Lab work shows advantages of liquid resin-coated proppants

Philip D. Nguyen, Brahmadeo T. Dewprashad
Halliburton Energy Services Inc.
Duncan, Okla.

For hydraulically fracturing a formation, liquid resin-coated proppants mixed on site have demonstrated, in laboratory testing, better performance than premixed resin-precoated proppants.

Liquid-resin coating (LRC) systems exhibit higher compressive-strength levels than RCPs at the following times:

• Immediately after treatment
• During stress-loading caused by production/shut-in cycling
• During the eventual depletion of the reservoir, when the failure of the proppant pack to support the perforations and formation in the near well bore region permits proppant flowback and the movement of formation fines into this area.

Proppant packs

The initial and continued performance of a resin-treated proppant pack placed near the well bore in hydraulic fracturing is largely determined by bottom hole temperature and the dynamic relationship of the cure kinetics with formation closure.

Retention of high strength for the life of the well is a key to the economic success of screenless fracpack completions and/or flowback control in hydraulic fracturing treatments. Repeated production/shut-in cycles create stressful loading conditions that can damage weak packs.

The cure-kinetics of resin-treated proppants has a significant effect on the proppant packs' compressive-strength properties. The interrelationship between the cure kinetics and the closure of the formation is a tenuous, intricate one, at best.

Ideally, the cure kinetics should allow operators adequate time to place the proppant pack and achieve a degree of fracture closure before hardening occurs without, at the same time, requiring an extended shut-in time.

The resin should cure very little until after formation closure is completed, then harden to produce a strong, consolidated proppant bed.

Conversely, the resin must cure at a rate sufficient to provide good compressive strength even in well bores with low bottom hole temperatures. It has to do this without an extended shut-in period.

Comparable cure rates

Evidence obtained through differential scanning calorimetry indicates that RCPs cure at a much faster rate than either the high-temperature LRC system (LRC-HT) or the low-temperature system (LRC-LT).

Measurements in the laboratory were taken at 250° F. for RCP and LRC-HT and at 150° F. for LRC-LT. The resin-precoated proppant cured within 5 hr, whereas the high-temperature LRC required more than 20 hr to cure completely.

The low-temperature system did not complete the curing process until more than 24 hr later (Fig. 1 [47,651 bytes]).

Compressive strength

An extensive laboratory study examined the various factors that affect the unconfined compressive strength and stability of RCPs and the LRC system, and the comparative impact that these elements exert on the two types of resin-coated proppant packs.

Experiments focused on the effects of temperature and cure kinetics on consolidation strength and stability. Temperature ranges were selected to simulate common downhole conditions to which resin-treated proppants are exposed during fracturing and placement procedures.

Results indicated that the accelerated cure rates of RCPs substantially reduce their consolidation strength. If the RCP proppants did not achieve grain-to-grain contact within a specific amount of time after exposure to downhole temperatures, the bonding strength of the proppant pack diminished as closure time increased, in spite of high closure stresses.

In contrast, both the high-temperature and low-temperature LRC systems were capable of maintaining much higher consolidation strengths despite a lengthy time-before-closure period.

Temperature and suspension time, and their correlation to formation-closure factors, have a significant effect on the consolidation strength of resin-treated proppant packs.

In experiments testing the impact of these elements, LRC-HT slurries and various kinds of premixed RCP slurries were suspended by slight stirring with paddles to minimize shearing effects. This was done at varied temperatures and time intervals before the samples were packed and cured.

In the first study, each sample was suspended at room temperature for 30 min and at 180° F. for 30 min and 1 hr. Control samples were packed for curing immediately after preparation, with no time allowed for suspension.

All the test samples were placed in Hassler sleeves and cured at 230° F. under a closure stress of 500 psi for 20 hr. After this time, unconfined compressive strengths were determined at room temperature.

Suspension time and/or temperature appeared to influence the compressive strength of most precoated proppant samples, some significantly. The consolidation properties of the LRC-HT samples, however, remained stable, maintaining a level of around 2,000 psi throughout (Fig. 2 [61,910 bytes]).

LRC-HT consolidation strength was tested against two RCP samples to examine the effects of lower-temperature suspension (100° F.) followed by high-temperature curing (230° F.). Although the precoated proppants experienced less compressive-strength loss than in the previous study, only the LRC-coated sand registered consolidation readings above 2,000 psi, attaining a level of almost 2,500 psi in one setting.

