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Abstract Formation damage caused by kill fluids invasion and filter cake depositionin the perforation tunnel has rarely been addressed in the scientificliterature. In this paper, a systematic experimental program using CaC03-HEC slurry to kill perforated wells is presented. The perforation target is simulated using a cylindrical core with a drilled perforation, and borescope has been used to observe the distribution of filter cake in the perforation tunnel. Important parameters affecting perforation plugging and cleanup have been identified. The effectiveness of perforation cleanup methods, such as backflush and backs urge have been studied and the mechanism of filter cake removal investigated. Experimental results show that among the various damage parameters investigated, only core permeability and fluid viscosity have big effects on Perforation Flow Efficiency (PFE), which is the comparative flow capacity of a damaged perforation to that of a clean perforation. Perforation Flow Efficiency decreases with increasing permeability. The mechanism of filter cake removal strongly depends on the perforation cleanup techniques. When backflush is used to clean the plugged perforations, a threshold differential pressure has to be reached before flow can be initiated and a minimum volume of fluid must be flowed to achieve optimum PFE. Depending on the damage and cleanup conditions, plugged perforations may be partially or completely cleaned by backflush, Sequential backflush is advantageous over single constant rate backflush for high permeability core, where matrix damage is severe. Increasing backflush rate increases PFE. When backs urge is used, all the filter cake deposited can be removed instantaneously, and the PFE obtain is very high. Backsurge pressure is more important than backs urge volume. The results obtained in this study will be very helpful in the design of kill pills and the design of perforation cleanup methods in the field. Introduction Whenever a workover operation is needed, a kill fluid is pumped into the well to obtain well control. Because the density of the kill fluid is high, thehydrostatic head in the well bore is greater than the pressure in the formation when the fluid reaches the formation face. Under this overbalance condition, the kill fluid in the well bore invades into the formation and this may cause formation damage. In a perforation completed well, fluid can only get contact with the formation through the perforations. Formation damage caused by kill fluid invasion results from the following processes: Invasion of filtrate into the formation Invasion of solids into the formation Deposition of filter cake in the perforation tunnel The invasion of filtrate and solids into the perforation matrix results in a zone of reduced permeability around the perforation tunnel and the deposition of filter cake in the perforation tunnel results in partially or completely plugged perforations. There have been many publications regarding the effects of filtrate and solids invasion on formation damage and the means to prevent or reduce them. However, there have been few publications in the area of filtercake deposition and removal from perforation tunnels. This paper will concentrate on the removal of the filter cake deposited in the perforation tunnel and the effects of various factors on perforation flow efficiency. Perforation damage can be minimised using solids-laden fluid systems which will quickly deposit a filter cake on the surface of the perforation tunnel and allow maximum cleanup once production is initiated. Although this idea is not new, the knowledge about the designing of this temporary system is very limited. Early works on mud design to minimise rock impairnent due to particle invasion can provide guidelines but their validity for perforated completions has not been established. Backflushing has been used in laboratory return permeability tests on linear cores, but its effectiveness in cleaning upplugged perforations is doubtful because the flow configurations are different.
