Physical Changes of Reservoir Properties Caused By Subsidence and Repressuring Operations
Authors: Allen D.R. 
Published: December 2007

The surface of the Wilmington oil field has now subsided as much as 29 ft at the center of an elongated depression roughly coinciding with the field productive area. To determine the compacting intervals, a method was developed to detect changes in length of individual casing joints. Both reservoir estimates and casing joint measurements indicated a pore volume loss of at least 3 porosity percent. Casing joint measurements, also were used to defect casing elongation in zones of water injection. The vertical expansion of the reservoir is also seen on the surface as uplift that now amounts to, as much as 8 or 9 in. in the areas of heaviest injection. Rate-pressure data from injection wells in Wilmington plot as a transitional curve on coordinate paper rather than showing a sharp change of slope as is normally seen when overburden pressure is exceeded. Engineers working in the field and familiar with the unconsolidated nature of the formations could not reconcile the rate-pressure performance with formation fracturing. By interrelating the zonal expansion and surface uplift with the rate-pressure curves and the radial flow equation, it was concluded that the formations experience a change in pore volume and permeability as pressures are increased.

Introduction

Wilmington field is located near the southwestern edge of the Los Angeles sedimentary basin. The structure forms a major part of the anticlinal trend that extends about 20 miles from Torrance field to the Huntington Beach offshore pool (Fig. 1). Wilmington produces from seven zones of Pliocene and Miocene age, spanning depths from 2,000 to 6,000 ft. The upper four zones, extending to a depth of 4,000 ft, are the intervals of primary interest (Fig. 2). They consist of arkosic sands, siltstones and shales with sand percentages varying from 25 to 65. The sands are unconsolidated and usually contain varying amounts of interspersed shale and silt material. Porosities range from 33 to 37 percent and permeabilities average 500 to 2,000 md in the various zones.

The problem of surface subsidence over Wilmington field has been well publicized and has attracted widespread interest. The surface now has subsided as much as 29 ft at the center of an elongated depression roughly coinciding with the field productive area (Fig. 3).

Much of the field is overlain by valuable industrial property in the Long Beach harbor district and by the City Civic Center. Surface subsidence in these areas caused millions of dollars of damage and posed a threat of inundation to the Long Beach Naval Shipyard, the harbor and the civic center. Concurrent with the construction of dikes and sea walls, the city and the other operators and landholders in the area of subsidence launched a program to determine the causes and possible remedies. The result is the largest waterflooding program in the world. Current field injection is about 800,000 B/D with ultimate plans calling for 1.5 million B/D. With this program in effect, subsidence now has been either stopped or reduced to about 1 in./year. In the areas of maximum repressuring the surface has regained some of the former loss of elevation.

Problems Related to Reservoir Changes

During both early field development and later waterflooding stages, it was evident that not only were surface elevations affected, but also physical changes were occurring in the reservoir. Changes of reservoir characteristics were indicated by measurements of subsurface pressures, plots of the relative permeabilities of gas and oil, and water injection rates vs injection pressure.

During early development, when the surface was subsiding rapidly, many studies were conducted to determine the reservoir mechanism of compaction. Most observers studying the problem related subsidence to compaction in the oil zones-compaction caused by a pressure reduction concurrent with the withdrawal of reservoir fluids.