Core Flooding as Related to Water Injection

Adrian C Todd
Department of Petroleum Engineering
Heriot-Watt University
August 1999

Foreword:

The paper has been written as a personal reflection on water injection related core flooding as presented in the mainly public domain literature. In putting it on the PWRI web site it is hoped the sponsors will respond with their in-house or subcontracted service company practice in core flooding with respect to water injection. This should include perhaps core flooding practice, which might have been carried out in the past but in recent years has not been used because of a lack of confidence in the application of the results obtained.

The document is not a statement of "good practice" but it is planned that, with the wealth of experience contained within the sponsor group, a good practice document will be forthcoming. I am not looking for a detailed comments on the paper since much of the material cited is of some age; but more for it to be used as a prompt to stimulate questions like:

"How do we carryout core flooding or water quality evaluation
and then interpret and apply the results?"

The response to these questions would lead to a more valuable document as compared to the limited historical perspective presented in the paper.

Introduction:

Water injection has been a long established practice in oilfield operations, where water is injected into the formation to provide the drive mechanism of a non-existent or limited natural water drive. Over the last thirty years as fields have been operating in offshore locations; sea water has been the prime injection water. Historically water injection has been a once-through operation, where treated injection water has been injected into the formation, and water produced with hydrocarbons, has been separated, treated and disposed back to the sea. In recent years this once-through injection process has become less favourable, as concerns over the impact of the oil content of the water disposed back into the environment have increased. Alternative systems are now becoming more common, where the produced water is not disposed back into the environment but is injected into the rock formation. Produced water re-injection, PWRI, is favoured, where the treated produced water is the injection water for the pressure maintenance process. The once-through injection system has therefore been replaced with a recycle system.

When water injection schemes were being designed in the early seventies for major offshore fields as in the North Sea, seawater injection systems were put in place to provide pressure support to these high rate, producing reservoirs. The uncertainty of natural aquifers being able to provide this support provided a good case for this development option. In designing these water injection schemes the challenge was to maximise injection into a limited number of injection wells in order to leave the other well slots for producing oil.1,2

One of the key parameters in designing these injection wells was their predicted injectivity with time. Would the wells be able to take the rates of water required, or would the formation act as a very effective filter and eventually block up and prevent further injection? A number of companies carried out core flooding tests by injecting samples of the recovered core with seawater. The permeability reduction in these short duration tests was significant and therefore fine filtration facilities were installed in most cases. In marine environments like the North Sea, although the water is very clean in its inert solid content, significant seasonal changes in overall solids content can occur as seasonal changes in plankton populations have their effect.3 It is interesting to note, however, the various schemes which were implemented in a producing area like the North Sea. The systems installed ranged from filtering the injection water down to 0.45mm to other installations where no fine filtering was used. The source waters were the same and the reservoir formations were not too different.4 The application of such diverse approaches to maintain injectivity confirms the opinion that well established design tools did not exist to relate well injectivity to formation and water characteristics. If there were a more common approach then such diverse treatments would not have been used by different operators. Twenty-five years on and with a wealth of water injection experience is industry much further advanced in its predictive capability in this area? After many years of water injection throughout the world it may have been expected for what, at first appearance, is a straightforward process, that standard practice would by now have provided validated tools to predict water injectivity.

Many of the water filtration systems have subsequently been removed from offshore platforms when it became clear that injectivities were being maintained at higher levels than had been predicted or expected based on these core tests. The most significant phenomenon was thermal fracturing occurring as a result of the cold water reducing the natural fracture gradient. The large pumps were such that injection pressures were well able to overcome injection resistance. The flow profile therefore was not restricted to the limited area presented in radial flow but a larger injection face presented as a result of an increasing fracture length.5 Another significant observation was that the quality of water being injected into the formation by one operator was worse than the quality of water being presented to the filtration system of the injection water treatment plant! 5

Core Flooding:

One of the tools that has been used prior to installation of water treatment facilities is core flooding, where operators or service companies have injected waters into rocks to provide data or give indications of permeability loss which can be used to predict injectivities in field injector wells. It is the purpose of this paper to review core flooding in this context and to suggest better procedures which might be used or to question the validity of core flooding in injection treatment design. In examining core flooding practice it is not the authorís intention to suggest that core flooding is a useful tool in well water injectivity predictions. It is considered that core flooding can be used to provide some evidence of outcome but because there are not well validated models that can be applied, core flooding is limited in its design application.