Abstract Filtration control has a considerable impact on drilling fluid properties & performance, on drilling costs and on well productivity. The requirements for filtration control, with regard to optimisation of the various stages of drilling a well, are ambiguous. This is largely due to the lack of understanding of the functionality of the mud components and on imprecise definition of demands. Multi-Core Dynamic Filtration equipment has been designed to study static- and dynamic filtration (including spurt loss conditions) and assess their effect on return permeability for up to 4 core samples at a time. Several core samples in one experiment will enable us to evaluate the effect of permeability variation on clean-up efficiency. In another development, equipment has been designed for continuous measurement of cake thickness during filtration. Polymer systems have been identified that effect acceptable fluid loss without solid particle additions. Apparently, the micelles formed have the dual functionality of solids to plug the pores and polymers to reduce cake permeability. This line of investigation may have potential in the development of truly solids free drilling fluid systems based on high density brines (e.g. Formates). Fluid loss, a key parameter in drilling fluid design Fluid loss (control) has an effect on a number of drilling and completion parameters. It is known to have a major effect on cake properties, penetration rate and costs to name a few. It can also have a major impact on impairment of the formation. The need to evaluate the inter-dependency of the fluid loss and these parameters has long been expressed, but so far little fundamental work has been published in this field. Research into the effects of fluid loss from drilling fluids (DF) was initiated in 1994. Initially the investigation was aimed at studying the relationship between fluid loss and impairment. It was soon realised that there were good arguments to extend the investigation to the wider field of the effect(s) of fluid loss on a number of drilling- and completion parameters. The decision was taken to design and build multifunctional experimental equipment that could be used to study a number of fluid loss related aspects of DF design. A description of the experimental equipment with some practical considerations is included. In this paper the rationale for the fluid loss related work is given based on a review of the suggested / assumed effects of fluid loss on various drilling- and completion parameters. Further, the results of the experimental effort and the resulting conclusions are presented. This paper discusses the "mechanical" DF properties; physico-chemical properties (chemical composition), very relevant for e.g. shale stability and clay "swelling" effects, are not discussed.
- Geology > Mineral (0.69)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock (0.49)
ABSTRACT Particle invasion and fines migration are among the major factors causing formation damage. Extensive studies of formation damage in laboratory and several modeling efforts for prediction of formation damage have been reported in the literature. However, a satisfactory model to simulate the near-wellbore formation damage in field conditions is still not available. In this paper, the linear flow core-scale model presented previously by Liu and Civan is converted into a radial flow field-scale model to simulate the formation damage near wellbore regions in actual field conditions. The radial flow field-scale model utilizes the values of model parameters obtained by a model assisted analysis of the laboratory core tests to determine the temporal and spatial variation of formation damage and the associated skin factor. Simulation results indicate that the overbalance pressure of drilling fluids is an important factor for formation damage control and that the formation damage due to constant-pressure mud filtration is less severe in two-phase flow of oil and water than single-phase flow of water in the formation. INTRODUCTION Formation damage occurs in almost every field operation. It is an adverse and complicated phenomenon caused by particle invasion, formation fines migration, chemical precipitation, organic deposition, and pore deformation or collapse. The production performance of a well is strongly affected by the magnitude of damage in the near-wellbore formations. Searching for methods to reduce the cost of formation damage is of continuing interest to the petroleum industry. Formation damage near wellbore can be determined by well testing techniques. However, these techniques can only provide the skin factor as an overall measure of formation damage, but they do not reveal any insight into the temporal and spatial development and causes of the damage for the assessment and control of formation damage.
- North America > United States > Louisiana (0.46)
- North America > United States > Oklahoma (0.46)
- North America > United States > California > Sacramento Basin > 4 Formation (0.99)
- North America > United States > California > Sacramento Basin > 2 Formation (0.99)
- Well Drilling > Formation Damage > Fines invasion (1.00)
- Well Drilling > Drilling Fluids and Materials > Drilling fluid selection and formulation (chemistry, properties) (1.00)
