Naturally Underpressured Reservoirs: Applying The Compartment Concept to The Safe Disposal of Liquid Waste

By

Jim Puckette, School of Geology, Oklahoma State University, 105 NRC, Stillwater, OK 74078, phone: 405-744-6374, fax: 405-744-7841, [email protected] and Zuhair Al-Shaieb, School of Geology Oklahoma State University, 105 NRC, Stillwater, OK 74078

The Oklahoma Panhandle region contains abnormally low-pressure reservoirs that are isolated from the shallow hydrostatic domain and overpressured reservoirs in the deep Anadarko basin. These underpressured reservoirs, which can be identified and mapped using available petroleum industry data, are potential repositories for liquid waste. They contain naturally low pore-fluid pressures and are completely sealed by thick confining units. Many of these compartments contain oil and gas. Fluid withdrawal during production has further reduced reservoir pressures, facilitating disposal by lowering injection and displacement pressures.

Types and sizes of compartments were strongly influenced by depositional environment. Individual channel-filling sandstones within valley-fill sequences form small-compartments, whereas sandstone reservoirs formed from sediments reworked during marine transgressions became large ones. Within the carbonate domain, thick accumulations of grain-rich sediment, subsequently altered by dissolution and/or dolomitization, form large- to regional-sized compartments. Selected sandstone and carbonate reservoirs have calculated disposal volumes ranging from approximately 0.5 million barrels to 21 million barrels per well.

Compartmentalized reservoirs with abnormally low fluid pressures offer an intriguing alternative for liquid waste disposal. They exist as self-contained vessels whose seals have confined pore-fluid pressures for durations of geologic time. Seal longevity and integrity are evidenced by the very existence of naturally occurring subnormal pressures that did not equalize with the normally pressured hydrostatic environment. These reservoirs, by virtue of their compartmentalized nature, fulfill two critical criteria for safe liquid waste disposal, (1) non-migration and (2) total isolation from the sphere of human activities.

AAPG Search and Discovery Article #90010©2003 AAPG Southwest Section Meeting, Fort Worth, Texas, March 1-4, 2003