The petroleum engineering challenge is considerable: it is one of predicting the filtration characteristics of a very heterogeneous porous system for a fluid whose solid content is complex. The upstream oil industry can take comfort, in another industry, the process industry, with more well defined systems, where tools for predicting the behaviour of fluid mixtures are more well founded than those where particulates are also involved. In our own industry, the behaviour of gas and liquids in reservoir rocks is far from understood and new insights into basic parameters such as relative permeability are still forthcoming.

Core flooding is still used in the particular application of water injection probably to give at least some indications of formation damage or water quality to use alongside prediction tools which are far from well established.

A common practice in general in petroleum engineering has been an empirical approach to prediction based on the 'giant leap' approach. The engineering of a reservoir is considerable when compared to that of a process plant; not only is the reservoir, (a downhole chemical factory), orders of magnitude larger in its overall size, but it is smaller in terns of the size of the process units which are beyond measure in number, the pores of the rock. The reservoir is also unique compared to process plants across the world. In process engineering a common practice is to take laboratory scale measurements, and scale them up to pilot scale (intermediate size), not only to provide confidence but more importantly to generate data for full-scale design purposes using well defined scaleup parameters. Reservoirs donít provide this luxury: there is not the opportunity for a pilot plant scale! Too often the small scale and short duration laboratory experiments have been expected to provide design data to enable field predictions to be made lasting years. Experiments have often been carried out with add-on 'field realistic' features and as a result revelation of basic phenomena has often been masked. Not surprisingly this 'giant leap' approach has unfortunately brought a lack of appreciation of those in the laboratory when laboratory results do not predict field outcomes. It is satisfying to see that in the industry, effort is now being made to move from this empirical approach with more emphasis being placed on seeking to understand some of the basic physics and chemistry of fluid movement in reservoirs. A principal objective is the progressive development of tools of confidence for ultimate field application.

The author considers core flooding in the context of water injection, to be one of these 'giant leap' approaches, where attempts have been made to carry out short-term laboratory scale experiments under 'simulated' conditions in order to predict injection well performance over periods of years.

Rationale for Core Flooding in Water Injection:

Core flooding in the context of water injection has been carried out for two basic reasons: as a basis for formation damage modelling and for field based injectivity predictions.

In the former, different workers have carried out experiments using different geometries of rocks to understand the various pore scale processes involved in the reduction of permeability as a result of injecting a defined dilute mixture into the rock. These basic studies have often been part of theoretical modelling to predict formation damage.4,6-20,32,33,34 In field applications, laboratory tests have been carried out, sometimes at field locations where waters similar to or the same as the field injection water have been injected into porous media to represent the 'field rocks.' 21-27 Results from these tests have then been used to anticipate performance of injectivity over the lifetime of a well.

Although the reasons for the core flooding have been different there are approaches which could form the basis of good practice for water injection related core flooding. In relation to standard practice, in 1991, the ASTM produced a document; "Standard Practice for Determining Water Injectivity Through the Use of On-Site Floods".28 Examination of this document however reveals that there are still improvements to be made which could lead to good practice procedures in relation to core flood tests.

It is important to distinguish core tests carried out to determine formation damage with those core tests that could be used as an assessment of the water quality. The author considers in some cases that the specific focus has not been identified and whereas core flooding has been carried out to determine the injectivity of a rock, the test has been an assessment of the quality of the water. Indeed the ASTM Standard Practice for core flooding does that when it suggests:

"If cores from the flooded zone are not available, choose another zone with similar properties as the next best alternative source. As a third choice use synthetic core material."

Some of the well-established approaches for particle impairment such as Barkman and Davidson29 are based on models related to behaviour on membrane filters. The author would question as others have,14 the value of such when the particle build up on filter membranes has no obvious correlation with particle deposition in rock. However there are some who still favour this approach by using filter membranes to lay down a filter cake which is then characterised for its permeability, the assumption being that it is the cake permeability which determines injectivity behaviour.35

The geometry of the experiments has also varied in relation to their application. Most common are linear core floods, but there have also been radial core floods. Recently because of the interest in fracturing associated with water injection simulated fracture core floods have also been carried out, but there are few cited in the literature.