- Reservoir Description and Dynamics > Reservoir Fluid Dynamics > Flow in porous media (1.00)
- Reservoir Description and Dynamics > Formation Evaluation & Management > Drillstem/well testing (1.00)
Abstract Most gravel packed completions are considered successful if the well produces sand free. However, many successfully gravel packed wells suffer reduced productivity as a result of formation damage induced by current gravel pack completion practices. Gravel packing even a slightly damaged formation can result in long term detrimental effects on production. The most predominate problem is that dehydration of the gravel carrier fluid will be restricted and perforation tunnels will not tightly pack. The areas of interest in which formation damage can be limited during a gravel pack completion include perforating, filtration, gravel carrier fluid selection, lost circulation control, and workstring hygiene. In addition, fluid leak-off during gravel packing may be optimized by proceeding the gravel pack slurry with low strength acid. Introduction Optimization of gravel placement during completion operations has been a widely discussed subject in recent years. The predominate objective has been to perform a gravel pack which results in little of no draw-down across the perforations. Numerous operators choose the position of attempting as clean a completion as possible; then, evaluate the well's performance following completion to determine weather or not a stimulation treatment required. An important observation of this practice is discussed by McLeod. If a well experiences excessive pressure draw-down following completion, the perforation tunnels may not adequately pack-off during gravel packing. Injecting acid into such a completion may result in creating or expanding existing tunnel voids and pack voids in the screen-casing annulus. The production decline following such a completion is often attributed to fines migration when in fact we have at least a partially failed gravel pack. The objective of gravel packing should be to eliminate all of the possible mechanisms which may lead to pressure draw-down prior to achieving a sand out. As an industry, we have made great strides in recent years to improve our performance in gravel packed completions. However, there exist areas which require clarification in order to further reduce formation damage during gravel packing. FILTRATION The benefits derived from optimized filtration practices for completion, workover and stimulation fluids has been extensively documented. In most cases, the formation damage caused by dirty fluids can never be completely removed. Therefore, damage prevention should be the paramount consideration. Achieving optimum damage prevention while maintaining cost control objectives requires a broad knowledge of filtration theory as well as the best demonstrated field practice. Filtration theory states that fluid clarity guidelines can be achieved by adjusting either flow rate or surface area. In the case of high fluid densities and/or viscosities, as are encountered during a workover, filtration may be controlled by:reducing flow rate, increasing the filter surface area, and a combination of flow rate and surface area. The level of filtration depends on reservoir plugging potential, practical limitations of filtration devices, physical properties of the influent, and cost. P. 197^
- North America > United States > California > Sacramento Basin > 4 Formation (0.99)
- North America > United States > California > Sacramento Basin > 2 Formation (0.99)
Abstract This paper presents an overview of the work carried out to date on the dynamic and static filtration characteristics of drilling muds. Factors controlling fluid loss to the formation that have been investigated include annular velocity, fluid temperature, hole angle, shear rate and the effect of the filter media. The effect of pressure on static fluid loss and filter cake permeability is also reported. Fluid loss during sequential filtration has also been evaluated. The fluids used comprised a calcium chloride brine with fluid loss additive and two water based muds. The filter media consisted of synthetic core material and a natural sandstone. Conventional photographic techniques have been used to identify filter cake response to backflushing. Introduction The occurrence of a change in permeability of the formation near the wellbore is most frequently related to the effects of the invasion associated with borehole filtration. As filtration proceeds the filtrate, with its accompanying fine particles, creates an invaded zone. Within that zone the productivity may be reduced by physical and/or chemical change to the formation rock or fluids and this impairment is commonly referred to as formation damage. Some appreciation of the complexity of this process can be obtained by considering the range of variables which could affect fluid loss in a borehole. Table 1 attempts to classify the more significant parameters into the following categories: parameters into the following categories:rock properties properties of the wellbore fluid physical conditions in the wellbore From the number of mechanisms shown it is clear that the control of formation damage is very difficult. It is likely that more that one mechanism may be operating at