Results from another study indicate that at low temperatures, RCPs do not perform well, nor do they possess sufficient compressive strength for use in screenless completions.

The slurries were suspended at 100° F. for various time intervals, then cured at 150° F. The LRC-LT samples exhibited compressive-strength levels consistently above 2,000 psi, compared with readings of less than 100 psi for the precoated proppants. These findings seem to indicate that RCPs perform poorly in downhole settings that fall at, or below, this temperature level.

The various test findings demonstrate that the benefits derived from the longer curing time of the LRC system are twofold as follows:

1. The slower rate works effectively within the dynamics of the cure kinetics/formation closure relationship.
2. The slower rate gives the system a tolerance to variations in temperature and suspension time.

The consolidation strength in LRC proppant packs was also found to be substantially improved by increasing the concentration of the resin coating and the length of the curing time.

Consolidation tests

Various types of RCPs were subjected to consolidation testing by suspending them at room temperature at 150° F. for various time intervals and then packing under 500-psi closure stress and curing the proppants at 200° F.

RCP-A and RCP-B lost considerable strength when suspended for time intervals of 30 min to 1 hr at 150° F. before being packed (Fig. 3 [55,451 bytes]). Furthermore, very weak consolidations were obtained with RCP-D and RCP-E (Fig. 3).

SEM exams

Scanning electron microscopy (SEM) was used to understand better the progressive loss in consolidation strength with increasing exposure to temperature and suspension time before the onset of closure with the RCPs and LRC-HT.

Slurries of RCP-A, RCP-B, and 20/40 Brady sand coated with the LRC-HT system were stirred for 0, 15, and 30 min at 150° F. and then cured under a closure stress at 200° F. for 20 hr, after which SEM photomicrographs were taken of the proppant packs.

The pictures reveal progressive decreases in the area and the number of contact points with respect to suspension time at temperature in RCP-A (Figs. 4a and b [298,025 bytes]) and RCP-B (Figs. 4c and d).

The data indicate that the cure advances caused the loss in consolidation strength of RCP-A and RCP-B before grain-to-grain contact was achieved.

Photomicrographs taken under the same conditions for 20/40 Brady sand coated with the LRC-HT system (Figs. 4e and d) show that there was no decrease in the number and area of contact points for these coated proppants.

These photomicrographs clearly show why losses in consolidation strength on exposure to temperature before application of closure were observed in the resin-precoated proppants, but not in the liquid-resin coating system.

The RCPs have faster cure rates than LRC-HT, and when pump times are extended or closure is delayed, RCPs exhibit considerable losses in consolidation strength.

Consolidation without closure

Consolidation without closure was examined through the tests to simulate downhole conditions in which formation closure is slow, or in which little or no closure stress is present. This is sometimes the case in the perforations and in certain sections of fractures.

The proppant slurries used in the consolidations featured 10 ppg of 20/40-mesh RCPs and Brady sand (coated with the LRC-HT system) prepared in a hydroxypropyl guar (HPG) gel at a pH of 10.2. The proppant slurries were stirred in separate baths at 150° F. for 30 min, after which they were allowed to settle and cure for 20 hr at 200° F. without confinement. For the controls, the proppant slurries were stirred for 30 min at room temperature before being allowed to settle and consolidate without confinement.

Core plugs were made from the cured proppant packs, and the compressive strengths were determined at room temperature. Test results are shown in Fig. 5 [56,919 bytes]. The LRC-HT system achieved consolidation strengths significantly higher than those of the RCPs, both at room temperature and at 150° F.

These findings indicate that in a propped fracture with a slow closure rate, or in perforation tunnels having little or no closure, LRC-HT would yield superior-strength consolidation. Conversely, since the RCPs exhibited much lower compressive strengths than the high-temperature LRC, especially at room temperature, the tests also showed that these precoated proppant systems would perform poorly in slow or minimal-closure formations.

The LRC-HT resin system demonstrated superior performance over the resin-precoated proppants because of the following factors:

An inherent capillary action promotes the flow of the liquid resin between proppant grains; consequently, the concentration of the resin at contact points is greater.

LRC-HT is "tacky," and this characteristic promotes grain-to-grain coherence. In contrast, the RCPs are less tacky, even when heated, and have little grain-to-grain contact without closure stress.

The LRC-HT system has a slower cure rate than the rates of RCPs.

