By Petroleum Technology Transfer Council

Lightweight proppants in slick water fracs boost flush output, cut post-frac decline

Early efforts with slick water treatments using lightweight proppants have proven to be equally successful for infill wells and recompletions in a mature reservoir.

Gary Feist, Cimarex Energy Co. Midland, Texas, and Mark Malone and Randy Kuiper, BJ Services Co., Midland, Texas

Over time, various hydraulic fracturing treatments have been favored in the Southwest Westbrook Unit’s Clearfork reservoir in Mitchell County, Texas, depending on various operators and stimulation technology evolution. There is a natural tendency to stay with the “tried and true,” as regards both the technology and the service provider.

While results with this philosophy may be acceptable, they are not necessarily optimum. Beginning in 2004, experimentation with slick water treatments using lightweight proppants began. Results showed that flush production following treatment was increased, and post-frac production decline was less severe. This applied to both new infill wells and recompletions.

Success in 2004 led to continued development in 2005 and 2006, and plans for 2007. Exploitation through 2007 should boost unit recovery by 6.6 million bbl of oil. Individual well incremental reserves, ranging from 35,000 to 50,000 bbl, are quite attractive, even with very conservative economics. Positive economic results have led Cimarex Energy to keep the property for its own development, rather than accept strong purchase offers.

GEOLOGIC OVERVIEW

Westbrook (Clearfork) is the Permian basin’s discovery field. It was found in 1920 on the Eastern Shelf in northwestern Mitchell County. The first well pumped 10 bopd from an openhole completion in the Upper Clearfork after shooting with nitroglycerine. Main pay in the 3,000-ft Lower Clearfork was found when this well was deepened in 1921. Three large secondary units now account for most Clearfork production.

The field is a stratigraphic trap, dipping west at 60 ft/mi. The Permian-age Upper Clearfork zone is a lenticular anhydritic fine-crystalline dolomite with occasional small vugs and rare fractures. The overall Upper Clearfork interval is around 560 ft thick, and continues to thicken toward the basin. Pay porosity ranges from 4% to 16%, with the average being 7.4 %. Permeabilities average around 0.5 md, but occasionally reach 50 md. Gypsum and silty laminations are common in the Upper Clearfork environment, indicative of a low-energy tidal flat or lagoon.

The Middle Clearfork zone has better porosity development and reasonable well-to-well correlation. Average dolomite porosity is around 8%, with average permeability of 1 md. The initial Middle Clearfork reservoir pressure was 1,100 psi, and it produces 25° API sour oil. Although an apparent oil/water contact exists around 1,000 ft, the Middle Clearfork pay produces by limited solution gas. Core interpretation suggests that the Middle Clearfork is a series of stacked shoal complexes. Breakthrough of fracture treatments and injection water along a N 75° E bearing to the adjacent 10-acre well occurs frequently, due to an East/ West preferential permeability trend.

BRIEF HISTORY, SOUTHWEST WESTBROOK UNIT

By 1964, 99 wells were on leases that would later be incorporated into the Southwest Westbrook Unit. The unit was formed in 1969 and operated by Union Texas. In its final form, the unit covers 3,833 acres, with calculated original oil in place of 61.6 million bbl. Union Texas immediately drilled injectors and converted producers to injectors to initiate injection. There were 26 injectors on 160-acre spacing.

Beginning in 1971, the unit’s south end was infill-drilled on a 40-acre five-spot pattern. This continued until 1986, when the 40-acre five-spot pattern in the unit’s north end was implemented in areas thought to be productive. In 1991, Meridian (Burlington) took over as operator and drilled, on 10-acre spacing, selected areas that its studies indicated to be the most productive. The injection pattern then became an east – west line drive, due to an east – west preferential permeability trend that was noted in several areas of the unit as it was down-spaced.

Gruy purchased the unit from Burlington in 1997 and continued to drill the 10-acre locations in what were thought to be the more productive, less risky areas. In 2004, a new team recommended a program of higher-risk drilling and recompletions, and it was initiated that same year. Cimarex bought the property from Gruy in June 2005 and continued this methodology at a much accelerated rate. Cumulative output is now 18.1 million bbl of oil.

THE SOLUTION

In 2003, a new team appointed to study the unit’s upside was presented with a pre-approved program that was carried out as directed. The new team was not familiar enough with the idiosyncrasies of the Clearfork formation within this unit. Thus, it was unable to make changes to the proposed drilling or recompletion candidates, other than to experiment with various forms of stimulation to optimize well productivity. It should be noted that this program was an experiment to improve conformance by drilling the proposed injectors on five-acre spacing north-to-south, and 20-acre spacing east-to-west, to make the east – west permeability trend work to improve recovery.

One of the shortcomings of this area’s Clearfork floods is that the injection-to-withdrawal rate averages 3.5 to 1. Since the Southwest Westbrook Unit’s well count in 2005 was 109 producers and 66 injectors, this is an uphill battle for operators trying to improve sweep efficiency.

During the Meridian and pre-2003 Gruy tenures, stimulation of the Middle Clearfork zone included small acid breakdowns, followed by cross-linked sand fracture treatments diverted with graded rock salt, and some surged acid treatments. Before Gruy became operator, producers and injectors were sand fracture-treated. However, Gruy was concerned about the ability to control water placement in the injection wells by using hydraulic fracture treatments.

Therefore, the firm looked at other stimulation forms that might keep the water in the productive zones and minimize the chances of stimulating into the aquifer or into so-called “thief zones” associated with wells in the east – west permeability trend. These thin (2-to-4-ft), high-permeability zones in injection wells could potentially take all of the injection water and immediately water out the producer to the east or west of the injector in an incomplete injection row.