any one time and, since formations are not homogeneous, there will be variations in the extent to which these individual mechanisms occur. Limiting the extent of filtrate invasion is important and over the past 3 decades or so a great deal of time and effort has been spent on this past 3 decades or so a great deal of time and effort has been spent on this problem. Among the reasons for attempting to quantify and reduce the problem. Among the reasons for attempting to quantify and reduce the volume of mud filtrate are the following:invasion of filtrate with associated fine particles may create a zone of reduced permeability around the wellbore which may not respond to backflushing and results in lower production rates than anticipated. filtrate that penetrated shale sections may cause swelling and subsequent sloughing into the wellbore. Stuck pipe may then be a problem. the location of a zone of mud filtrate around the wellbore will affect the response of electrical logging tools. Correct interpretation of the logs requires accurate knowledge as to the extent of this zone. knowledge of the depth of invasion is required for the determination of perforation depths and possibly for the selection of stimulation treatment. Utilising muds with improved filtration control characteristics provides an efficient method of limiting the extent of the invaded zone. provides an efficient method of limiting the extent of the invaded zone. However it is impossible to eliminate fluid loss entirely since the formation of a low permeability filter cake necessarily involves invasion. Therefore attempts to minimise fluid loss must be accompanied by obtaining information as to the permanency of damage and possible techniques for its removal. p. 283
- North America > United States (0.68)
- Europe > Norway > Norwegian Sea (0.24)
- Geology > Mineral (0.71)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock (0.54)
- Geology > Geological Subdiscipline > Geomechanics (0.48)
- Well Drilling > Drilling Fluids and Materials > Drilling fluid selection and formulation (chemistry, properties) (1.00)
- Well Drilling > Drilling Fluids and Materials > Drilling fluid management & disposal (1.00)
- Reservoir Description and Dynamics > Reservoir Fluid Dynamics > Flow in porous media (1.00)
Abstract The use of solids free brines as completion/ workover fluids has given rise to the need for methods of fine solids removal. Filtration of clear brines has become a major concern to operators. This paper will address the problem of removal of fine suspended particles in the .5–50 micron range. particles in the .5–50 micron range. The body of the paper deals with the removal of fine solids by flow through porous media. The mechanics of removal discussed include sieving, bridging, adsorption, and settling within the porous matrix. Parameters such as flow density or velocity and pressure drop will be defined. Each of the removal mechanisms will be discussed in terms of how the operational parameters affect filtration efficiency. A discussion of surface and depth filtration will be included. Also, performance parameters such as effluent quality, dirt capacity, and clear pressure drop will be discussed. Design of the pressure drop will be discussed. Design of the porous media will be covered along with a discussion porous media will be covered along with a discussion of how filters are rated. Current filtration techniques used in the oilfield will be reviewed and classified. The classification will include the type of removal mechanism whereby the specific filter operates, the effect of operating conditions on efficiency, and what kind of performance can be expected. Introduction Considerable attention has been paid to the potential for fine suspended solids to cause formation potential for fine suspended solids to cause formation damage by impairing the permeability of oil or gas bearing formations. It is not the purpose of this paper to add to that body of knowledge. Rather, paper to add to that body of knowledge. Rather, it is intended to provide a basis for understanding some aspects of the practical problem of economically removing these solids (particulates) from oil field brines. The degree of particulate removal (i.e. size and amount of particles remaining in the fluid) desired will vary with the situation, but a process for particle removal should be able to reliably meet any specifications set. Generally, filtrations is the final step in particulate removal. However, the major question is: is it economically feasible to use filters and how can their use be optimized in solids removal? To try and help answer these questions this paper proposes to review theories of filtration paper proposes to review theories of filtration mechanisms and in that context the current state of the art as practiced with oil field brines. Filtration Mechanisms Filtration can be regarded as the removal of suspended solid particulates from liquids (i.e. primarily aqueous brines) by a porous solid medium primarily aqueous brines) by a porous solid medium which can be fabricated in a number of ways. There are three basic idealized methods by which filter medium can remove suspended solids (Figure 1). The first (1a) is direct interception of the particulates at the medium surface since all