Linear Core Floods:

These are the most common floods and are the basis for the current ASTM Standard Practice. 29 They generally involve the confining of a cylindrical core in a resin or flexible sleeve and either measuring the pressure drop over the core length for a fixed rate of injection, or measuring the rate of fluid exiting the core for a fixed injection pressure.

Core Length:

It is in the aspect of core length that the credibility and broader value of many low concentration formation damage core floods are in question. The diameters of the cores vary but are generally between 19 mm and 38 mm (0.75 and 1.5 inches). Lengths of the cores also vary, from around 2.5 to 10 cm (1 to 4 inches) and sometimes longer. This lack of standard in length is the first shortfall in standard practice, particularly for those tests where the permeability is obtained by measuring exit flow at constant pressure (ASTM practice) or overall DP at constant rate. The formation damage behaviour is illustrated in Figure 1 below for a linear core flood.

Figure 1.  Potential errors in permeability associated with core length.

Basic equations of resistances in series would indicate that the average resistance, permeability, for the top core, which has the same formation damage characteristics as the bottom core would yield a higher average permeability than the bottom core. This basic concept seems to have by-passed many in formation damage core flooding where there is no standard length, making it impossible for other workers to make use of the data generated. Indeed the ASTM standard suggests that the core length should be based on a length to diameter ratio of 1! Permeability ratio, which is the ratio of the permeability at any time to the initial permeability, is used as a measure of formation damage. In the above illustration then a different permeability ratio would result for cores having the same formation damage

A number of workers have recognised that the depth of invasion of formation damage is both a function of the fluid injected, in regard to particle sizes and concentrations and also the characteristics of the core. In their experiments they have installed pressure monitoring along the length of the core to measure this depth of invasion.6,7,12,13,14,16,19,27,32,33,34

The author's recommendation is for a fixed length to be defined or for pressure taps to be installed at fixed intervals along the core for longer core. The author in research in the eighties used pressure taps at 5mm, 12.5mm, 33mm,and 56mm for a 75mm core, recognising that the major impairment was towards the injection face end of the core. (Figure 2). An example of the resulting permeability profiles in a linear core flood is shown in Figure 3.

Figure 2.   Schematic diagram of core plug and holder.

Figure 3.   Pressure profile for a cut-faced core.

Core Preparation:

The preparation of the cores for most workers appears to follow conventional core analysis practice, where the cores are cut with a core barrel and trimmed to length with a saw. Indeed this is the recommendation of the ASTM standard. The author used this procedure for a number of years taking great care by cutting the core ends against a flow of water out of the face of core to prevent ingress of fines. Using low concentration dispersions of solids in water, as low as 1 ppm, external material would build up on the face of the core after 7 hours of injection. Using this preparation technique there was always a concern that the results of the experiments were a reflection of the characteristics of the machined face rather than the formation damage characteristics of the rock. Similar results therefore might have been generated by putting a piece of filter paper at the face of the core, perhaps not unlike membrane filter tests!

Core preparation was changed and a broken face preparation procedure was used. In this case the ends of the core were not trimmed by sawing but were fractured along a plane perpendicular to the longitudinal axis of the core. The author considered that this preparation technique was more likely to generate data pertaining to the deposition and entrainment behaviour associated with low concentration invasion of a solid in water dispersion.

Results of this cut face versus broken face investigation were reported in the open literature in 1988,13 where it was demonstrated that there was considerable impact of this preparation procedure on the permeability variation of the cores. Visual observations of the inlet faces showed that whereas an external material had built up on the face of the core for the saw trimmed material, for the broken face core an external build up was not as obvious. SEM images again confirmed the external deposits on the trimmed core and a relatively clean surface for the broken face. Pressure taps enabled a depth of invasion comparison to be made between the two procedures and these again confirmed a significant impact on the depth of penetration of particles and the face damage due to the preparation procedure. Figures 3 and 4 from the 1988 paper show a comparison of behaviour. Because of the serious impact of this preparation procedure the author chose to repeat the majority of the previous experiments, which was no small effort.

Figure 4.   The pressure profiles obtained for a broken-faced core.

It is a concern that some researchers are still preparing the faces of the core by sawing.34 In this very recent paper on oil and solids invasion using a transparent end piece van den Broek et al. were able to observe the build up of material on the face of the core. It would have been interesting to see the impact of core preparation procedures using this visualisation method.

