Fluid design impacts drilling and production operations

Syed Ali
Chevron U.S.A. Production Co.
New Orleans

Louise Bailey, Hemant LadvaSchlumberger Cambridge Research
Cambridge, U.K.

Lindsay Fraser
Dowell Drilling Fluids
Houston

Whether back producing or stimulating the filter cake in an open-hole completion, the nature and location of the filter cake can profoundly impact drilling and production operations.

The filter cake must quickly and effectively shut-off mud invasion into the reservoir, provide an unimpeded pathway for hydrocarbon flow into the well, and facilitate flow back through the completion assembly.

Selection of the filter cake-removal method, either through physical or chemical means, must address not only the facilitation of reservoir flow, but also the outcome of contact between back-flowed materials and the completion assembly.

In addition, the size distribution of solids must be closely examined, since this is critical in both the shut-off process and the ability to flow back cake solids without screen damage.

This first part of a two-part series addresses the relationship of the external filter cake to the formation and screen assembly while addressing the inherent (internal) strength of the filter cake itself. Clean-up techniques and implications for maximizing production will be discussed in the conclusion.

Filter cake issues

The advent of open-hole completions, as commonly encountered in horizontal wells, has caused the industry to scrutinize the role played by the filter cake in the process of exploiting reservoirs. Reference to Gray and Darley's classic text, "Composition and Properties of Oil Well Drilling Fluids," illustrates the traditional view taken towards the place of filter cakes in the drilling process. ¹

Little time is spent in dealing with the issue of filter cake removal prior to producing the reservoir. The reason is simple. Until recently, cased and cemented completions dominated the industry's efforts to deal with filter-cake problems while open-hole completions occupied only a minor role.

Gray and Darley slanted their discussion on filter cakes heavily towards limiting formation invasion during the drilling process while controlling the cake thickness to avoid instigating occurrences of pipe sticking.

Creation of the well-bore seal was, at the time of their writing, of paramount importance. When it was necessary to breach the seal, as in the reservoir, perforation activities comprised the accepted technique. While the filter-cake characteristics identified and discussed by Gray and Darley remain as relevant today as they were decades ago, current open-hole completion practices also require the assurance that removal of the cake from the production path is achieved in an efficient manner.

Not only is it important to address the removal of the cake from the face of the well bore, but it must be performed in a fashion that achieves the continued protection of the reservoir matrix while avoiding destructive blockage of the sand-screen assembly and gravel pack.

An efficient seal

In the case of purpose-designed, water-based drill-in fluids (DIFs)-and perhaps to a lesser extent, compositions based on a hydrocarbon continuous phase-the selection of bridging materials and associated filtration control additives occupy a major role in the fluid design exercise.

While other aspects of the fluid performance need to be addressed, such as the ability to drill the hole while maintaining a stable state, great emphasis is placed on achieving an early and efficient seal at the well-bore interface with a view towards minimizing the invasion of foreign materials such as filtrates, drill-in fluid additives, and drill solids.

In low-density, water-based fluids, the filtration control package usually consists of ground calcium carbonate (limestone or marble) with a cross-linked starch or starch derivative. The original rationale for a calcium carbonate bridging material centered on acid solubility, which provided an easy route to removal.

Efficient methods of removal for the starch derivatives were less easily defined in early applications. A variation on the principle involves use of saturated brine bases, polymers, and sized salt particles for bridging. Undersaturated brines are used to dissolve the bridging salt.

Some success has also been achieved under fairly specific field conditions using low-solid polymer fluids with abnormally high viscosities. The approach is to use the fluid viscosity rather than physical bridging to impede flow through the narrow pore networks. However, this method is not considered to be main stream and will not be addressed further in this article.

Fluid design

By focusing on the industry's general approach to the early and efficient placement of a physical seal on the reservoir, it is possible to identify three major aspects of the fluid design-bridging materials, starches, and viscosifiers-that influence the outcome of drilling and production operations.

Bridging materials

Bridging materials have commanded an abnormal amount of the industry's attention over recent years. Most considerations have been concerned with the type, size distribution, and loading of the aforementioned materials.

Inherent in the discussions is the need to remove these particles from the reservoir face prior to or during production. Thus, quite appropriately, the focus has been comprehensive in terms of the total process requirements.

While sized salt particles have been widely and successfully used as a mainstay bridging material, the industry has been moving to sized carbonates as the materials of choice for lower-density fluids.

If the variability in acid solubility for various carbonate sources is set aside, the issue distills down to selecting the right grind and establishing the threshold treatment to achieve rapid bridging.

Results of unpublished work as part of an EC (European Community) funded consortium between Schlumberger Cambridge Research (SCR), Statoil AS, and the Institute Francais du Petrole throws new light on the whole cake-forming process. Using simple polymer-based fluids, it was found that under static conditions, the presence of both fines and bio-polymers reduced the initial and critical spurt loss, or the initial rush of fluid prior to pore plugging, to the matrix.

