Water Control and Relative Permeability Modifi ers – Laboratory Screening for Improved Results in a Middle East Context

Water Control and Relative Permeability Modifi ers – Laboratory Screening for Improved Results in a Middle East Context

By Clive Cornwall, Corex (UK) Ltd.

Introduction

Relative Permeability Modifi ers (RPMs), after a period of extreme user scepticism, are once again being considered as an eff ective method of controlling unwanted water production in both oil and gas reservoirs. The inclusion of RPMs in the conformance engineer's portfolio of possible remedial solutions is being driven, in part, by the introduction of new and improved products by a number of major chemical suppliers. These companies are responding to the ever growing number of maturing fi elds, in all hydrocarbon provinces including the Middle East, which are plagued by excessive water production and to the demands of clients to extend viable hydrocarbon output beyond current predictions.

The RPM's original fall from grace was driven by extravagant claims for universal applicability, in essentially all lithologies. The point has been made that if the majority of RPM treatments had met with measurable and general success, their deployment would not have declined quite so dramatically. It may be argued that this pattern of initial misplaced optimism, followed by a broad rejection of the products and the techniques, could ensnare the present range of products, thereby dissipating their believed potential.

There is a signifi cant body of research concerned with the theoretical mechanisms that enable RPMs to suppress water fl ow while permitting hydrocarbons to move without undue interference. A common thread running through this work is a realisation that the success or failure of an RPM treatment is dependent on the unique combination of factors found in the target well and reservoir. Of the myriad of contributing elements to be considered, wettability is of particular importance, as it directly determines the level of polymer attachment and subsequent retention.

A realisation of the importance of wettability to the success of an RPM treatment would surely lead to a requirement for the determination of the wetting preference of the formation to be a key component of the pre-injection planning. Unfortunately in the majority of cases this measurement step is missing from the sequence. In fact in correspondence with one of the major suppliers the point was made that a formation is presumed to be water-wet unless advised otherwise by the client. Where the wetting preference is predicted not to be water-wet, the standard advice is to "clean" the formation, to achieve the desired water-wet preference. Such assumptions are not routinely checked to ensure the formation can be prepared to maximise polymer adsorption and to minimize displacement during subsequent production.

This paper seeks to make the case for an integrated core analysis and petrographic screening study, in conjunction with one or more RPM formulations, to determine which product is most suited to the prevailing conditions. It also aff ords an opportunity to modify a particular RPM formulation to maximise its potential and where a clean-up chemical is to be used, to ensure it will have the desired eff ect.

Background

The wide gamut of test formats available to the core analyst is many and varied, and provides a powerful device in defi ning the original condition of the rock material and the saturating fl uids. The inclusion of petrographic examination, in the form of thin section analysis, SEM and XRD enhances this original assessment process. Thereafter, the ability to perform testing following on from treatment with both conditioning and RPM chemicals generates comparative data sets for the determination of "before and after" trend patterns, thus suggesting the eff ectiveness of each selected product.

The Relative Permeability Modifi ers that would benefi t from an integrated package of core analysis screening are the non-sealing, water soluble polymers, whose purpose is to reduce the fl ow of water into the wellbore, without unduly suppressing the production of the oil phase. The manifestation of the alteration in the fl ow patterns is found in the eff ective water and oil permeabilities, and in the fl uid saturation profi les. Such parameters can be measured under reservoir conditions of pressure and temperature, with crude oil and simulated formation brine, as part of the core analysis screening.

The eff ective oil and water permeabilities, on a before and after treatment basis, are translated into (Residual) Resistance Factors to water and oil, which are taken as a measure of the success of the RPM in controlling water production while maintaining the passage of oil. The (Residual) Resistance Factor values available in the literature supplied by the chemical companies are encouraging, although the use of standard sandstones, which are naturally water wet, reduces their applicability to specifi c well conditions. The impact of wettability is discussed later in the paper and its importance is demonstrated in terms of the relevance to polymer attachment and the movement of fl uids within the pore system.

Candidate Test Formats

A typical screening exercise would draw upon the following test formats to defi ne the important petrophysical and petrographic characteristics that have a potential bearing on the success of the RPM treatment.

• Combined Amott/USBM Wettability, together with sourcing of suitable core material and its possible restoration.
• Eff ective Water Permeability at Residual Oil Saturation and Eff ective Oil Permeability at Irreducible Water Saturation for (Residual) Resistance Factor to Water and Oil
• Specifi c Gas Permeability, Pore Volume and Porosity, with Dean Stark Extraction
• Mercury Injection Capillary Pressure for pore throat size distribution
• Petrographic Analysis - Th in Section, SEM and XRD

Each of these formats is discussed in the subsequent subsections, culminating in a generic preparation and measurement sequence, presented in Figure 1.

