Workshops Provide Insights On Horizontals

(Tech Connections Column, July 2005, American Oil and Gas Reporter)

In a meaningful coincidence, PTTC’s regions sponsored three horizontal-oriented workshops in May.

“Cost Effective Horizontal Well Technology,” presented in Lafayette, La. (Central Gulf Region), by Bob Knoll with Maurer Technology Inc., conveyed global insights and lessons learned from Maurer’s extensive horizontal experience.

In “Horizontal Drilling, Updates on the Permian Basin,” held in Midland, Tx. (Southwest and Texas regions), various speakers provided a concise summary of Permian Basin horizontal trends and operators shared experiences in individual projects.

A third workshop titled, “Geology of Horizontal Reservoirs: A Core Workshop,” was held in Denver (Rocky Mountain Region) in conjunction with the Rocky Mountain Association of Geologists and the U.S. Geological Survey.

It is well known that effectively functioning multidisciplinary asset teams are a must for horizontal wells. Experience confirms there is a site-specific learning curve, and beginners are urged to keep it simple.” Horizontals are best designed backward; that is, establish the “where” (geology, structure), “why” (reservoir, drive mechanism, profile) and “what” (production engineering, reservoir management, EOR, completion) before designing “how” to drill the well. Horizontals require 3-D visualization, and tools are becoming easier to use and less costly every day.

The deeper the reservoir, the less incrementally expensive a horizontal is. The incremental cost of additional horizontal length within the reservoir is minimal.

Although re-entries appear attractive on first look, there are inherent downsides. Among these is the cost of well preparation, which can be prohibitive in shallow applications.

As local teams develop site-specific expertise, costs should be reduced and productivity ratios should increase.

Geologists become involved in actively geosteering horizontal wells. This goes well beyond logging-while-drilling and requires a willingness to rethink the geological model as data from multiple sources continually arrive. Completion design starts in the curve. It is recommended that the curve be drilled into the target and immediately cased in the first horizontal well in a field.

Completion designs can range from open hole to uncemented preslotted liners to cemented liners. Globally, industry is becoming more comfortable with the integrity and flexibility that open-hole completions provide. Regardless of completion type, damage is a major concern in horizontals.

As with vertical wells, cores provide answers that no other tool can. Special steps and contingencies can be employed to minimize the perceived risk of horizontal coring. Those attending the core workshop in Denver learned how important “knowing the rocks” could be to successful horizontal development.

Most of the 830-plus horizontals in the Permian Basin are in carbonates that have “good” permeability zones and low-permeability “tight” zones. Goals in good-permeability reservoirs include controlling coning or changing/improving sweep efficiency. Laterals are generally less than 1,200 feet and are completed open hole or with uncemented, preslotted liners. Reentries are more common than new wells. Laterals in tight zones are generally new wells 4,000 feet or more long. Most often they are completed with cemented liners (or heel-cemented, preslotted liners). Hydraulic fracturing is common in tight completions with a trend toward limited entry and individual zonal stimulation.

Operator case studies confirmed the learning curve. In Yates Petroleum Company’s Dagger Draw project in New Mexico, horizontals were used to access the Upper Penn reservoir (vugular porosity) at 7,700 feet, drilling from both re-entries and new wells. Yates experienced serious mechanical problems drilling two of the five curves and laterals from re-entries; the reservoir configuration necessitated windowing below the pay and approaching from below to avoid the shale above. Yates also encountered problems keeping the laterals within the desired pay zones. The laterals drilled from re-entries were not economically successful, but the two laterals drilled from new wells were.

In a Department of Energy-supported Class III project at New Mexico’s Nash Draw (Brushy Canyon Delaware Sand), playa lakes and a potash mine necessitated using horizontals with relatively long laterals. At Nash Draw there was a steep learning curve for stimulation practices. Fracture gradients were substantially higher (by 0.2 or 0.3) than those experienced in offset vertical wells. Screen-outs with more than four pounds-per-gallon proppant also were a problem. Initial production declines were steep and “crushed” sand was recovered with production. Wells were subsequently refractured and propped with high-strength material with favorable results.

In another DOE-supported Oklahoma project using laterals for Bartlesville Sand production, Grand Resources’ experience demonstrates how critical it is to understand the reservoir, its saturations, etc. The initial pilot was relocated when the reservoir was found to be too tight, and the laterals in a second pilot had to be redrilled higher in the reservoir when the initial laterals found too much water.

The good news is that although there is pain and expense going through the learning curve, answers can be found with profitable horizontal solutions as the prize. Success rates are rising as experience increases, making horizontals a viable option for field development.