes and which wells should be stimulated in another way.

PLANNING THE ACID TREATMENT

The first step in planning an acidizing treatment is to determine whether the well is damaged and how much. One should determine the production potential of the well to see whether removing the damage will provide enough production increase to pay for the acid treatment in a reasonable period of time.

The question to be asked in evaluating well damage are: (1) when was the well damaged, (2) how was it damaged, and (3) what caused the damage. Production history curves often show when a well was damaged unless the damage occurred during drilling and completion. it is important for the engineer to understand all aspects of formation damage in order to interpret the records that exist in well files.

Formation Damage Analysis

Certain types of damage consistently occur in the three major phases of a well's life: (1) drilling and cementing, (2) completion and (3) production. Information exists or can be obtained to show whether damage could have occurred and its mechanism.

Drilling and Cementing Damage

One of the most important sources of damage is that from drilling mud filtrates which usually have a high pH. Several authors, have shown that pH's above 11 are damaging to formations with significant quantities of clay, i.e. 5-20% by weight of clay. A recent paper, showed the variation of permeability with pH. This curve can be used as a first estimate to determine the reduction in permeability by invasion of formations with high pH mud filtrates.

Abstract The Bachaquero field is located on the east side of Lake Maracaibo, where oil exploitation has been occurring for more than 50 years. Primarily composed of sandstone, most of the producing reservoirs in this formation are from the Tertiary period and can be found in the Miocene Epoch. Nonconsolidated and poorly consolidated sands are also common in this field. Complex mineralogy has been identified as the primary cause of production decline for wells in this field, with fines migration being the principal mechanism of formation damage. Other factors also influence the decline in production rate, including high-permeability formations, salinity, pH changes, and drag forces caused by fluid-flow velocity, multiphase flow, turbulence, and fluid viscosity. Chemical stimulation has become a useful technique for enhancing production, and matrix acidizing with hydrofluoric-acid (HF) systems has proven to be very effective in this field. Matrix stimulation is a technique that has been used extensively since the 1930s to improve production from oil and gas wells and to improve injection into injection wells. Matrix stimulation is accomplished by injecting a fluid to dissolve and/or disperse materials that impair well production in sandstones or to create new, unimpaired flow channels between the wellbore and a carbonate formation. In matrix stimulation, fluids are injected below the fracturing pressure of the formation. Substantial production improvements can be achieved with matrix stimulation if treatments are engineered properly. It is well known that HF reacts with clays present in the reservoir to dissolve them and restore original permeability, but some of those reactions are not always desired. Secondary and tertiary reactions of HF with aluminosilicates can promote nonsoluble fluorosilicates precipitation, which requires that fluids be tailored for compatibility with the formation's mineralogy. Variations in mineralogy determine which fluid performs better, and a high presence of feldspars requires more conservative treatments to avoid undesirable precipitations. A stronger retarded HF (RHF) has also been used to treat wells that are deeper in the formation. Other good practices in addition to primary acid selection are also applied to help ensure treatment success. The stimulation treatment design includes pumping formation conditioning fluids before and after the main acid; using different types of organic solvents to dissolve asphaltene deposits in the well; performing NWB, HCl, and HCl/organic acid blend preflushes and post flushes to treat calcium deposits and control pH and iron precipitation in the reservoir; achieving short-term clay inhibition and long-term clay stabilization; and using other fluids, such as relative permeability modifiers (RPMs) for water-control applications and diversion of treatment in laminar reservoirs with petrophysical heterogeneities. Each of these combined practices have resulted in successful stimulation of the field. This paper discusses a comprehensive approach that has been successfully applied in wells located in the Bachaquero field in the Maracaibo basin. The workflow includes a candidate analysis, from the reservoir description, mineralogy, and identification of the formation damage mechanism, to stimulation treatment design, laboratory fluid-systems tailoring, placement and diversion techniques, pretreatment operational task fulfillment, field execution, quality control, and post-job evaluation through analysis of records and statistics.

