A Method For Projecting Full-Scale Performance Of CO2 Flooding In The Willard Unit
Authors: Bilhartz H.L. , Charlson G.S. , Stalkup F.I. , Miller C.C. 
Published: December 2007

A non-producing CO2 flood tertiary recovery test was recently completed in the Willard Unit of Wasson Field. Flood responses during waterflood and alternate injection of CO2 and water were monitored at a logging observation well using compensated neutron and pulsed neutron logs. A pressure core was taken to measure residual oil saturations at the test conclusion.

The overall objective of the testing was to obtain information for evaluating the potential for full-scale CO2 flooding in the unit. Our method for making this evaluation involves:defining CO2 flood displacement efficiency and representing this efficiency in a miscible flood reservoir simulator;defining a representative average reservoir description; andprojecting full-scale CO2 flood performance with the simulator.

The paper provides a performance with the simulator. The paper provides a status report on progress to assess CO2 flooding potential for the Willard Unit in this manner. potential for the Willard Unit in this manner

Introduction

A tertiary recovery test of CO2 miscible flooding was completed recently in the Willard Unit of Wasson Field in Yoakum County, Texas. The test involved time-lapse logging an observation well and pressure coring behind the CO2 front. It was designed to give the following information:

reduction in waterflood residual oil saturation caused by alternate CO2/water injection;

extent of gravity segregation across the entire pay interval and within porous zones;

importance of stratification and crossflow.

This information is needed to assess the potential for full-scale CO2 flooding in the Willard Unit. Volumetric sweepout by CO2 and the volume of displaced oil that actually can be recovered, also needed for the assessment, were not determined directly in the test. Instead, these important measures of CO2 flood performance will be projected with a numerical reservoir performance will be projected with a numerical reservoir simulator from test performance using a reservoir description representative of the Willard unit.

This problem of scale-up or projection of fullscale flood behavior from pilot test behavior is common to all small pilot tests whether they be minitests, such as the one described in this paper, or producing pilots. The test area usually does not producing pilots. The test area usually does not have reservoir properties representative of the reservoir on average. Well pattern and spacing may also be different. Reservoir simulators can be a useful tool to accomplish this scale-up. In this paper we describe a method for evaluating CO2 flooding in the Willard Unit using mini-test data as well as additional data from primary production, waterflooding, laboratory core flooding, and coring. Results from a preliminary analysis are discussed to illustrate this method.

DESCRIPTION OF MINI-TEST

Fig. 1 shows the location of wells in the minitest area. The 32A injection well was completed by Setting casing through the formation, perforating in selected intervals, and fracturing. This was the typical completion technique for injection and producing wells in the Willard Unit. The fracture was producing wells in the Willard Unit. The fracture was propped with sand and had an estimated half fracture propped with sand and had an estimated half fracture length of 200 feet. Orientation of this fracture was nearly perpendicular to a line through the four minitest wells (Fig. 1). A logging observation well (32AO) and a fluid sampling/pressure monitoring well (32AS) were located 100 feet and 125 feet north of the injector. Both wells were cored to get reservoir description data. Well 32AC, located 35 feet north of the injector, was drilled with the pressure-retaining core barrel to obtain a core behind the CO2 front.

At the start of the mini-test, the test area had been depleted by solution gas drive to a pressure of 600 psi.