Wettability – Comments and Determination

It is generally accepted that the water soluble RPM products being off ered by the major chemical suppliers require the rock matrix to be preferentially water wet, to ensure robust adsorption and attachment, and prolonged adhesion. If all reservoirs had an affi nity to water in the presence of oil, the need for screening would be removed. The assumption that all reservoirs are very strongly water-wet has formed the basis of a signifi cant body of reservoir engineering practice for some considerable time. The rationale for this assumption being that water originally occupied the reservoir trap and that as oil swept the formation the water phase would be retained by capillary forces in the fi ner pore spaces and as fi lms on grain surfaces overlain by oil. However, there is a growing movement away from this viewpoint, based on published evidence into the eff ects of crude oil on wetting behaviour, towards an acceptance that most reservoirs are at wettability conditions other than very strongly water wet. Extensive testing has shown that reservoir wettability can cover a broad spectrum of wetting conditions from very strongly water wet to very strongly oil wet. Within this range complex mixed wettability conditions given by combinations of preferentially water wet and oil wet surfaces have been identifi ed. Mixed wettability in the reservoir often results from surface-active molecules in the crude oil adsorbing onto grains over time.

The point has been made that rock properties such as relative permeability and capillary pressure, depend on the distribution of water and oil in the pore space. In order for laboratory measurements to be representative, it is necessary for the pore level distribution of the fl uids and the wettability to be the same in the laboratory as in the reservoir. Unfortunately much, if not all, of the laboratory based proving of the RPM chemicals has been conducted on standard sandstone plugs, such as Berea (see table 1). These sandstone plugs are cleaned with solvents prior to saturation with brine and oil, which creates a uniform wetting preference. Anderson in his review of the technical literature with respect to rock-oil-brine interactions and wettability observes that when all surface contaminants are carefully removed, most minerals, including quartz, carbonates and sulphates are strongly water-wet.

The requirement for a water-wet surface casts doubt on the use of RPMs in carbonate reservoirs. Published studies have concluded that carbonate reservoirs are typically more oil-wet than sandstone. Such diff erences are linked to diff erent adsorption characteristics of silica and carbonate surfaces, in terms of simple polar and crude oil compounds. In recent fi eld investigations performed on carbonate cores from the Middle East, the trend was for a general intermediate to slightly oil wet preference in the oil column. However, the ability to screen carbonate core provides an opportunity to test whether the presumption is valid or whether a modifi - cation of the formulation would be benefi cial for RPM adsorption.

The preferred method for measuring the wettability of the core samples, whether fresh, cleaned or restored state is the Combined Amott/USBM method. The combination of the two techniques ensures all possible wetting preferences can be determined, including mixed and non uniform wettability.

The generation of the wettability indices would be undertaken on a specifi c suite of plugs. For the determination of the inherent wetting preference of the reservoir, the plugs would be in either a fresh or restored condition (see below). When it is necessary to assess the impact of the cleaning chemical, the plugs would be fl ushed with the selected chemical prior to the wettability testing.

The Condition of the Test Core

The screening process is dependent on the securing of representative core material, in terms of its petrophysical and petrographical properties.

The preferred option is for the cutting and trimming of plugs at wellsite at selected depths across the reservoir section. Ideally the core should be taken with a low invasion coring system, to minimise mud fi ltrate invasion. It has been shown that oil based mud fi ltrate will alter the wettability of the reservoir rock. Emulsifi ers and surfactants included in these fl uids, even at low concentrations, have been shown to be responsible for a change in wettability. Water base mud will artifi cially enhance the residual water saturation present in the pore space and may contain wettability altering surfactants. The plugs, taken from the "undisturbed" centre of the core would be preserved in a closed environment pending analysis.

If it is necessary to use archived core, which is either inappropriately preserved or has suff ered long term atmospheric exposure, it is advisable to restore the core to re-establish the representative wettability and saturation profi le. In the laboratory the reservoir wettability of cores can usually be restored by duplicating the process that established the wettability in the reservoir (31).

• Establish a water-wet state by solvent cleaning. A clean mineral surface is indicated by a water-wet condition since clean silica and calcite are strongly water wet. This is a typical sequence but there will be exceptions, for example when fatty acid emulsifi ers are added to an oil base mud (32). In such cases the cleaning sequence must be adapted to the specifi c case.
• Establish saturations representative of the reservoir, or more precisely, a representative pore-level distribution of water and oil.
• Ageing in the presence of a high saturation of crude oil at reservoir temperature.