The LRC-HT resin is not removed from the proppant surface during stirring (simulated pumping) because the system has been specially formulated to coat proppants preferentially in gel. The resin used in the RCPs, on the other hand, has been shown to leach off into the fracturing fluids.¹²

Stress cycling

In addition to its success rate in fracturing treatments, a high-performance proppant pack has the capacity to withstand stress-loading caused by the cyclical occurrence of productive periods and shut-in times.

Confined cyclic-loading tests were conducted on the high-temperature LRC material and various RCPs under simulated loading and unloading conditions using common stimulation and production practices. Samples were placed in a cell with a confining stress level of 1,000 psi and an axial load that was increased from 1,100 to 3,850 psi.

The highest axial level was maintained for 10 min, after which the load was reduced to its original level and maintained for another 10 min. This process was performed 20 times.

The LRC-HT exhibited the least amount of permanent alteration, and less creep than the RCPs. The precoated proppant samples, in fact, did not have sufficient consolidation strength to survive even the first cycle.

Reservoir depletion

The reduction in drawdown during a fracturing treatment deters sand production early in the life of the reservoir; but as depletion of the reservoir progresses, additional stresses are imposed on the formation, finally reaching a point at which any drawdown will initiate failure and sand production from the formation. When this happens, the mechanical failure of the proppant pack in the near well bore region is the primary cause.

Once the proppant begins to produce out of the perforation tunnel, the formation starts to lose its support and to create fines. Furthermore, in a screenless frac-pack completion, no screen or annular gravel pack exists to sustain the external, or perforation, pack.

For a screenless completion to succeed throughout the life of the well, the perforation tunnel leading from the well bore to the fracture, along with its supporting proppant pack, must remain stable during production and drawdown. These two areas, however, are the most vulnerable, and therefore, the site of the initial failure, if there is one.

The first purpose of the consolidated proppant pack is the prevention of proppant flowback, a task requiring relatively little consolidation strength. When stress-loading caused by reservoir depletion begins, however, this role is converted into the support of the perforation and formation in the near well bore region, a function demanding substantially more consolidation strength to withstand the added stress.

Failure in this region initially results in proppant flowback, followed by the exodus of formation fines from the fracture.

Studies indicate that failures caused by depletion-associated drawdown do not occur in perforations located in areas having rock strengths in excess of 2,200 psi.³ Considering this fact, the likely source of unsuccessful screenless frac-pack completions is the disintegration of the proppant pack because of stress loading.

With compressive-strength levels consistently recorded at the 2,000-psi mark and above in the aforementioned tests, and with an obvious capacity for remaining stable in spite of environmental variations, the LRC system appears to be ideal for helping to supply this needed support.

On site customizing

A major advantage of the on site LRC system is the fact that the resin is formulated for the specific job, which allows for customizing the mix for the downhole conditions of a particular formation.

For temperatures below 180° F., resin and a hardener, or curing agent, are mixed at a fixed ratio and injected into the blender tub that contains the proppant slurry. During the treatment, this process requires special metering equipment to ensure that the correct ratio is obtained and that the proper concentration of resin has been metered to the frac blender. The proppants are then continuously coated in the blender during the fracturing operation.

For temperatures above 180° F., a different mixture of resin and hardener can be prepared and stored successfully for an extended time, to be used when needed. This system does not require special metering equipment.

RCPs, on the other hand, are precoated with a partially cured phenolic resin and brought to the well site as bulk sand. Operators use conventional equipment to mix the proppants with the fracturing fluid and pump them into the well bore.

Because they have a single, predetermined formulation, these RCPs are not as versatile as the LRC system and perform at an optimal level only under certain conditions.

References

1. Norman, L.R., Terracina, J.M., McCabe, M.A., and Nguyen, P.D., "Applications of Curable Resin-Coated Proppants," SPE Production Engineering, November 1992, pp. 343-49.
2. Almond, S.W., Penny G.S., and Conway, M.W., "Factors Affecting Proppant Flowback with Resin-Coated Proppants," Paper No. SPE 30096, European Formation Damage Conference, The Netherlands, May 15-16, 1995.
3. Morita, N., Burton, B., and Davis, E., "Fracturing, Frac-Packing and Formation Failure Control: Can Screenless Completions Prevent Sand Production?" Paper No. SPE 36457, Annual Technical Conference and Exhibition, Denver, Oct. 8-10, 1996.

The Authors