Abstract

Roberts Unit is a mature San Andres waterflood project located in Wasson Field, Yoakum County, Texas. Texaco, as operator, is evaluating the reservoir for CO₂ flooding, and a four-phased CO₂ project is being designed for the Unit. A critical aspect of CO₂ flood design is the development of a geologic reservoir model. The model also contributes to other areas of reservoir management such as flood monitoring and evaluation of infill drilling. This paper describes the geologic reservoir model established for the southeastern portion of the Unit (the CO₂ Phase III Area).

The reservoir consists of stacked carbonate depositional sequences. The cyclic nature of these depositional sequences is reflected in both core-defined lithofacies and porosity log character. Sequences consist of basal mudstones, restricted shelf skeletal wackestones, open shelf skeletal wackestones and packstones, solution and brecciated zones and peloidal packstone caps. Intertidal mudstones and wackestones occur at the top of the reservoir and in the overlying reservoir seal. Porosity distribution is controlled by diagenetic events, but these events are closely related to depositional facies.

Reservoir geometry and reservoir quality are interpreted from study of carbonate lithofacies, porosity/permeability relationships, and production and injection characteristics. Depositional sequences are subdivided into layers (“flow units”) for use in reservoir simulation. Log normalization, core description, reservoir mapping, 3-D modeling, and joint effort between project geologists and engineers contributed to the development of the reservoir model.