The use of core plugs that have been vigorously cleaned with solvents, such as xylene and methanol, without subsequent ageing in crude oil, is not advisable as it will skew the following Permeability Resistance Factors toward an overly optimistic position. It will also prevent an unbiased assessment of any clean-up chemicals that may be recommended by the RPM suppliers.

Determination of the (Residual) Resistance Factors for Water and Oil from Effective Permeabilities to Water and Oil

The plugs, whether in a fresh or restored condition, will be at irreducible water saturation with the remaining pore space fi lled with oil. They are mounted in individual hydrostatic core holders, a representative eff ective reservoir overburden pressure is applied and the assembly placed in an oven set at the reservoir temperature (see Figure 2). Following a period of stabilisation the following conditioning and measurement sequence is undertaken:

• Eff ective oil permeability at irreducible water saturation (Formation to Wellbore)
• Flood to residual oil saturation (Formation to Wellbore)
• Eff ective water permeability at residual oil saturation (Formation to Wellbore)
• Flushing with cleaning chemical (if required by chemical supplier and operator) (Wellbore to Formation)
• RPM treatment (Wellbore to Formation)
• Flood to residual water saturation (Formation to Wellbore)
• Eff ective Oil Permeability (Formation to Wellbore)
• Flood to residual oil saturation (Formation to Wellbore)
• Effective water permeability at residual oil saturation (Formation to Wellbore)

This generic sequence can be modifi ed to refl ect the specifi c requirement of the chemical supplier, in conjunction with the operator, in terms of fl ow rates, shut-in periods and quantities injected. The selection of the fl ow rates is of particular importance to minimize polymer stripping during subsequent formation to wellbore production. The fl ushing cycles can also be altered should there be a need for over-fl ushing or focusing on a single phase rather than both water and oil. Additional analysis may also be included to monitor the amount of RPM polymer present in the effl uent to chart the rate of displacement associated with prolonged fl ushing after treatment.

The pairs of eff ective water and oil permeability values provide a measure of the effi ciency of the RPM treatment in terms of (Residual) Resistance Factors (see Table 2):

(Residual) Resistance Factor (to water and oil) = Permeability (mD) Before Treatment / Permeability (mD) After Treatment The end point saturation values of irreducible water and residual oil may also be taken as indicators of the impact of the cleaning chemical and RPM in terms of water retention and the production of the oil phase.

Specifi c Gas Permeability, Pore Volume and Porosity, with Dean Stark Extraction

On completion of the fresh or restored state testing, the end point water and oil saturations are fi xed by Dean Stark extraction. Th e base parameters of gas permeability, pore volume and porosity of the clean and dry plugs are measured under ambient conditions. Th e end point fl uid saturations are expressed as percentages of the measured pore volume.

Mercury Injection Capillary Pressure and Petrographic Analysis

To understand the interplay between the RPM and rock matrix, an in-depth characterisation of a representative range of samples from the available core material is recommended. Mercury Injection to 60,000 psi will fully quantify the pore throat size distribution, which is a key factor in the control of fl ow in general, and the subsequent suppression of water movement within the pore system.

Thin Section, SEM and XRD analysis assists in the interpretation of the data generated by the screening process. For example, core material containing coal can be naturally neutrally wet, as suggested by an inability to achieve a strongly water wet preference during prolonged solvent cleaning. Another example may be found in the oil wetness of the North Burbank unit, which is caused by a coating of chamosite clay on the grain surfaces. Clays in general can adsorb asphaltenes and resins that can make the clays distinctly oil wet. Once attachment has occurred, as found with kaolinite and montmorillonite, it is diffi cult to remove. Th e presence of carbonate cements may also be detected through petrographic examination and will add an appreciation of the contributing elements in a sample's wetting preference.

Conclusions

To maximise the benefi ts of RPM treatments, whether directly into the matrix or associated with fracturing, an understanding of the properties of the target formation is vital. Th is knowledge base will also assist in selecting the pre-treatment processes and solutions that are applied to create a water wet preference for polymer attachment. Screening of the representative reservoir rock plugs under simulated reservoir conditions diminishes uncertainty. It also removes the current reliance on standard sandstones or cleaned reservoir core, with its encouragingly water wet preference, to predict the successful suppression of unwanted excessive water production.