In the low concentration formation damage literature there is considerable reference to external and internal filter cakes. This terminology probably comes from the more concentrated systems associated with drilling fluids, where the effectiveness of the drilling fluid is in part dependent on its ability to reduce solids invasion by the laying down of an effective filter cake. It is interesting to note that an overview on formation damage in 1989 made very little reference to water injection formation damage.31 Some prediction models used for water injection use this external and internal filter cake concept. The author's work with very low concentration systems typical of injection waters would question this approach.

Flow Rate:

Experiments carried out by those investigating the influence of velocity on formation damage have indicated that there is a velocity effect.6,9,13,14,15 An example of the impact of velocity is shown in Figure 5. In carrying out injectivity tests for immediate field application core injection rates have often been based on the flux of water into an estimate of the area presented to the water, based on a simple radial expression of the well and its injection interval. In reality the water would generally penetrate through regular perforations. In laying down good practice for core flooding a series of fixed flow rates might be used, to enable greater flexibility in application of the data. Whereas this might be straightforward for constant rate tests, it would be more difficult to conduct for constant pressure experiments that are the basis of the ASTM standard practice procedure.28

Figure 5.   Exponential relationship between flow velocity and permeability.

Core Saturation:

A number of workers have recognised the importance of fully saturating the core prior to injecting a dispersion - to ensure that permeability measurements are not influenced by gas blocking due to trapped.25 A normal procedure is to apply a vacuum to the core and then allow water to penetrate the evacuated core. This is the recommended ASTM method.28 The author suggests that this might not always provide effective saturation. For many years the author has carried out a procedure which ensures 100% saturation. The dry core is successively evacuated and saturated with carbon dioxide gas. After the final evacuation of carbon dioxide from the core, the deaerated water is allowed to penetrate the evacuated core. If any residual gas is present in the core after successive evacuation then this would dissolve in the water, carbon dioxide being very soluble.

Fluids for Injection:

One of the more difficult areas in water injection core flooding concerns the injection fluids. Now that produced water injection has become a major focus, as opposed to seawater injection, the issue is even more complex. The field objective is to determine the impact of different water qualities on injectivity when injected into rock representing the properties of the injection intervals.

On-site core flooding is favoured in this consideration since the actual water is likely to be available. The ASTM procedure does not make much reference to the injection water except to the use of a series of pre-filters for examining the impact of water filtered to different levels.

More difficult is produced water injection where there are at least two components to consider, the solids concentration and oil concentration. Another perspective of produced water reinjection, PWRI, is that whereas conventional water injection is a once-through system, PWRI it is a recycle system. The produced fluids are therefore quite a chemical cocktail of natural chemicals and chemicals added to the systems for a wide range of reasons.22 One component of waters which is often neglected are the live species which could have a major impact on injectivity.26,30 Good practice in assessing this aspect is still not well established and is worthy of more effort. In field operations these materials are treated by various chemical methods.

In laboratory studies for basic evaluations or for field applications, fluid preparation can be more readily controlled, and cleanliness can be ensured. Cleanliness is essential in examining the impact of water that is often cleaner (from a solids loading perspective) than domestic water. Preparing, maintaining and monitoring a consistent suspension for a long duration core flood is no small task. In the context of PWI, fluid preparation is a big issue. In our experiments16 on this topic where over 100 core floods involving different combinations of solids and oil, and solids with oil concentrations have been carried out, the work was preceded by a considerable effort to refine the ability to generate and maintain a consistent dispersion of oil in water. In the field there is considerable energy and shear available through pumps and other systems to ensure such stable dispersions. In the laboratory some measure of 'realistic' improvisation is required. To obtain a stable dispersion of oil in water, surfactants may be required. Natural surfactants are present to an extent in reservoir oil. In the laboratory there is varying practice in the use of added surface active agents to generate stable oil in water dispersions. A very recent paper on a laboratory study of the external filter cake characteristics in PWI systems has also used a surfactant in preparing the oil in water dispersion.35 Clearly, there is some question concerning the impact of the surface active agent on the formation damage interaction of 'particles' and the rock pores.

