Abstract

The successful design and implementation of any improved oil recovery project in a fractured reservoir depends on an accurate characterization of the fracture system. This is especially true in a steam pilot project currently underway in the Yates Field of West Texas. This pilot will assess the economic viability of accelerating gravity drainage in the gas cap of the fractured San Andres reservoir. From the conceptual phase of the project through implementation and monitoring, fracture characterization in the pilot area has been critical to pilot design and success. Key decisions have depended on an accurate assessment of fracture density, orientation, flow capacity and connectivity to other portions of the reservoir. Many geologic and engineering methods have been employed to understand the fracture system. Flexure mapping, tracer testing, pressure interference testing and reservoir simulation were employed in the design phase of the project. Fluid sampling and passive microseismic monitoring have been employed to monitor the project. This paper will discuss each of these methods, field results, and key decisions that were based on the analyses.

Introduction

The Yates Field is located at the southern tip of the Central Basin Platform in the Permian Basin of West Texas (Figure 1). The majority of production comes from a series of stacked carbonate shoals in the San Andres formation at an average depth of 1500 feet1. Reservoir quality dolomite in the Yates Field has average porosity of 15% with average matrix permeability of 100 millidarcies. Reservoir flow is dominated by fractures caused by deformations and surface exposure in the geologic past. Fracture porosity averages 2% with permeabilities exceeding 1 darcy. The dominant mechanism for oil recovery is gravity drainage, employing the Double Displacement Process. This process has been defined2 as the gas displacement of a water invaded oil column. At Yates, nitrogen gas is injected into the reservoir to recover more oil by expanding a gas cap and allowing gravity drainage of liquids to occur.

A steam pilot was initiated in December 1998 to assess the economic viability of improving the vertical gravity drainage process. This pilot, implementing the TAGS (Thermally Assisted Gravity Segregation3) process, is unconventional in a number of ways. The pilot utilizes innovative treating processes to generate steam from poor quality produced water4. Steam is injected into a fractured secondary gas cap at high rates, not as a displacing agent, but to heat oil, reduce viscosity and improve gravity drainage from dolomite matrix toward highly conductive fractures. Oil mobilized due to steam injection drains vertically to the oil column, then laterally via fractures to offset producers (see Figure 2). Economic viability will ultimately depend on the incremental oil production due to steam injection versus project costs.

Many issues arise in the design and implementation of an EOR project of this type, which ultimately depend on an accurate characterization of the fracture system. Issues in the design phase include the location of the steam pilot, the rate of steam injection into the gas cap, the number of injectors required to achieve this rate and the ultimate placement of these steam injectors. Once the project is implemented, a successful monitoring program depends on an understanding of how steam and heated fluids will move through the reservoir. Poorly positioned monitoring wells that lack communication with the fracture system are not only expensive, but may not provide critical data necessary for an economic assessment of the project.

Many geologic and engineering tools were employed throughout the course of the project. This paper will now focus on these tools, how and when they were used and key decisions that were based on their analysis.