With this in mind, and the lackluster success of the surge acid jobs previously attempted, the team tried simple acid stimulations, chemically retarded acid stimulations, foamed CO₂ cross-linked acid fracture stimulations, and foamed CO₂/ acid fracture stimulations. Because nine of the 11 wells to be drilled in 2003 were slated to be injectors, the decision was made to produce them prior to putting them on injection.

Results were disappointing, with most wells producing at an initial flush rate of 25 bopd and quickly declining to 5 to 8 bopd. Injection rates were equally disappointing, once the wells were put on injection. Most would only take 75 to 100 bwpd initially, and then decline to 0 to 40 bwpd in the unit’s northern end. However, the unit’s southern end has better reservoir characteristics, and most injectors in that area are able to take 300 to 400 bwpd.

Burlington’s previous work was reviewed, along with that of the offset operators. Two things became apparent. First, mud logs on previously drilled wells showed that there were more Upper Clearfork zones that could be exploited. Second, the team noticed that offset producers had developed both the Middle Clearfork and Upper Clearfork zones on leases offsetting undeveloped acreage, in the unit that had been previously written off as not having economic productive capacity. A drilling and recompletion program was put together and submitted to Gruy’s management to exploit these areas.

In the interim, the team learned about BJ Service’s LiteProp fracturing technique. LiteProp is a proppant used in hydraulic fracturing that has a specific gravity of 1.25 and a closure range tolerance up to 5,000 psi. When it is pumped in a near-saturated brine water with a specific gravity of 1.20, the result is a near-buoyant proppant. This buoyancy allows the proppant to be placed farther out in the induced hydraulic fracture without viscosity-enhanced fluids for proppant transport.

These non-gelled or slick brine water fluids are essentially non-damaging compared to polymer-laden fluids that have been pumped historically at Westbrook. The lack of viscosity also works favorably in controlling fracture height growth development, therefore increasing the potential to stay in zone (and out of water at Westbrook), thus creating a longer effective fracture length. It was also believed that a proppant partial monolayer could be created in this low-closure reservoir to further enhance conductivity in the induced hydraulic fractures. The effects and theory of proppant partial monolayers are well documented and were first introduced to the industry in 1959.

All factors considered, the team believed the Clearfork formation to be an excellent candidate. This, combined with having no other ideas for new or innovative stimulation treatments, led to implementation of slick water hydraulic fracture treatments using lightweight proppants.

Treatments yielded 80 to 100 bopd during flush production, and declines decreased significantly after six months so that, after a year-and-a-half on production, the rate was still 20 bopd, Fig. 1. It should be noted that most of the wells that were drilled or recompleted in 2004 are in areas not supported by injection. Unit production (Fig. 2) shows output at about 480 bopd prior to initiation of the 2004 program, and peaking at ±1,200 bopd before falling off, due to lack of activity during the marketing process and a lack of injection support. The unit was marketed, but the offers were not significant enough to merit consideration. In 2005, no drilling was scheduled, although 20 recompletions were performed, with results similar to those of 2004.

Fig. 1. SWWU stimulation effect on well production.

Fig. 2. SWWU production.

As Cimarex personnel became acquainted with the Gruy properties after the June 2006 merger, they recognized the unit’s potential. The team once again evaluated the properties, based on the 2004 and 2005 programs, and developed a go-forward agenda to enhance both primary and secondary recovery of the Upper Clearfork and Middle Clearfork zones. The base case used for evaluation was the drilling of 50 wells in 2006 and 43 in 2007. The base case also included performing 43 recompletions in 2006 and 39 in 2007. At current commodity prices, the economics far exceed Cimarex’s requirements. Even at $30/bbl held flat, the project still surpasses Cimarex’s economic hurdles substantially.

Prior to kicking off aggressive exploitation, Cimarex once again tested the market. Even though a strong bid was made to purchase the property, the firm believed in the project so strongly that it declined the offer and initiated the 2006 program. Results to date have been far above expectations. Work has included six wells that have produced from 110 to 185 bopd initially, while three wells have held output above the 100-bopd level for 45 days. Planned exploitation for 2006 and 2007, which includes more treatments, should yield 6.6 million additional bbl of oil. 

THE AUTHORS
	Gary Feist is a senior production engineer with Cimarex Energy Co., responsible for the firm’s holdings in Texas, north and east of the Midland Division office, which includes numerous waterfloods. His experience encompasses drilling, completion and operational aspects, both on primary and secondary properties, with companies ranging from small independents to major producers. He holds a BS degree in petroleum engineering from Texas Tech University. Email: gfeist@cimarex.com
	Mark Malone is the Permian basin region technical manager for BJ Services Company. He has held various engineering and sales positions during his 19 years in pressure pumping services, working in Colorado, New Mexico and West Texas. Mr. Malone’s focus has been on practical application of new pumping services technologies. He is of the board of directors of the Southwestern Petroleum Short Course, an SPE member and author. He holds a BS degree in agricultural engineering from Texas Tech University. Email: mark.malone@bjservices.com
	Randy Kuiper is an executive account manager with BJ Services Company in Midland, Texas. After teaching school for eight years, he began his oil field career with the Western Company in 1981. Since then, he has worked in the Texas Panhandle, Wyoming, Colorado and the Gulf Coast region. Mr. Kuiper has held various operations, sales and management positions throughout his pumping services career. He earned a BS degree in education from the University of South Dakota. Email: RKuiper@bjservices.com