Produced Water Injection Core Floods:

The paper so far has focused on the general issue of water injection core flooding. The current major interest is produced water injection where for environmental emissions reasons PWI is a preferred alternative to once through sea water injection. Open literature papers have been presented since 1982 on this topic.16,18,19,22,24,27,32,34 The papers are similar to those for conventional water injection in relation to core types and flowing procedures, and clearly differ with respect to the nature of floods and, in particular, the nature of the injected fluids. As mentioned previously, produced water injection fluids are a combination of different dispersed phases, solids and oil. From the papers cited above only the papers of Zhang et al.,16 and very recently van den Broek et al.,34 have taken account of this oil and solids consideration, by attempting to examine the impact of solids, and oil on their own and when together as part of a dispersion. Zhang's extensive research on this topic,19 followed up by that of Al Homadhi32 in this Department, has demonstrated the challenge in understanding the damage mechanisms of dilute solid and oil low concentration dispersions.

Radial Core Floods:

Various authors have sought to examine the impact of changing velocity resulting from a radial flow injection by using radial flow geometry. Such geometry does not lend itself to simple on-site core flooding tests but is a valuable contribution to fundamental behaviour studies. Some authors have used a full radial configuration and machined out a central hole in the rock to simulate the injection well.11 A concern could be the realism of the radial flow being compromised by the reality of the injection face obtained as a result of drilling out the 'injection' face. A geometry which others have used incorporates a segment of the full radius.14 The confining of the segment is not simple but pressure tappings can be included to monitor the impact of the invading solids.

Cross Flow Configurations:

In order to simulate field realistic situations in core flooding, an approach which is practised in relation to drilling and fracture fluids is where the main fluid flow is parallel to the injection face of the core, although the author is not able to site a reference for water injection. In drilling fluid systems this is the mode of filter deposition on the wellbore, where the main flow is in the circulating fluid with filtrate leakoff into the porous walls of the open hole. A similar configuration can be established in water injection simulation where the face of a core plug is parallel to the flow and in a constant pressure mode fluid leaks through the core. The permeability is determined from the declining rate of fluid passing through the core. Some have suggested that such a configuration might be appropriate for simulation of the leakoff along a fracture. The author sees this as somewhat of an unnecessary complication since the behaviour at the face will not be straightforward. The fluid dynamics of cross flow filtration is complex and the sudden change of fluid flow profiles and particle capture at the face of a core arising from some of the flow suddenly diverting into the core would be difficult to model. The author suggests keeping flow normal to the injection face and when understanding is achieved consider the more complex cross flow configuration. Experimental design will also not be straightforward in order to eliminate entrance effects.

Simulating Behaviour in Fractured Systems:

The impact of thermal fracturing in many seawater injection locations has generated an interest in examining at small scale the process of low concentration formation damage applicable to predicting formation damage where the flow geometry is not radial but that of a fracture. There is a convention developing in water injection circles where radial flow geometry is called matrix flow. The good injectivities achieved as a result of thermal fracturing are such that in injection practice, intentional hydraulic fracturing (in addition to thermal fracturing) is likely to be implemented to achieve injectivities that are not possible with matrix injection. The question then in this context is: "What is the appropriate geometry of core-related tests for application in fracture flow situations?"

There is very little published on this. In research carried out in this Department over ten years in PWI, both linear and fracture geometry configurations have been used to examine permeability reduction through a combination of solid and oil concentrations through different core permeabilities.

In designing the core system to simulate fracture flow behaviour an attempt was made to generate flow profiles which would enable permeabilities to be calculated along the progressive length of the fracture face. The core configuration is shown in Figure 6a. The core is prepared by taking a cut slab of the rock and then splitting it into two halves along its length. The two broken faces are then separated by plastic spacers to enable a fracture gap to be maintained between the broken faces. The core is mounted in resin and the faces opposite to the fracture faces are machined open to enable flow through, normal to flow along the fracture. The fluids exiting from these faces are collected in sections along both sides to enable permeabilities to be measured along the sample's length. The geometry and the fluid capture system are considered suitable to generate permeability information. If a cylinder of rock is used, Figure 6b and the two broken halves are mounted in a similar way, it is not possible to get as good an indicator of permeability because of the converging flow profiles for the example shown (Figure 6b).

Figure 6.  Fracture flow core configuration, (a) rectangular and (b) cylindrical.

