SERVICE COMPANY ALLIANCE REDUCES TIGHT SANDS FRAC COSTS

Jeffrey L. Hunter
Pennzoil Exploration & Production Co.
Houston

Stephen G. Stuchly
Halliburton Energy Services
Houston

Smaller, multiple-stage fracture treatments, worked out by an alliance between a producing and a service company, were a significant element in reducing costs for fracturing Carthage Cotton Valley infill wells in Panola County, Texas.

The 1991-1992 study 1 made by Pennzoil Exploration & Production Co. and Halliburton Energy Services, allowed Pennzoil to lower fracturing costs by 36% /well.

Pennzoil's infill drilling program takes advantage of the Texas Railroad Commission's (RRC) ruling 2 that allows optional 80-acre well spacing in this tight gas-sand reservoir. Pennzoil spudded 29 wells between September 1992 and December 1993 and expects to spud 20 more in 1994. Fig. 1 shows the 1992-1993 drilling activity.

This well schedule illustrates the significantly reduced completion time after procedures were solidified and efficiencies were realized at about the seventh month into the alliance.

The completion team coordinating the drilling program consists of two Pennzoil engineers, one Pennzoil geologist, one landman, and one technical advisor from Halliburton. Halliburton's advisor operates from an office alongside Pennzoil's project engineer in Pennzoil's Houston headquarters.

During fracturing the project engineer and technical advisor monitor the jobs via a cellular phone (voice and data) link from Houston to the on site automatic and remote control center. Changes in job design and conduct are discussed with on site supervisors and may be made in real time.

Fig. 2 shows typical data transmitted to Houston. The data can be recorded in Houston for later analysis by the team.

In the case shown in Fig. 2, engineers in Houston noticed that the job's surface pressure had increased by 200 psi and the slope on the net pressure plot had been increasing for 60 min. After discussions with Pennzoil's on site supervisor, a decision was made to increase the pump rate by 1 bbl/min at 11:58 (Fig. 2) and monitor pressure after the rate increase. The surface pressure continued to increase, and at 12:09 the slurry rate was increased by 2 bbl/min. Because the surface pressure continued to increase at a rapid pace, the job was halted.

The decision to stop pumping allowed the proppant to be displaced into the formation without screenout. This avoided having to dean out sand before the next stage of treatment.

Fig. 3, also transmitted to Houston by cellular phone, shows the increase in net bottom hole treating pressure that corresponds to the slurry rate increases in Fig. 2.

COTTON VALLEY

The Cotton Valley formation in the Carthage field is a transgressive-regressive marine sequence bounded by the Bossier shale, below, and the Travis Peak formation, above. This upper Jurassic formation is about 1,400 ft thick at depths of 8,200-9,600 ft.

The formation is divided into two major intervals: the upper Cotton Valley and the Taylor sand. Generally, the upper Cotton Valley is subdivided into several intervals. 1

Productive Cotton Valley sands usually have permeabilities between 0.01 and 1.0 md. Porosities range up to 15%, but generally are 6-8%.

Gas production from the Cotton Valley formation began in 1958 and has evolved through many innovations and experiments. Some milestones are as follows:

1958--A few wells were completed in the Cotton Valley.
1978--Large-scale drilling began as a result of Natural Gas Policy Act of 1978. Proppant (sand) concentration was about 3 ppg. Well spacing was 640 acres.
1980--A surge of drilling resulted from two rule changes. The Federal Energy Regulatory Commission (FERC) classified the Cotton Valley as a "tight gas sand," qualifying it for incentive gas prices. The RRC changed Carthage field rules to allow 320-acre spacing.
1982--Massive hydraulic fracturing became standard practice, with 2-3 million lb treatments common and record-setting jobs of about 6 million lb.
1988--The RRC changed field rules to allow optional well spacing of 160 acres. Federal tax incentives further encouraged drilling activity.
1992--The RRC again changed field rules to allow optional well spacing of 80 acres. Small, multiple-stage fracturing treatments, with strict attention to individual producing zones, became the new standard . Economized stimulation received more emphasis and treatment size decreased.

THE ALLIANCE

Under terms of the alliance, Halliburton furnishes drilling fluids, cementing, cased hole logging, perforating, fracturing, coiled tubing services, production packers, and post-fracture evaluation.

Fig. 4 shows the typical cement and casing program in a Pennzoil well. The 4-1/2-in. OD casing is run to TD at about 9,600 ft. Automatic fill equipment in the string allows the casing to fill with fluid as it is run in and helps prevent pressure surges that could cause lost circulation to weak formations. Forty centralizers are nm to centralize the casing. Centralization helps in obtaining turbulent flow and displacement of mud with cement.