In addition, under dynamic conditions, it was found that the spurt increased with increasing shear as the capture of blocking particles was hindered. This brought into question the relevance of at least one segment of laboratory-based work concerning shear-related studies.

Filtration control

Of late, a belief has developed that the vast majority of reservoir matrices can be successfully bridged using a single carbonate blend having a log-linear grind and a low d50 (median particle size distribution). The EC work highlights the shortcomings of applying this approach. Fig. 1 [40,686 bytes] illustrates the outcome of attempting to use such a fine material to bridge a coarse limestone.

Spurt volume for the test in question was high as was the matrix invasion. When a coarse bridging agent was used, the solids were more efficiently excluded; however, the spurt loss was not controlled. Optimum filtration control and solids exclusion were achieved using a mid-range, median-size grind. It is interesting to note that high spurt loss and the invasion of solids and polymers translated into poor return permeabilities for these tests.

Additional observations made by Fraser, et al., addressed the mechanical degradation of bridging solids and the incorporation into the DIF of formation solids during the drilling process, both of which caused the particle size distribution to deviate quickly from the presumed ideal.²

The EC study concluded that for efficient bridging to occur, solids should have diameters ranging from about 0.2 to 2 times that of the average rock pore size. Varying the particle shape was also found to have a significant effect on both the spurt loss and the steady-state filtration rate.

Although it was not included in this study, the question of minimum bridging-material loading in relation to effective performance needs to be addressed. While there are occasions when the density requirement dictates the carbonate loading, there are other occasions when it is necessary to find the best compromise between achieving effective bridging and minimizing increases in density.

The industry norm seems to have moved from minimum treatments of about 50 ppb to about 25 ppb. In principle, the minimum treatment required to achieve effective bridging is felt to be optimum, particularly if the intention is to use an acid-based cleanup.

Starches

In water-based DIFs, starches of one form or another are commonly used as filtration-control additives. Starches normally work in conjunction with the bridging materials to form an efficient seal.

By cross-linking the molecules, filtration control efficiency increases, and it is often possible to use such products to augment bio-polymers to develop the much sought-after low shear rate and suspending fluid performance properties.

Further advantage has been found by reacting the starch in order to attach substituent groups. This can have the effect of rendering the products immune to attack by common bacteria and enzymes, which can also have negative connotations for subsequent cleanup. However, some of the substituent groups are sensitive to salts, most particularly those containing divalent cations. Matching filtration control additives to the base salinity may therefore be critical.

Viscosifiers

Finally, perhaps the most obscure and least publicized of the factors influencing cake formation is the viscosifying mechanism. The dominant viscosifiers include bio-polymers with xanthan and scleroglucan. MMH (mixed metal hydroxide)/bentonite-based adducts are also used.

Fraser, et al., showed that the unique MMH associative mechanism is capable of bridging over large pore spaces, even in the absence of conventional bridging particles.² The result was shown to be a well-ordered cake, which remained external to the matrix (Fig. 2 [35,177 bytes]).

Polymer fluids treated with appropriately sized bridging solids were found also to be capable of laying down an external cake, although the appearance was different. Externalization of the cake is an important consideration when it comes to cake removal by whatever means.

Sized salt fluids were somewhat less successful at developing an external cake, affecting the choice of clean-up methodology.

Facilitating reservoir flow

Not all reservoirs are created equal and damage to subsequent production operations, resulting from intrusion of foreign matter, can be quite variable. However, there is one inescapable fact. For production to occur, the filter cake must be breached to allow the hydrocarbon to flow.

In principle, there are two routes by which this can be achieved. First, reservoir pressure can be used to disrupt the cake, pushing the residue out into the well bore. Second, from an environment of overbalance, solutions can be used to remove at least a portion of the cake, allowing for flow across the entire length of the exposed hole section.

These approaches are fundamentally different and may induce different effects on subsequent events, for example, the retention of the flow capacity of the sand control assembly.

Back flow

Bailey, et al., studied critical aspects of the filter cake composition and structure as it relates to back flowing the cake without chemical stimulation. ³ They found that during back flow, it was possible to distinguish two distinct phases in the cleanup process.

During the first phase, as flow was initiated, the filter cake was found to fail. This was associated with a maximum pressure commonly called the cake lift-off, or more correctly, flow-initiation pressure. The flow-initiation pressure (FIP), as shown by a wide variation in FIPs with different mud types, was observed to depend on the filter cake strength and thickness.

Subsequent unpublished work carried out at Schlumberger Cambridge Research also showed a dependence on formation types supporting findings originally published by Browne, et al.⁴ A second phase of cleanup was observed in some cases as the established flow increased the ruptured zones in the filter cake, scouring out some of the invaded solids and polymer from the near-well bore pores (Fig. 3 [35,267 bytes]).

