Friendswood Field - A Case Study in Reservoir Management
Authors: Brady Teresa R. 
Published: December 2007

This study of Friendswood Field reviews a 60-year history of reservoir management that includes an efficient depletion of the oil column and a successful high-rate blowdown of the gas cap. Although many of today's new development opportunities are far removed from the era of Friendswood, the focused and committed reservoir surveillance and the benefits of unitized operations associated with this field serve as examples for the next generation of engineers.

Introduction

Friendswood Field (including the Webster Field Unit) is located in the upper Texas Gulf Coast salt dome trend southeast of Houston, Texas (Fig. 1). Humble Oil and Refining Company (predecessor to ExxonMobil) discovered the field in 1937.

Geology and Reservoir Description

Friendswood Field is moderately faulted with 3 main fault blocks (Fig. 2): the east fault block (EFB), the far west fault block (FWFB), and the southwest fault block (SWFB). The structure has a favorable 4° dip in the two west fault blocks (FWFB and SWFB) and 10° dip in the EFB. The Frio reservoir is Oligocene in age and begins at a depth of approximately 5500 feet. This reservoir consists of 17 sand members with over 700 feet of section (Fig. 3). The Frio in Friendswood Field is a high-quality unconsolidated sandstone, with an average porosity of 30% and permeabilities that range from 300 to 1000 md. Average connate water saturation is 25% and residual oil saturation to water drive is 12%, based on chemical tracer tests performed in watered-out zones in two wells. The reservoir is a mixed wettability rock, leading to the relatively low residual oil saturation to water displacement.

Original Fluid Distribution.

At discovery, there were common contacts in all sands. The original gas oil contact (OGOC) was 5640 feet subsea and original oil water contact (OOWC) was 6040 feet subsea resulting in an original oil column height of 400 feet (Fig. 2). Original oil in place (OOIP) as 900 million barrels, and original gas cap in place (OGIP) was 210 billion cubic feet (BCF).

Reservoir Data.

The oil gravity is 29°API and due to its very low wax content, has been in high demand for lube oil stock. The original gas/oil ratio (GOR) was 385 scf/stb. Oil viscosity is 1 cp and gas gravity is 0.6, which provide a very favorable mobility ratio and density difference for water drive recovery and gravity segregation. Reservoir drive energy was supplied from a combination gas cap expansion and water drive. There is a strong water drive in the EFB, a moderate-to-strong water drive in the FWFB, and a moderate water drive and gas cap expansion in the SWFB.

History of Oil Production

The history of oil production operations at Friendswood is illustrated in Fig. 4 with a series of schematic cross sections. The cross section runs west-east through the west fault blocks (FWFB and SWFB) to the EFB.

Original Fluid Distribution.

At discovery, there were common contacts in all sands. The original gas oil contact (OGOC) was 5640 feet subsea and original oil water contact (OOWC) was 6040 feet subsea resulting in an original oil column height of 400 feet (Fig. 2). Original oil in place (OOIP) as 900 million barrels, and original gas cap in place (OGIP) was 210 billion cubic feet (BCF).

Reservoir Data.

The oil gravity is 29°API and due to its very low wax content, has been in high demand for lube oil stock. The original gas/oil ratio (GOR) was 385 scf/stb. Oil viscosity is 1 cp and gas gravity is 0.6, which provide a very favorable mobility ratio and density difference for water drive recovery and gravity segregation. Reservoir drive energy was supplied from a combination gas cap expansion and water drive. There is a strong water drive in the EFB, a moderate-to-strong water drive in the FWFB, and a moderate water drive and gas cap expansion in the SWFB.