Abstract

Rienecke field is one of a series of fields in the Horseshoe Atoll of the Midland Basin. This carbonate reservoir has fairly good lateral and vertical continuity and is confined in a dome-like structure. A gravity-stable crestal carbon-dioxide flood was begun in 1997 by injection into five wells at the high point of the structure. Pulsed-neutron logs run through casing provide a method to monitor reservoir dynamics and locate oil production targets.

The first step in developing a cased-reservoir analysis model is to estimate the carbon dioxide content. This is done by correlation of the cased-hole neutron and density porosities from the pulsed-neutron system. In developing a three-phase saturation model (oil, water, carbon dioxide)the effects of the carbon dioxide on the oil and water saturation measurements must be determined.Computer simulations of tool response are utilized in conjunction with field data to develop the model and the uncertainties in the saturations.

An example log and analysis demonstrate the capabilities of the reservoir analysis model. The analysis results are compared to production and historical data for the field.

Introduction

The Reinecke field is one of a series of fields along the Horseshoe Atoll of the Midland basin. The structure of the hydrocarbon entrapment is a dome shape created by phylloidal algal build-up1.Originally discovered in the 1950's, the field has produced over 82 million barrels of oil. Given the dome structure and the good permeability, the remaining oil is being produced by injecting carbon dioxide into the crest of the dome.Cased-hole logging is one of the available analysis tools to monitor the flood progress and select oil production targets2.During the initial production and water drive production, many of the wells were cased and completed after drilling only a few feet into the reef. Now with the crestal CO2 flood, the wells are drilled or deepened into the reef structure so the oil below the gas cap is produced.Along with providing petrophysical analysis, pulsed-neutron logging can also provide geological correlation across the structure. Figure 1 shows the cased-hole neutron and density porosities set into a geological cross-section.

Specific-purpose response models for the pulsed-neutron measurements are developed for monitoring the oil and CO2 content. Along with determining the gas cap location, the effects of the moved oil are considered. The miscible oil is generally considered to be a mixture, approximately 80% oil and 20% carbon dioxide3.The response of the pulsed-neutron measurements for the oil-rich liquid phase and the CO2 rich gas phase are described in the sections that follow.

Quantifying the Carbon Dioxide Content

The response of the neutron porosity can be used to quantify the amount of CO2 in a reservoir section. As the CO2 displaces water or oil, the drop in the hydrogen content is reflected in the neutron porosity. To quantify this effect a series of points were generated using the computer code POROSITY 4.This code calculates the apparent neutron porosity for a given mixture of rock and fluid.

Figure 2 is a graph of the pulsed neutron ratio for porosity and CO2 content. The splines in the graph represent porosity values. The expected ratio response for various amounts of CO2 and porosity can be read from the graph. To apply this linear relationship in time-lapse monitoring, the current neutron porosity is compared to the original fluid-filled base run of pulsed neutron or compensated neutron porosity. When no base logs exist, the cased-hole, density-neutron crossplot porosity can be used to estimate the original fluid filled porosity.

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