pores are always smaller than all particulates. The second (1b) is direct interception of particulates through the medium thickness due to pore size variation through the medium. The third (1c) is adhesion to the filter matrix when particles are smaller than the pore sizes of the medium. In a practical filtration situation all three ideal cases will occur for the following reasons:Distribution of particles shapes. Distribution of particles sizes. Possibility of particle bridging. Possibility of particle flocculation. Notice in Figure 1, that the membrance medium is represented as having variable pore size through its thickness. Very few filter media can claim to be true surface filters with a sharp particulate cutoff and a truly uniform pore size. Achieving these characteristics are woven mesh filters (available down to 8 microns) and Nuclepore filters which are plastic sheets with uniform holes drilled by a combination of ionizing beam and etching processes. Such media are usually characterized by a relatively low void volume yielding a high resistance to fluid flow which is compensated for by making them as thin as possible. P. 139
- Water & Waste Management > Water Management > Lifecycle > Treatment (1.00)
- Energy > Oil & Gas > Upstream (1.00)
Abstract For several years the chemical industry has used diatomaceous earth system as a standard in effluent dewatering. Recognizing the similarities in clarity requirements between effluent dewatering and brine completion-fluid filtering, a new diatomaceous earth filtration system has been designed, tested, and is successfully being used in Gulf Coast operation.* The system consistently shows a one-pass solid efficiency of greater than 98%, at varying flow rates. Resulting solid concentrations average 74 parts per million (ppm, 0.007% by volume) and have been recorded as low as 10 pgm. This is a 40% improvement in removal efficiency and a filtrate that is 17 times cleaner, after one pass filtration, than the average of several existing cartridge units. The efficiency of filtering high density brines is dependent upon several factors including the effectiveness of the filtration system, the physical characteristics of the brine and suspended solids, and operator experience. Filtration removal efficiency and Industry standards for brine cleanliness can to modified to become more realistic and field-oriented. In future filter unit evaluations or comparisons, an absolute term of removal efficiency, which is defined by multivariable correlations through time, should be utilized. And rather than a maximum particle size standard, the Industry standards for brine cleanliness should be field-oriented and can be based on a concept of clarity and degree of contamination. CONCEPT The Diatomaceous Earth (DE) system is based on the concept of bed filtration. Bed filtration uses a porous and permeable bed as a filter media. The porous and permeable bed as a filter media. The concept is; that suspended solids will be removed by this media as the fluid passes through. The DE system includes the premise that the contaminating solids can themselves be used as part of the filter media. FILTRATION RESULTS Test results on brines ranging in weight from 11.1 ppg, CaCl2, to 14.4 ppg, CaCl2-CaBr2, demonstrate ppg, CaCl2, to 14.4 ppg, CaCl2-CaBr2, demonstrate an average removal efficiency of over 98%. The filtrate averaged 74 ppm remaining suspended solids after one filtration pass at varying flow rates. ibis is a 40% increase in removal efficiency and a filtrate that is 17 times cleaner, relative to remaining parts per million suspended solids, than recently tested per million suspended solids, than recently tested cartridge filters. A detailed comparison twill be presented later in the text. presented later in the text. The individual fluid samples recorded mean solids concentration values, removal efficiencies, and relative particle size distributions as shown in Table 1 as well as particle size midpoint average of contamination data as shown in Table 2. Figures 1 and 2 are field photographs illustrating the degree of particle removal by fluid clarity comparisons. particle removal by fluid clarity comparisons. Whereas, Figures 3 and 4 are photomicrographs of the 13.2 ppg filtration test comparing the solids contamination and relative size distribution of the particles in influent are effluent samples. A study of particles in influent are effluent samples. A study of the photomicrographs and the fluid analysis reports indicates that the DE filtration system removes the suspended solids immaterial of the particle size. Figures 5 through 7 are graphic illustrations of midpoint averages of contamination in terms of parts per million versus particle size, in microns, for each per million versus particle size, in microns, for each of the fluids. It should be noted that the up and downstream curves are very similar in configuration. This presentation also illustrates that approximately 90% of the suspended solids will range between 5 and 25 microns in size. UNIT DESIGN The DE unit presently utilized in Gulf Coast operations is a filter press design. The flow path of this type design is illustrated by Figure 8. p. 29
- North America > United States > Louisiana (0.47)
- North America > United States > Texas (0.29)