Oil Recovery by Imbibition Water Flooding in the Austin and Buda Formations
Authors: Hester C.T. , Walker J.W. , Sawyer G.H. 
Published: December 2007

In Texas along the Balcones fault trend, which extends from Eagle Pass to Talco, many oil fields produce from the Austin chalk and Buda limestone. These carbonate formations of Cretaceous age vary in character from tight to impermeable, with and without fractures. and with varying amounts of shale inclusion. Completions in these formations may potential at high rates where naturally occurring or induced fractures are present; but primary oil recovery is invariably low because of the loss permeability of the matrix. During the last four years, field trials of imbibition water flooding have been conducted in several wells in the Darst Creek and Salt Flat fields using wells completed in the Austin chalk and Buda limestones at depths of from 2,200 to 2,500 ft. Except for the initial and one subsequent trial, the wells have not noticeably responded to the imbibition process. The causes of failure have not been determined, but are probably due to

insufficient expulsive energy to transmit the released oil to the wellbore after water has been imbibed, uncontrolled water entry into zones of very high water saturation, and wellbore damage in the zone of interest due to solids contained in the water.

Introduction

Numerous oil fields produce from the Austin chalk and Buda limestones, along the extensive Balcones fault trend which extends from Eagle Pass to Talco, Tex. Most of the wells in these fields currently produce at near uneconomical rates. This low average productivity is caused primarily by the low permeability of the matrix of the Austin chalk and Buda lime. The result is a prevalence of dissolved-gas-drive mechanisms and reduced reservoir pressures. Ultimate primary oil recoveries are abnormally low in terms of oil originally in place, and for a long time operators have considered ways of improving recoveries. However, the only widespread technique employed so far has been acid treatment or a very large fracture using sand for a propping agent. Conventional water flooding has not been considered feasible because of the highly varying permeability, discontinuities in the formation, and naturally occurring fractures, all of which lead to low conformance. For this reason, using capillary pressure to absorb water and release oil from the tight matrices has been investigated. Field trials have been conducted in the Salt Flat and Darst Creek fields.

Reservoir and Fluid Characteristics

The Austin chalk and Buda limestones, which are normally very tight, occur along the Balcones fault trend at depths from 2,000 to 8,000 ft. They have oil-saturated porosities ranging from 8 to 30 per cent, with natural fractures sometimes occurring near fault planes. The remainder of this discussion will be limited to the Austin and Buda formations found in the Salt Flat and Darst Creek fields, located near Luling. Tex. The Austin chalk occurs at a depth of approximately 2,200 ft and is usually about 180 ft thick. The formation is interspersed with stylolites, with streaks of shale, and with many dense impermeable streaks of limestone, so the net effective thickness is only about 90 ft. As shown in Table 1 and Fig. 1. the porosity of this effective section varies from 8 to 30 per cent and averages 12 per cent. For the most part, this porosity is in the form of intergranular pores, with only a minor contribution made by vugs and fractures. Matrix permeabilities for the Austin chalk range from 0.01 to 15 md and average approximately 0.12 in the net effective pay. These low permeabilities indicated by measurements are substantiated by well performance. With the exception of a few so-called "crevice wells". Austin-chalk completions produce little if any oil until the wells are stimulated.

JPT

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