Abstract Oil exploitation in the Bachaquero field in east Maracaibo Lake has been occurring for more than 50 years. Sandstone is the primary formation type, and nonconsolidated and poorly consolidated sands are common in this field. Complex mineralogy and fines migration have become root causes of production decline and formation damage. This paper describes a comprehensive approach to reservoir characterization that has contributed to the successful stimulation of the sandstone formations in the field. Chemical stimulation, specifically matrix acidizing with hydrofluoric (HF) acid systems that are customized and tailored to reservoir characteristics, has proven to be effective at enhancing production in this field. The types of clays that are present include kaolinite, illite, smectite, chlorite, and mixed-layer clays; feldspars are also present. An adequate analysis of each well helps to ensure that HF acid dissolves the clays to restore permeability without promoting nonsoluble fluorosilicates precipitation through reactions with aluminosilicates. Variations in mineralogy determine fluid performance and make customized fluid selection necessary. The high presence of feldspars requires more conservative treatments to avoid undesirable precipitations. Reservoir characterization and fluid tailoring has helped ensure treatment success, but other good practices also have been applied to help achieve production goals. The stimulation treatment design includes pumping formation-conditioning fluids before and after the main acid; using different types of organic solvents to dissolve asphaltene deposits in the well; performing near-wellbore (NWB), hydrochloric (HCl) acid, and HCl/organic acid blend preflushes and post-flushes to treat calcium carbonate and control the pH and iron precipitation in the reservoir; achieving short-term clay inhibition and long-term clay stabilization; and using other fluids, such as relative permeability modifiers (RPMs) for water-control applications and diversion of treatment in laminar reservoirs with petrophysical heterogeneities. All of these combined practices have resulted in successful stimulation of the field. This paper discusses in detail this comprehensive approach to reservoir characterization applied successfully in wells in the Bachaquero field. The workflow includes candidate analysis, from reservoir description and mineralogy and formation damage mechanism identification to stimulation treatment design, laboratory fluid systems tailoring, placement and diversion techniques, pretreatment operational task fulfillment, field execution, quality control, and post-job evaluation through analysis of records and statistics.

Abstract Berkine basin is one of the main oil producers in Algeria. The upper, middle, and lower TAG-I are the target oil-bearing sands. In this basin, the ROD field is under pressure maintained mainly through water injection together with, to a lesser extent, gas injectors. The southern part of the field, "ROD Tail" has four water injectors targeting the middle TAG-I. In recent evaluation conducted through pressure measurement and an interference test, reservoir pressure was found to have declined by 35 bar within 2 years. This has prompted questions about reservoir management, mainly about the effectiveness of injector well capacity in maintaining reservoir pressure. Extensive data were gathered through well intervention; cleanout, perforation, and a caliper log. Many failed acid jobs were also noted in the history of these wells. An engineered high-pressure jetting operation via coiled tubing was executed, but injectivity could not be restored. A methodology and workflow were adopted to identify the source of formation damage and scale deposition in the near-well area and around perforations. Solid samples were collected from the well and sent to laboratory to characterize formation damage type. The injection water was also analyzed by performing a standard 12-ion concentration analysis. An aqueous model simulator was used to confirm that the water was supersaturated with CaSO4 and CaSO4.2H2O. Finally, clay acid treatment was found to be effective. The treatment fluid was designed to prevent proppant dissolution and to clean fracture matrix interface. This was the first time this type of operation was executed after many unsuccessful conventional acidizing operations. Excellent results were obtained after the acid stimulation treatment. The injection rate was found to increase significantly from 120 m/d to 360 m/d. Water injection pressure was also found to decrease from 243 bar to 220 bar, and the injectivity index increased by three times. Near-wellbore formation damage was removed, and formation permeability recovered. The clay acid treatment was applied to other wells in the field and similar results were obtained.