VaporFrac treatments minimize the fluid pumped during the frac, which also significantly reduces the equipment required on small, remote locations. (Images courtesy of BJ Services.)

Nearly all shale reservoirs require hydraulic fracturing to stimulate economic gas production. However, the fluids that are often used in these treatments can cause problems after the frac.

For fluid-sensitive and low-pressure formations, a new technique for energized fracture stimulation has shown potential for enormous benefits by enabling production optimization with minimal post-frac cleanup.

Eastern US shale gas

The Big Sandy gas field is a productive field dominated by the Devonian Ohio shale. Shale thickness ranges from zero on the western edge to more than 3,600 ft (1,098 m) at the eastern part of the field, typically running 200 to 1,600 ft (61 to 488 m) thick in the productive Kentucky and West Virginia areas. Shale depth ranges from surface to 4,000 ft (1,220 m). More than 25,000 wells are producing gas from this field.

Typically, eastern Kentucky and western West Virginia wells commingle production from several zones, including the Berea tight gas sands and the Ohio shale’s Cleveland and Huron formations. The shales, in particular, are classified as having extremely low pressure and permeability. The tight sand and shale formations require fracture stimulation for economic production. Post-frac commingled production typically ranges from 20 to 500 Mcf/d.

A number of stimulation designs have been used in the area over the years in an effort to optimize hydrocarbon recovery. The most successful balance of stimulation cost vs. production enhancement has come with high-velocity nitrogen fracs or nitrogen foam fracs using large volumes of proppant. Reservoir pressure provides guidance as to which type of stimulation is likely to provide the best result.

Historically, typical fracs in the Lower and Upper Huron intervals have used varying foam quality with 120,000 lbs of 20/40 mesh sand, 46,000 gal of nitrogen foam, and 7,350 gal of liquid frac fluid per stage. Foam quality begins at 90% for the pad phase and drops to 85% as proppant concentration increases. Proppant concentration is ramped from 1.0 to 4.5 ppa at rates from 40 to 48.2 bbl/min.

Studies in 2002 and 2003 concluded that production was better in area gas wells stimulated with nitrogen and proppant than those stimulated with nitrogen alone (MacDonald et al., SPE 77464 and SPE 84834). The latter study also concluded that cleanup effectiveness was critical to maximizing net present value.

Lighter proppant

Unfortunately, the low-pressure formations in the area tend to unload frac fluids slowly and inefficiently. In addition, the area’s terrain does not lend itself to stimulation operations; well pads typically are remote, difficult to reach with equipment, and compact, at best (Figure 1). In 2004, to improve both well cleanup and logistics, a major operator in the area began using LiteProp ultra lightweight proppants (ULWPs) in place of frac sand in some wells.

ULWPs have much lower specific gravity than conventional proppant, reducing their settling rate in water and providing unprecedented proppant transport and longer effective frac length. This transportability allows the creation of proppant partial monolayers. Compared with conventional multilayer proppant packs applied typically at 1 to 2 lbs/sq ft, partial monolayers can provide superior fracture conductivity with much less proppant (Darin and Huitt, SPE 1291).

The ULWP frac designs typically use 6,000 lbs of 14/30 mesh ULWP followed by 4,000 lbs of 18/12 mesh ULWP with 48,000 gal of nitrogen foam and just 6,846 gal of liquid frac fluid per stage. Foam quality was designed to be 90% for the pad and 85% during the proppant phase. Proppant concentrations were .25 ppa, with the job pumped at 40 to 40.5 bbl/min. during the pad and proppant phases.

In a 2005 study of five wells treated with LiteProp proppants and offsets treated with conventional sand, production in four wells was equal or higher in the ULWP-frac’d wells (Kendrick, Puskar and Schlotterbeck, SPE 98006). Cleanup time was reportedly reduced and frac sand production virtually eliminated. The reduced proppant volume eliminated the usual large proppant bins and reduced the number of frac tanks required. The operator also saved about 8% on pumping services charges.

Keeping the well dry

Advancing the technology further, the most recently introduced ULWP is LiteProp 108, which has a specific gravity of just 1.056. This makes the proppant nearly buoyant in water, leading to new options for transporting and blending it during frac operations. For smaller treatments and those using energized fluids, conventional on-the-fly blending with dry proppant can be replaced with a pre-mixed proppant-bearing Liquid LiteProp slurry in a non-damaging slickwater fluid. Even compared with prior ULWP fracs, this approach reduces the equipment and personnel required on location while ensuring that proper proppant concentrations are pumped.

In low-pressure and water-sensitive formations, Liquid LiteProp technology also provides a safe and efficient means to use ULWPs in fracs with very little water. In VaporFrac operations, Liquid LiteProp slurry is pumped directly into a high-pressure nitrogen or carbon dioxide gas stream. Energized fluid qualities range from 93% to 99% of the total fluid volume; the remainder comprises the Liquid LiteProp slurry. The technique virtually eliminates post-frac cleanup time and water disposal costs.

The VaporFrac treatment requires no frac tank rental, water hauling, chemicals, or water heating. It also eliminates the need for a sand hauler and a proportioning unit; instead, the proppant slurry is added to the nitrogen stream with a small acid pump. This enables operations in a much smaller footprint and fewer personnel on location than typical fracture stimulations.

Kentucky case histories

In early 2008, two vertical wells near the Kentucky/West Virginia border were stimulated for Equitable Production Company using the VaporFrac process.
Well A was drilled to 4,100 ft (1,250 m) in Perry County, Ky., and completed in the Berea sandstone and in the Cleveland and Upper and Lower Huron shale formations. Well B was drilled to 5,400 ft (1,647 m) in Mingo County, W.Va., and completed in the Berea sandstone and in the Upper and Lower Huron shale formations.

The optimum areal proppant concentration range for a partial monolayer of 14/40 LiteProp 108 proppant is .01 to .04 lbs/sq ft. The rule of thumb for VaporFrac treatments to date has been to reduce the traditional frac sand requirement by a factor of ten. The VaporFrac treatment design used 5,080 lbs of 14/40-mesh LiteProp 108 proppant with 50,900 gal of nitrogen and just 2,005 gal of liquid frac fluid per stage. Foam quality was designed to be 100% for the pad, dropping to 94% during the proppant phase. Proppant concentrations were ramped from .05 to .20 ppa with the job pumped at 50 bbl/min.

To assess the utility of the VaporFrac technology, production from the wells was compared to offsets in the field, normalized for days post-stimulation (Figure 2). The offsets for Well A are vertical wells completed and stimulated in 2007 and 2008. The offsets for Well B are predominantly horizontal wells completed and stimulated between 2001 and 2006, and some of them include commingled production from additional formations.

For Well A, the 30-day cumulative production was nearly twice (188%) the average cumulative production of 11 offsets. At 30 days, Well B was producing formation fluids with the aid of surfactants for artificial lift; nevertheless its 30-day cumulative production was more than twice the average cumulative production (259%) of the 15 offsets provided by the operator for comparison.

As this article went to press, BJ Services was in the midst of a seven-well horizontal VaporFrac treatments campaign in the area.