Extensive experimentation using different oil and solids in this fracture configuration provided some interesting results.32 Flow through the injection face along the core was not uniform. This flow was clearly influenced by the irregular build up of oil and solids along the fracture faces. At times an irregular wormhole effect developed in the material deposited in the fracture. Although the experiments provided interesting behaviour, it is still difficult to extract specific mechanistic information of generic formation damage understanding and data. One might consider that linear core floods are just as appropriate in this configuration as in matrix flow. The results of the linear core floods could be then built into a fracture flow model to predict injectivities in fracture configurations.

An obvious consideration in relation to fracturing is the need to simulate stress considerations in laboratory scale experimentation. As an industry there is some comfort when laboratory studies are carried out under "reservoir conditions." So, for example, for permeability measurements core holders can load core specimens under various stresses. It is unfortunate that conventional biaxial loading of cylindrical cores for horizontal permeability measurements necessitates that the two of the three principal stresses are the same. Such restrictions could question the effort of carrying out such additional "realism." If stress conditions are being applied then attempts should be made to apply stress in more realistic ways (Brian Smart, personal communication).

There is little published in relation to carrying out formation damage studies under stressed conditions. Although stress related experiments might have value in relation to rock fluid interaction from dissolution effects, (Brian Smart, personal communication) it is questionable whether it is necessary at this stage in our understanding to simulate stress conditions in experiments to generate formation damage criteria. Clearly stress related experiments are necessary when understanding failure in the context of water injection fluids.

Other Injection Considerations:

Not all injection processes can be considered as radial matrix injection or, at the other extreme, fractured systems. Some formations are very unconsolidated, sometimes termed soft formations, and the injection flow geometry for these materials is not well understood or documented. The formation damage behaviour in such formations is open for investigation, and associated experimentation is far from established. Consideration may be given to filtration know-how in other industries, where filtration in not dissimilar materials takes place. However the confined nature of formation formations and their behaviour under high injection rates are such that flow profiles into soft formations are far from developed. When such understanding is available the incorporation of a formation damage function may not be too great a challenge!

Horizontal Well Applications:

It is clear that well injection geometries for both conventional water injection and produced water injection will increasingly include horizontal wells. Horizontal well flow models are improving and well intervention aspects are a subsequent objective. The type of core flooding simulation appropriate for such configurations has not been discussed, but it is the authors opinion as for many of the field injection geometries that once the formation damage rules have been determined from a combination of generic laboratory studies and field injectivity experience, injectivity predictions will be generated based on horizontal well flow models.

Analysis Methods in Injectivity Related Core Flooding:

A number of analysis method are used in core flooding, with respect to the fluids and rocks injected, the produced fluids and the rock fluid interactions.

A key factor in the fluids' analysis is the nature, amount and size of the constituents of the water. For produced water injection there is the additional perspective of the concentration and size of oil droplets.

Particle sizing and counting systems have gone through considerable development since the 1960ís. It should be recognised that the size of an irregular object is not easily defined. The definition of size therefore is often related to the application for which it is applied and the appropriate measurement method in that context. Most particle sizing procedures relate the size to an equivalent sphere dimension. In many fields of technology there are accepted sizing protocols. For example, in defining the size of particulates in paint, the size related to surface area is likely to be appropriate to reflect its coating ability. In the context of formation damage in rocks the author is not aware of any protocol. Clearly an object might pass through a pore even if one of its dimensions is larger than the pore: the principle of size definition as determined by sieving. For many years in water quality applications, the Coulter Counter has been used as a size-defining instrument. It measures the size of a pulse of non-conducting fluid and relates this to an equivalent size according to volume. In recent years optical instruments based on measuring the scattered light from a particle have been used, e.g. Malvern. The number against size from these machines is likely to give two distinctive results for the same fluid. There is a need therefore for a standard to be established in this area so that when size and number are being defined the definitions are clear. Many of the instruments are not able to distinguish oil droplets and particulates. In formation damage it is important to distinguish the two since their behaviour will vary not least because the oil droplet is capable of deformation when passing through a pore opening.

The characteristics of the rocks are well established in relation to permeability and porosity. Permeability is not an absolute property: it is a function of pore size, porosity and pore shape all of which will have an impact on plugging with invaded material. In water quality work, pore size distribution measurements have been used to characterise the rock and then mean dimensions of pores related to the size of invading particles. Mercury injection porosimetry is commonly used in this context but there are still questions on the validity of this method for defining pore size.

