Advanced hydraulic fracture diagnostics optimize development in the Bossier sands: a series of studies for Anadarko Petroleum on the Bossier formation optimized frac treatment designs and improved well economics. Two-stage frac jobs resulted in better output than single-stage efforts

World Oil, July, 2005 by Stephen Wolhart

Anadarko Petroleum Corp. has conducted a series of studies to improve field development and stimulation practices in the Bossier formation of East Texas. Advanced hydraulic fracture diagnostics were used to better understand fracture geometry and well performance. The objective of the diagnostics was to measure hydraulic fracture geometry, improve estimates of propped fracture length and optimize fracture designs.

Diagnostics included microseismic and surface tiltmeter fracture mapping, radioactive tracer logging, production logging, fracture modeling, pressure buildup testing and production data analysis. As a result, changes have been made to completion and stimulation design, resulting in improved well performance.

BOSSIER OVERVIEW

The Bossier play is on the western flank of the East Texas basin, Fig. 1. Bossier wells generally produce dry gas from overpressured sands contained within the Bossier shale. (1) Productive sands are found at depths from 12,000 to 15,000 ft and generally occur in the upper 500 to 600 ft of the Bossier shale.

[FIGURE 1 OMITTED]

Five separate sand units (Taylor, Shelly, Moore, Bonner and York) have been identified. They are found as lenses, segregated pods and channel trends. Reservoir heterogeneity is high (Fig. 2), which shows logs from three wells in Dowdy Ranch field. The APC Anderson 2 and APC Anderson 3 wells are only 495 ft apart, but the thickness of the Bonner and York sands changes significantly between these wells.

Porosity in these sands varies from 6% to 20%. Permeability ranges from 0.001 md to 1 md but is usually below 0.1 md. The upper three sands, Taylor, Shelly and Moore, are typically of lower reservoir quality than the Bonnet and York. Gas has been produced from the Bossier since the 1970s, but current activity began in 1996 and ramped up in 1998. Anadarko is the most active operator in the Bossier, but other operators work this area, as well. All producing wells drilled in the Bossier are hydraulically fractured. Stimulation is key to the play's economics.

Water fracs and hybrid fracs are common stimulation methods in many low-permeability reservoirs. These treatments involve creating fractures, using slickwater rather than cross-linked gels. The proppant is carried into the fracture, either with a gelled fluid (hybrid frac) or slickwater (water frac). The theory is that low-viscosity pad fluids result in longer, more confined fractures. However, low fluid viscosity can result in poor proppant placement, and narrower fractures can cause proppant bridging. Both water fracs and hybrid fracs have been used in the Bossier.

There have been five stages in the evolution of frac design in the Bossier:

* Early wells were stimulated with conventional cross-lined gel and sand treatments. These wells produced about 12,000 cfgd per net foot of pay during the first six months online. Job cost ranged from $200,000 to $350,000. Other treatments were evaluated, due to marginal well economics.

* The next set of wells was treated with water fracs without proppant. Output rate for these wells was initially higher, but rates dropped quickly as the fracture began to close. Overall, well economics did improve with these inexpensive ($50,000-to-$100,000) treatments.

* Wells were then treated using water fracs and 20/40-mesh sand as proppant. Output rates increased, and treatment costs ranged between $100,000 and $150,000.

* The next evolution was to pump water fracs with 40/70-mesh sand as proppant. Proppant and slickwater were pumped in alternating stages. Total proppant volume was typically 200,000 lbs. Well economics improved again, with long-term production averaging 16,000 cfgd per foot of net pay. Costs were similar to previous water frac treatments with 20/40-mesh sand.

* The final treatment type has been the hybrid frac. Slickwater is pumped first to generate length, followed by a cross-linked gel pad and then by the proppant stage with 20/40-mesh sand with cross-linked gel. Cost for these jobs typically ranges from $175,000 to $225,000. These wells have long-term output rates of about 18,000 cfgd per foot of net pay.

HYDRAULIC FRACTURE MAPPING

Pinnacle Technologies performed microseismic imaging to measure overall fracture geometry on four wells during 2001 and 2002. Two of the mapping wells were stimulated with water fracs, and two were stimulated with hybrid fracs. Microseisms are micro-earthquakes induced by changes in stress and pressure associated with hydraulic fracturing. (2) These earthquakes are slippages that occur along pre-existing planes of weakness (e.g., natural fractures) and emit seismic energy that can be detected at nearby seismic receivers.

With an array of tri-axial receivers situated at depth, near the hydraulic fracture, compressional (primary or P) and shear (secondary or S) waves can be detected. The location of any individual microseism is deduced from arrival times of the P and S waves (provides distance and elevation) and from particle motion of the P-wave (provides azimuth from the receiver array to the event).