Use Of Time-Lapse Logging Techniques In Evaluating The Willard Unit CO2 Flood Mini-Test
Authors: Charlson G.S. , Bilhartz H.L. , Stalkup F.I. 
Published: December 2007

Abstract
This paper describes how compensated neutron and pulsed neutron logs were used to monitor performance pulsed neutron logs were used to monitor performance of a tertiary recovery minitest of CO2 miscible flooding. The paper contains (1) a description of procedures used to take and interpret the logs, procedures used to take and interpret the logs, (2) a discussion of the significance of the log responses, and (3) a discussion of the accuracy of calculated saturation changes.
The experiment was run in the San Andres dolomite formation in the Willard Unit, Wasson field. Logs were taken in an observation well, completed with unperforated steel casing, and located 100 ft from the CO2 injector. Water was injected first to thoroughly waterflood the region near the observation well. This was followed by injection of CO2 and water in small, alternate slugs. Logs were taken regularly during both the waterflood and the CO2 flood.

Introduction
In 1972, field testing of the carbon dioxide miscible process was begun in the Willard Unit of the Wasson field in Yoakum County, Tex. Concurrently with a flood on normal well spacing, a nonproducing tertiary recovery minitest was run to get the following information:change in residual oil saturation caused by alternate CO2/water injection;extent of gravity segregation within the entire productive interval and within porous zones; and productive interval and within porous zones; andimportance of stratification and crossflow.
To obtain this information, saturation changes during waterflooding and CO2 flooding were calculated from compensated neutron and pulsed neutron logs taken in an observation well; a core was taken behind the CO2-flood front with the pressure core barrel; and a history match of observation well saturation changes was made with a miscible flood simulator. This paper describes the observation well logging and its value.

RESERVOIR DESCRIPTION
The San Andres formation in the Willard Unit is characterized as an anhydritic, finely crystalline dolomite interbedded with subtidal or marine wackestones and packstones. Porosity and permeability development is dominated by diagenetic permeability development is dominated by diagenetic processes including dolomitization, leaching, and processes including dolomitization, leaching, and emplacement of anhydrite. Vertical flow is restricted by mudstones or by wackestones and packstones with lime mud or anhydrite filling the packstones with lime mud or anhydrite filling the pore space. Locations of wells in the minitest are pore space. Locations of wells in the minitest are shown in Fig. 1. Fig. 2 is a cross-section through these wells illustrating the continuity and highly stratified nature of the reservoir within the 160-ft pay interval. The six vertical flow barriers pay interval. The six vertical flow barriers indicated in this figure are mud-rich zones with permeabilities less than 0.1 md. We believe they permeabilities less than 0.1 md. We believe they extend throughout the minitest area. Fig. 3 shows porosities estimated from core and log data at the porosities estimated from core and log data at the 32AO well. The intervals shown by alphabetic designations are major porous zones.

DESCRIPTION OF MINITEST
The injection well (32A) was completed by setting casing through the formation, perforating across selected intervals, and fracturing - a typical completion technique for both injection end producing wells in the Willard Unit. The fracture was propped open with sand and had a calculated half-fracture length of 200 ft. A logging observation well (32AO) was drilled 100 ft north of the injector. In this well, casing was set through the formation but was not perforated. Another well (32AS) was drilled for fluid perforated. Another well (32AS) was drilled for fluid sampling and pressure measurements 25 ft north of the observation well. It was perforated selectively, but was not fractured. At the test conclusion, cores were taken through the entire injection interval with a pressure-retaining core barrel. The coring well pressure-retaining core barrel. The coring well (32AC) was located between the injection and observation wells.