Upon reaching TD, the casing is reciprocated while mud is circulated around. Then, while pumping cement, the casing is rotated. Cement pump rates of 10-12 bbl/min cause the lead slurries to go into turbulent flow, enabling good bonding between the casing and formation.

Mud conditioning, casing movement, and other good cementing practices have helped achieve cement coverage over more than 5,000 ft. These good cement jobs have contributed to the success of the stimulation program.

Fig. 5 illustrates a typical Pennzoil fracturing program. Fracturing is done in stages from the bottom up. Each fractured zone is then covered with a sand plug to create a new well bottom for stimulating the zone above.

After the last fracturing stage, coiled tubing is used to wash sand out the well and clear all zones for production. Sand plugs are preferred over bridge plugs because high treating pressures can distort the casing which possibly could cause bridge plugs to stick.

Pennzoil uses a two-dimensional (2D) fracture design program, Because the company has a large data base of after-frac logs run on more than 150 Cotton Valley wells, it inputs fracture height as a parameter rather than using a 3D program to compute height. Pennzoil's typical treatment scheme closely follows the practices recommended by the Cotton Valley Production Enhancement Team (Cvpet). The Cvpet was made up of 19 Cotton Valley producing companies that agreed to share information on drilling, completion, and production practices in the Cotton Valley formation. A typical treatment is as follows:

Four or five fracture stages per well
Fracture height of 100-300 ft
Fracture length design of 600-1,000 ft
Limited entry perforating when necessary
Treatment rate of 10-35 bbl/min down 4-1/2-in. casing
Resin-coated proppant used as tail-in near the well bore
Frac sand and resin-coated proppant tagged with different radioactive isotopes
After-frac logs for estimating actual fracture height
Sand plugs for isolating zones
Production logs for determining production contribution from each zone
Remote data transmission for real-time treatment monitoring, real-time changes during treatment, and optimization of future treatments.

As the alliance team optimized fracture treatments, several procedural improvements were made during the infill drilling program. These included:

Pad volume decrease from 30 to 15%
Late tip screenouts for better proppant packs
Controlled fracturing rates for limiting frac height to help prevent fracturing into water zones, prevent communication between reservoirs, and obtain anticipated fracture length
Maximum proppant loading decrease from 8 to 5 ppg
Resin-coated proppants for tail-in to provide high conductivity at the well bore and to help prevent proppant crushing, flowback, and displacement during sand plug setting.

Using results of the ongoing infill drilling program' Pennzoil is conducting a volumetric analysis to delineate several geological aspects of the Carthage Cotton Valley field such as definition of stratigraphy, creation of detailed maps based on stratigraphy, and computer-generated log analysis.

Data are used to calculate the original gas-in-place of each zone and producing unit in Pennzoil's areas of interest. The full scope of the infill drilling program will be realized when production and reservoir engineering data are combined with results of the geological aspects to assess remaining gas potential and distribution through the formation. 3 After-frac analysis includes running radioactive tracer logs, running production logs, evaluating fracture treatments, and analyzing actual production. Also analyzed are the production characters such as producing zones, decline rates, and cumulative production.

ALLIANCE

The Pennzoil-Halliburton alliance began in September 1992 for the purpose of drilling and completing Cotton Valley infill wells through 1993. The two companies share the cost of new technology development, with Pennzoil providing the rig time to test Halliburton technology. To date, the alliance has experimented with anelastic strain relaxation, a six-arm extensometer, and a water-recovery surfactant.

Because of the results achieved, the project has been extended an additional year.

Some of the features of the alliance are:

Halliburton guarantees the availability of crews and equipment to meet Pennzoil's drilling and completion schedule.
Halliburton's technical advisor studies existing wells to find candidates for workover or refracture.
The technical advisor analyzes, plans, and evaluates the ongoing program.
The alliance is not rigidly structured, and other service companies perform part of the work.

Both parties have benefitted financially from the alliance and well performance has met or exceeded expectations. The alliance has enabled Pennzoil to stay on a rigid and aggressive drilling schedule and through efforts of the alliance, fracture orientation has been confirmed.

REFERENCES

Hunter, J.L., et al., "Cotton Valley Production Enhancement Team Points Way to Full Gas Production Potential," Paper No. SPE 24887, 67th Annual Technical Conference and Exhibition of SPE, Washington D.C., Oct. 4-7,1992.
Railroad Commission of Texas Oil & Gas Division, Docket No. 6-96,060, Commission-Called Hearing to Determine the Effectiveness of Temporary Field Rules for the Carthage (Cotton Valley) Field, Panola County, Tex.
Hunt, Ercil, Texas Railroad Commission Docket No. 6-96060: TRC Hearing to Determine the Effectiveness of Temporary Field Rules for the Carthage (Cotton Valley) Field, Panola County, Tex. Exhibits and Testimony of Ercil Hunt, February 1992.