Although ultimate productivity is generally determined by the residual damage once the well is flowing freely, the flow-initiation pressure is considered to be critical in bringing the well onstream.^{3 4} If the FIP exceeds the expected drawdown, chemical breakers used to degrade mud polymers or solids will be required.

The filter cake was found to fail by either detachment from the formation (lift-off) or by rupture (blistering and pinholing), following the flow into the well bore after drawdown (Fig. 4 [17,286 bytes]).

Formation detachment

In general, formation detachment requires overcoming adhesive forces. It would appear that in this case, the filter cake is not cleanly removed from the rock. Instead, the external filter cake was parted from the internal filter cake formed within the surface pores during the initial stages of filter cake growth.

Thus, on detachment as well as in rupture, the filter cake suffers tensile failure. The forces required to fail the filter cake and initiate flow were shown to be related both to the thickness of the cake and to its yield strength. The cohesive forces between particles that make up the filter cake were found in this study to determine the strength of the cake. Therefore, they must be considered important in determining the ease of cake removal and well productivity.

Factors that contribute to filter cake strength, and which must be considered if the mud is to be designed to reduce flow initiation pressures, include not only the mud composition but also the filtration conditions in the well.

It was observed that a high differential pressure not only increased the thickness of the filter cake as it increased the filtration rate, but also affected the state of filter-cake compaction, increasing its solids content and yield strength. Drilling near balance, consistent with well safety measures, would be expected to reduce the strength of the filter cake as well as the extent of filtrate invasion.

Yield strength

Mud type was found to have a strong effect on yield strength. Systems such as MMH-bentonite with fragile gels were generally found to have lower yield stresses. Oil-based systems were also shown to have much weaker filter cakes by virtue of their weak interparticulate association and inherent lubricity.

Manipulation of the interactions in the filter cake through addition of dispersants and lubricants could be used in the field to reduce yield strength. However, these classes of additives could also adversely affect the formation damage characteristics of the filtrate, so caution is advised.

Weighting agents were shown in the study to play a key role in determining filter-cake strength and permeability. The weighting agent is usually the major component of the filter cake, and it was concluded that the particle size, concentration, and surface chemistry of this additive all have an effect on cake strength.

A compromise must be reached between minimizing yield strength and minimizing overall formation damage. The trend to lower bridging-material loadings as noted earlier will reduce invasive damaging fines while possibly creating a stronger cake, making it more difficult to remove.

Particle distribution

A balanced particle-size distribution gives more-effective packing, reducing fluid loss. It was found that yield stress increased with increasing fines content. Tight control of drill solids will be required in the field to reduce skewing the particle size distribution downward, thereby minimizing the filter-cake yield strength, filtrate volume, and fines invasion that would otherwise occur as a result of disrupted particle packing in the filter cake.

Browne, et al., who were first to study the FIP phenomenon, also noted a dependence of fluid type on the FIP for cakes laid down on substrates having the same permeability.⁴ The study showed that formation permeability had a profound effect on the ease of cleanup achieved by exclusively physical means.

This would appear to have implications for production from heterogeneous formations, or where the well intersects more than one producing formation. Back production may only initiate flow from the higher-permeability zones, leaving the filter cake on the tighter formations untouched. This could have deleterious consequences for production and any subsequent chemical treatments.

Acknowledgment

The authors wish to thank the members of the EC consortium JOF3-CT95-0020, Schlumberger Cambridge Research, Statoil, and Institute Francais du Petrole for sharing their findings with the authors, as well as Schlumberger Dowell, and Chevron U.S.A. Production Co. for granting permission to publish this article. Thanks also to Paul Reid of Schlumberger Dowell and Keith Morton of Chevron Petroleum Technology Co. for their valuable contributions.

References

1. Gray, G.R., and Darley, H.C.H., Composition and Properties of Oil Well Drilling Fluids, Fourth Edition, Gulf Publishing Co., 1980.
2. Fraser, L.J., Reid, P., Williamson, L.D., and Enriquez, F. Jr., "Mechanistic Investigation of the Formation Damaging Characteristics of Mixed Metal Hydroxide Drill-In Fluids and Comparison with Polymer-Base Fluids," SPE paper 30501, presented at the SPE Annual Technical Conference and Exhibition, Dallas, Oct. 22-25, 1995.
3. Bailey, L., Meeten, G., L'Alloret, F., and Way, P., "Filtercake Integrity and Reservoir Damage," SPE paper 39429, presented at the SPE International Symposium on Formation Damage Control, Lafayette, La., Feb. 18-19, 1998.
4. Browne, S.V., and Smith, P.S., "Mudcake Clean-Up to Enhance Productivity of High-Angle Wells," SPE paper 27350 presented at the SPE International Symposium on Formation Damage Control, Lafayette, La., Feb. 7-10, 1994.

The Authors