Abstract

The benefits of horizontal fractures to steamflood performance of shallow, heavy oil reservoirs are demonstrated through reservoir simulation. Two cases were considered:

  1. an unpropped fracture to improve mobility of oil ahead of the steamfront, and

  2. a nearly impermeable (cement-slurry- filled) fracture to improve vertical sweep efficiency.

Horizontal fracturing in shallow reservoirs (<1,000 ft) has been previously demonstrated.

Reservoir data for the low thickness areas of the Slocum (Texas) field were used to evaluate the potential of the unpropped fracture. On initiating steamflooding, the steam was injected above formation parting pressure while the oil production rates remained high. The fracture was introduced at the top of the reservoir (i.e., away from the underlying water zone). Subsequent steam injection pressures were below the fracture reopening pressure. The results were compared to those of steamflooding where pressures were always maintained below the formation parting pressure. Steam soaking the formation with the fracture accelerated the recovery process. For the 2.5-acre pattern, project time decreased by 40%, cumulative oil steam ratio (COSR) increased by 20%, and production cost decreed by 16%. Production costs (per bbl of oil) for formations with and without underlying water zones were $13.80 and $12.30 when produced-water heat and 5-acre patterns were used.

Published data for a thick Kern River (CA) reservoir were used to evaluate the benefit of the impermeable fracture. The fracture was introduced into the reservoir, immediately below the elevation of the steam override zone, after 5.6 years of steamflooding The steamflooding was simulated to continue in the zone below the fracture for four years, and the results were compared with those of the low-quality steamflood conducted in the field over the same period. Steamflood performance improved significantly. Oil recovery increased by 10% OOIP (30% of no-fracture production). mainly because of higher temperatures and steam saturations in the layers below the fracture.

Introduction

About 122 million bbl heavy oil resource exists in the Slocum (TX) and Camp Hill (TX) reservoirs of the Carrizo formation in the U.S. Gulf Coast region. Predicted reservoir performances of steamflood operations in the thin undeveloped areas of Carrizo reservoirs are poor. Field steamflood performance of Groves Edison (CA) field where the sand thickness is 18 ft and permeability is 1,300 md was poor. A low cumulative oil-steam ratio (COSR) of 0.05 was observed for a 5-spot pattern, 5-acre area test site with steam injected at high rates (800 to 1,000 B/D). Closer spacing cannot be economically justified in thin reservoirs because the revenue generated would be insufficient to meet the high capital costs. Advanced technology is needed to improve recovery from this type of reservoir.

Currently, most of the heavy oil production in the United States comes from steamflooding of excellent quality reservoirs, such as the Kern River reservoirs (CA), where permeabilities and thickness are high >500 md and >35 ft, respectively). Although gravity drainage reduces residual oil saturations to very low levels in the steam override zone, one major problem with this type of reservoir is the low vertical sweep efficiency.

This paper describes engineering assessments for introducing horizontal fractures during steam injection EOR in the Slocum (TX) and Kern River (CA) reservoirs.

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