The scanning electron microscope, SEM, has been a powerful tool in the visualisation of particles trapped and many core flooding papers have used SEM images in their analyses. The sample preparation techniques for conventional SEM however have limited the scope of this visual method. The recent introduction of the environmental SEM, where a conductive coating is not required and analysis can be done on 'wet' specimens, may prove to be a more powerful visualisation method of produced oily water invasion into porous media.

The Use of Core Flooding Data:

As indicated at the introduction of this paper. core flooding has two applications: as a tool in understanding basic formation damage phenomena and for use in predicting well injectivity performance. In the published literature there is little foundation for the value other than in qualitative indications of core flooding as a design tool for injectivity predictions. Indeed how can flooding tests lasting a matter of hours on a limited number of core lengths, questionably representative of a large injection interval, be able to predict the loss of injectivity of a well made up of a number of heterogeneous layers of rock of different compositions and structure? It is a classic example of the 'giant leap' approach

ASTM Standard 28 is more confident in its conclusions, suggesting that core test results are used to determine the treatment volumes or concentrations required to treat the formation around the injection wellbore. To quote Section 10.4 in the standard in relation to application:

The interpretation of the test results as applied to actual injection conditions depends on the proper choice of test cores in terms of permeability and mineralogy. Real formations contain a variety of permeabilities and mineralogies. The greater use of the test core results can now be made as filtration and rock-water interaction data are related to the rock properties and then in turn to the injection well performance.

In addition, Section 10.3 states:

There are geometry effects that must be considered in comparing core test results with well performance. In a core test the permeability is measured over a few centimeters. Consequently, permeability changes are amplified. In an injection well permeability changes (as seen by changes in injectivity) are averaged over large distances. The geometry of the wellbore (openhole, cemented and perforated, etc) must also be taken into account.

In the concluding comment the ASTM's Standard Practice document (Section 11) admits the limited value of core flooding tests when it comments:

Due to the large number of variables encountered in the practice no statement of precision can be made. Interpretation of results is subjective. Results are a function of individual equipment assembly, variability of natural core material, core handling, and sample preparation.

The Reservoir - The Best Approach:

Papers continue to acknowledge the frustration of simple interpretation of core floods in, for example, commenting that

"on-site core floods give a loss of injectivity prediction for one day as 90%. This is much more severe than what is normally observed in the field, thus suggesting that either the model is flawed or a much large surface area exist in the wells to take the water." 27

History matching is a proven technique in developing and designing reservoir behaviour models. The suggestion that short-term core floods can provide a predictive basis for long-term injectivity predictions is somewhat unrealistic and should not be expected. Well-validated scale-up rules do not exist in this sector. Good quality core floods carried out to conform to standard practice however could be used to understand the basic physics of formation damage, so that predictive models can be developed which when incorporated into well constructed reservoir flow models might be capable of giving more confident predictions than is the case at present. In the end the reservoir and its wells are the best 'pilot plant' and although perhaps not benefiting the field itself could provide design know-how for future injection design.

Bridging the knowledge gap in predicting well injectivities in various aspects of conventional and produced water injection will not be achieved by the 'giant leap' approach based on small-scale, short-term laboratory experiments. The gap has to be bridged from both sides. From one side, generic work carried out in carefully controlled experiments alongside validatable numerical simulations should provide mechanistic understanding, and from the field side, the collection of good injection field data from which injectivity models could be extrapolated.

A useful illustration of this is the Forth Bridge, a rail bridge near Edinburgh spanning the Forth estuary, a magnificent steel structure made up of three main sections each with major structural elements and supported with underpinning foundations. In this analogy, three main elements could cover models of science predicting generic behaviour, models to provide effective tools for use in technological application, and the third, specific models of the 'field' system. An excellent example of this analogy is the industry-funded PWRI project, where operators, recognising that the structural elements of PWRI know-how and design tools were not in place, are collaborating by pooling their water injection field knowledge and experience with a view of backing out models and good practice, at the same time, from the other side, identifying the generic science components or the lack of such important elements.

It is hoped that the integration of these elements will lead to the development of design tools based on good science and which can be used with confidence in future field applications.

References:

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